Subfloor: Filling Floor Ridges vs. Leaving Them Empty

Introduction

Building a campervan subfloor in a van involves dealing with the factory metal floor, which has raised ridges (corrugations) with valleys in between. A common question among van builders is whether to fill those low valleys before adding insulation and subfloor. The decision can impact thermal performance, noise, and floor height. In fact, there are “two schools of thought” on this issue and endless debates online (Bedrug VanTred Cargo Liner: Drop-In Floor Insulation for Vans?). This guide will break down the pros and cons of filling vs. not filling the floor corrugations, and recommend best practices given your priorities:

1. Maximize thermal insulation
2. Reduce sound (noise)
3. Minimize overall floor height buildup

By drawing on van conversion experts and community experiences, we’ll help you decide the optimal approach for your Transit subfloor.

Understanding the Transit Floor and Your Priorities

The van’s cargo floor is thin metal stamped with corrugated ridges for strength. The ridges (~0.3–0.4″ high) create an uneven surface with hollow channels in between. When insulating and building a subfloor, you have to choose how to handle these channels (“valleys”). Key considerations include:

• Thermal Insulation: The metal ridges act as thermal bridges – they conduct heat/cold easily. Insulation (like XPS foam boards) is used to cover the floor and slow heat transfer. Any uninsulated air gaps or exposed metal will reduce overall R-value. A continuous layer of insulation is ideal to keep the floor warm.
• Sound Reduction: The van’s metal floor can resonate road noise and vibrations. Hollow cavities can amplify sound (drum effect), and wood-on-metal contact can lead to squeaks. Adding mass or damping material and decoupling layers can significantly cut noise.
• Floor Height: Every inch of subfloor (insulation + plywood + flooring) eats into interior headroom. Many vanlifers aim to keep the floor build thin. However, going too thin can sacrifice comfort (cold, noisy floor) or structural stability. The challenge is to achieve good insulation and sound deadening with minimal thickness.

Understanding these factors, let’s examine the two main approaches for the ridged floor: filling the valleys versus leaving them open under your XPS insulation.

Option 1: Filling the Floor Valleys (Insulation Strips)

What It Involves: Cutting pieces of insulation (or other material) to fit snugly in the low channels so they are flush with the tops of the metal ridges. Commonly, builders use strips of rigid foam (XPS or polyiso) or a closed-cell foam like minicell, glued in place. Some older methods use wood furring strips, but as we’ll see, that has drawbacks.

Pros of Filling the Valleys:

• Better Insulation & Sound Damping: Packing foam into the corrugations adds a bit more R-value and eliminates empty air cavities. This can slightly boost thermal performance and reduce noise compared to leaving them empty (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). The foam in the channels is additional insulation and also contacts the metal, potentially damping vibrations. One van builder notes it “gives you more insulation than just leaving air voids” (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife).
• More Support for the Subfloor: With the valleys filled level, you create a flat, fully supported platform. Your top foam board and plywood will sit on continuous support rather than spanning gaps. This can reduce flexing and potential squeaks. FarOutRide (a respected van conversion blog) filled the ribs in their first van and noted it provided more support surface for the next layer (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). If your ridges are uneven height, filling makes sure there are no low spots.
• Less Chance of Foam Compression: Although quality XPS foam is very rigid, having support in the valleys means any point load on the floor is distributed. This gives peace of mind that the foam won’t be crushed in the long term. Some builders who are “really paranoid about squeaks” or compression choose to fill because it’s a “proven process” (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). (As we’ll discuss later, high-density foam can usually handle the load even without fillers.)
• Leveling for Finished Floor: A filled floor is completely level metal-wise. This is helpful if you plan to glue down thin flooring directly. It also means any spills won’t immediately run to the center – though this is a double-edged sword (more on moisture below).

Cons of Filling the Valleys:

• Significant Extra Work: Nearly everyone agrees that cutting and fitting strips for every channel is tedious and time-consuming (How to Build an Insulated Subfloor in Your Van Conversion | by Jeremiah Luke Barnett | Medium). There can be dozens of strips to measure and cut. A DIYer who filled all grooves with 0.5″ Foamular board said “This was a very tedious process” (How to Build an Insulated Subfloor in Your Van Conversion | by Jeremiah Luke Barnett | Medium). It’s a lot of effort for what some consider marginal gains. FarOutRide ultimately decided the “return on investment [was] not worthwhile” for a full rib fill (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide).
• Potential Moisture Trap: By filling the valleys, you essentially block those channels. Any water that does find its way under the floor (from condensation or a spill) can’t easily escape. The trapped moisture could sit against the metal and promote rust (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) (Bedrug VanTred Cargo Liner: Drop-In Floor Insulation for Vans?). In contrast, an open channel could allow water to flow to a drain point or evaporate. One vanlifer mentioned they skipped filling because “if I ever have a major spill or leak it might be nice to have all those channels open for the spill to drain”, which indeed helped when a clogged drain caused a leak (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). Thus, filling requires you to be confident in your water sealing and perhaps add drain holes or desiccants to mitigate corrosion risk.
• Minimal Height Impact, But…: Filling the ridges itself doesn’t add height (you’re just bringing the low spots up to ridge level), but there’s a subtle effect: some people who fill with wood strips then lay plywood directly on the ridges/strips. This saves height but means no continuous foam above the ridges. If you instead fill with foam and still add a top foam board layer, you haven’t saved any height – you’ve just done more work to fully support it. In short, filling doesn’t really allow a thinner build; it’s usually done in addition to your planned insulation thickness.
• Avoid Wood Furring for Insulation: A popular older method was to use wood slats in the valleys (for example, 1/4″ or 3/8″ wood strips) as anchoring points for the subfloor. However, wood is a thermal bridge – it conducts heat/cold better than foam. Using wood breaks your insulation layer and creates hot/cold stripes through the floor. As one guide cautions: “using wood furring strips to fill the corrugations… creates ‘thermal bridges’… basically a path for the cold from your metal floor, through the wood and into your finished floor. You need a solid layer of insulation between the metal and your finished floor.” (Insulating and Installing a Campervan Floor). If you need hard attachment points (for bolting down furniture), it’s better to use limited wood or metal only where required rather than across every rib. We’ll cover a hybrid approach in the best practices section.
• Cost of Materials: Filling the ribs usually doesn’t require much additional material (foam board or strips are relatively cheap), but specialty foam like minicell can be expensive. Some builders like the FarOutRide “4-season” method use 1/4″ minicell foam sheets in the valleys for sound and anti-squeak. Minicell is great (doesn’t squeak, hydrophobic), but if you have to import it, it can be pricy. In one case, a builder in Canada found it would cost $300 extra to get minicell for filling, and was looking to skip it (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). If cost or availability is an issue, you could use strips of the same XPS you already have (many have done this successfully (How to Build an Insulated Subfloor in Your Van Conversion | by Jeremiah Luke Barnett | Medium)). Just be aware to secure them well (spray adhesive works) so they don’t rattle loose.

How to Fill Properly (If You Choose To): If you decide the benefits are worth the work, here are a few tips from the community:

• Use foam, not wood: Preferably use closed-cell foam strips (XPS or minicell polyethylene foam). XPS has higher R-value (~5 per inch) (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide), while minicell has ~3.5 per inch but is softer and guarantees no squeaks (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). Both are closed-cell (won’t absorb water). Cut strips of the appropriate thickness (usually 1/4″–3/8″) to match the ridge height (Insulating and Installing a Campervan Floor). Glue them to the metal with a strong spray adhesive (3M 90 or similar) for a firm bond (Insulating and Installing a Campervan Floor). Ensure no gaps for best support.
• Pre-template the floor: Use a cardboard template or the factory mat (if you have one) to mark where strips need to go. Some builders lay big sheets of foam first, then trace ridges, but it’s often easier to place strips first, then add the top layer.
• Consider BedRug VanTred: If time is more precious than money, there’s a product called BedRug VanTred (or VanRug) which is a pre-shaped plastic/foam floor liner. It’s essentially a drop-in layer cut to your van’s exact floor shape, with a foam backing that fills the corrugations and a rubbery top surface. This can save hours of work: “Cutting out each individual piece of minicell foam and gluing them down is a tedious process… [BedRug gives] a custom fit floor template” out of the box (Insulating and Installing a Campervan Floor). The VanTred is about 1/2″ thick and provides around R-3 insulation (Insulating and Installing a Campervan Floor). The downside is cost (~$200-300) and slightly lower R-value than thicker foam boards. Still, many find the convenience worth it. (Image below: a VanTred cargo liner being installed – it unrolls to fill the ridges with its foam backing.)

(DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) A drop-in BedRug VanTred cargo liner being installed in a Ford Transit. The foam-backed mat fills the floor’s corrugation valleys, providing a flat insulated surface. This is an alternative to cutting individual foam strips by hand.

• Don’t block weep holes: Ensure any factory drain/weep holes in the floor remain clear or are relocated. If you fill over them, drill a small hole through the foam or floor layer so that if water does get under, it has an exit.
• Treat the metal floor: Since filling can trap moisture, it’s wise to rust-proof the metal first. Clean any existing rust and apply a coat of rust-inhibitor paint (e.g. POR-15 or Rustoleum) (Van floor tips, climate considerations? | Expedition Portal). This extra step ensures that even if a bit of water sits in a valley, your metal is protected from corrosion.

Option 2: Leaving the Valleys Unfilled (Floating Floor)

What It Involves: In this approach, you do not put anything in the low grooves. You lay your insulation board (XPS, polyiso, etc.) on top of the ridges, effectively “bridging” over the valleys. Then you add your plywood subfloor on top. The foam and plywood span the gaps created by the ridges, and the space in the channels remains an air gap. In essence, the subfloor is supported only by the metal ridges and wherever the foam touches those high points.

Pros of Not Filling:

• Simpler and Less Work: The biggest advantage is simplicity. You can cut your foam boards to fit the van’s floor outline and drop them in, without fiddling with dozens of small pieces. It’s “less work, cheaper” as FarOutRide summarizes (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). Many DIYers report that skipping the filler strips made the floor install much faster with no ill effects. This approach also uses slightly less material (you’re not buying extra foam for the channels).
• Preserves a Drainage Path: Leaving the channels open can be beneficial for moisture management. Any condensation on the metal or minor leaks/spills have a path to evaporate or migrate instead of being trapped against the metal. The valleys create little gutters under the floor. FarOutRide notes this approach “allows condensation/spill water to evacuate” (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). In real-world examples, people have indeed found that an open channel helped alert them to water and guided it out. For instance, one builder mentioned a leak from a clogged vent: because he hadn’t filled the valleys, the water ran along the groove to where he could see and address it (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). In a filled floor, that water might have been absorbed or hidden under the insulation. In short, an open design is a bit more forgiving if moisture sneaks in.
• Nearly Equal Insulation (with Air Gap): It might seem counterintuitive, but an enclosed air gap has insulation value too. The hollow space in each valley is trapped air once the foam board and flooring are on, which provides some thermal break. One van professional argued that “the still air gap is going to work nearly as well as filling it” in terms of insulation (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). The main insulation is from your continuous foam layer above, which is the same in either case. The difference in overall R-value between filled vs. unfilled grooves (especially if the grooves are only ~0.3″ high) is quite small – on the order of R-0.2 or so. Many builders report warm floors even without filling, as long as a decent thickness foam board is used. Thermal priority can still be achieved without fillers, by using, say, 3/4″ or 1″ XPS across the whole floor.
• Adequate Strength – Foam Can Handle It: Modern rigid foam boards (XPS, polyiso) have high compressive strength (15–25 psi or greater). Spread over even a small area, they can support a tremendous load. For example, one van builder noted that a sheet of XPS has a weight capacity of over 6 tons – “Your van will be crushed before the XPS [fails]” (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). In practice, people have seen that the foam does not collapse or permanently compress in the gaps. A Sprinter van owner who did 1″ polyiso with 3/4″ plywood, no strips, reported after a year: “the XPS had not really compressed much into the corrugations… Our floor is solid and stable” (Insulating floors…really necessary for conversion? | Sprinter-Source.com). Another who full-timed for 2 years with just 1/2″ foam and no plywood (only laminate on top) found “No compression. No damage to the van’s floor” after heavy use (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers). These real-world cases show that as long as you use high-density foam (XPS or polyiso, not cheap white EPS) (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers), the foam and subfloor can easily bridge the 4-5″ wide gaps between ridges without issues. (Do avoid the low-density beaded polystyrene; it’s not durable enough.)
• Avoids Thermal Bridges: By not adding wood furring, you maintain an all-insulation layer between the metal floor and your living space. Aside from the thin metal ridges touching the insulation, there are no additional conductors. This is actually a more thermally efficient design than using wood strips. As one van converter put it, “The framing [wood strips] is more work and introduces thermal bridging”, whereas just foam and plywood avoids that (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers). In your case, with insulation as a top priority, skipping wood makes sense – and an unfilled floor by definition uses no wood in the field (except perhaps at edges or for specific anchors, which we’ll address).
• No Height Penalty: This method doesn’t add any extra layers beyond what you planned. You’ll likely use the same thickness of XPS foam board either way, so the total build height remains minimal. In fact, some people have gone ultra-minimal with this approach: for example, laying 1/2″ or 1″ foam and then installing thin laminate or vinyl flooring directly on top, with no plywood, to save height (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers). (This can work if the floor isn’t bearing heavy furniture, but most prefer at least a thin plywood for secure anchoring and weight distribution.)

Cons of Not Filling:

• Slightly Reduced Insulation & Soundproofing: Leaving the valleys empty does mean there’s a bit less material between you and the outside. You’ll have small air pockets instead of foam there. The metal ridges themselves are still exposed on the inside (they touch the underside of your foam board). So, in theory, a filled floor has marginally higher R-value and a bit more sound damping. FarOutRide notes the trade-off: not filling yields “a little less insulation [and] noise reduction” (Bedrug VanTred Cargo Liner: Drop-In Floor Insulation for Vans?). If you plan to camp in extreme cold, you might care about every fraction of an R-value. That said, many vanlifers (even in winter climates) report no noticeable difference – other factors (overall insulation thickness, etc.) have a bigger impact. For noise, the hollow channels could very slightly amplify sound, but in practice the difference can be mitigated (see best practices below).
• Less Direct Support (Theoretical Concern): One worry is that without fillers, the plywood and foam are not supported in the middle of each span between ridges – potentially causing flex or foam compression under heavy loads. In practice, with quality materials and correct install, this has proven to be a non-issue. For instance, a van builder on Reddit pointed out that XPS is used under concrete slabs and doesn’t compress under a van’s furniture and occupants (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers). Another said, “Filler strips seem like a lot of work, but I don’t see the value… our floor is solid and there is no way it could wobble.” (Insulating floors…really necessary for conversion? | Sprinter-Source.com). The key is to use sufficiently thick foam and a strong top layer. If you use a very thin plywood (or none) and have a heavy point load, then yes, you could feel some flex. But with, say, 1/2″ plywood screwed down into the foam, the floor will feel firm. One caution: during installation, before the plywood is in, avoid stepping directly on the foam spanning a gap – you could crack it under your foot. “Just be somewhat careful to not crack it by walking or kneeling on the XPS on a rib until you install plywood,” one installer advises (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). Once the subfloor sheathing is on, the load spreads out and this concern disappears.
• Possible Squeaks or Movement: If the foam and plywood are not secured, there’s a chance of the assembly moving or rubbing slightly on the metal ridges over time. This can cause squeaking noises when driving or walking. Rigid foam against metal can squeak if there’s any flex. Without filler strips, you have fewer contact points (just the ridges). However, this can be addressed by proper installation: using adhesive or a thin cushioning layer so nothing is loose (we’ll detail this in best practices). FarOutRide mentions that XPS floors can squeak if no care is taken, but they solved it by adding glue and would still choose not to fill the whole floor (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). In short, an unfilled design might require a bit more attention to prevent noise, but it’s manageable.
• Edge Support Gaps: In a Transit, the pattern of ridges might leave some spots (especially along the edges or corners) where there isn’t a ridge directly under part of the subfloor. For example, near the walls or the sliding door threshold, you could have an overhang. In a filled scenario, you likely would have placed a strip there; in an unfilled scenario, that spot would be unsupported. The solution is simply to add support only where needed (e.g. a small block or strip at the perimeter) without filling every valley. Even professional upfitters do this: “Some vans like the Transit also need some furring strips around the perimeter since the raised corrugations are not very close in these areas” (Van floor tips, climate considerations? | Expedition Portal). So, you might still add a few strategic supports – which isn’t really a “con,” just a modification to the pure empty approach.

How to Succeed with an Unfilled Floor: If you choose not to fill the valleys, follow these best practices to ensure a quiet, sturdy, and well-insulated result:

• Use High-Density Rigid Foam: You’re already planning XPS, which is perfect. XPS (extruded polystyrene, e.g. Owens Corning Foamular) has ~15-25 PSI compressive strength and R-5 per inch (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). Polyiso boards also work (slightly higher R (~6 per inch) but can lose R in very cold temps). Stick to 1/2″ or 3/4″ minimum thickness (many do 1″ for more insulation – according to one guide, the cost and headroom difference between 1/2″ and 1″ isn’t huge (Insulating and Installing a Campervan Floor)). Cut the foam boards to closely fit the floor shape and around wheel wells, etc., so they don’t shift. Do not use low-density EPS (the white beaded foam) – it can crush and squeak.
• Consider a Thin Decoupling Layer: One clever trick to avoid squeaks is to lay a thin layer of foam or fabric over the metal floor before the rigid foam boards. For example, a 1/8″ closed-cell foam (Minicell or even a yoga mat) or strips of it on top of each metal rib. This thin foam will compress slightly and act as a cushion so that the plywood/foam above isn’t rubbing hard on metal. It also adds a tiny bit of insulation over the ridge peaks (since those are essentially uninsulated contact points). Some van builders roll out a layer of 3M Thinsulate acoustic insulation (which is about 1/4″ thick and compressible) across the floor for the same purpose (Van floor tips, climate considerations? | Expedition Portal). “Roll it out then put your floor on it” – it’s super easy, reports one engineer, noting that “Thinsulate is adequate and offers better noise reduction” while Minicell is a bit more supportive (Van floor tips, climate considerations? | Expedition Portal). The Thinsulate will crush down where the ridges are, but still provide a buffer. This step is optional – your floor will work without it – but if road noise and squeaks bother you, it’s a good addition with negligible height impact.
• Adhere the Foam to the Metal: Another way to eliminate movement and noise is to glue the foam boards down to the metal floor (at least along the ridges). Using an adhesive (like 3M spray or even construction adhesive in a zigzag) between the XPS and the metal will stop the foam from shifting or bouncing in the gaps. FarOutRide emphasizes this in hindsight: “Don’t cheap out on glue at the metal/XPS interface (top of grooves); use lots of it to achieve uniform coverage” (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). By effectively bonding the foam to the floor, you create a laminated structure that is less prone to squeaks. In FarOutRide’s second van, they did not fill the ribs and reported a couple of squeaks, but they chased them down by adding more adhesive, and then it was solid (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). They still “wouldn’t fill the valleys” completely in the future, preferring strategic support only where needed (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide).
• Screw the Subfloor Securely (but floating is okay): Once the foam is down, lay your plywood on top. Many builders actually “float” the floor – meaning they do not bolt or screw the plywood into the metal; it’s held down by its own weight and the furniture on top. This floating approach further decouples vibrations (less noise transfer) (Van floor tips, climate considerations? | Expedition Portal). If you go this route, just make sure the plywood pieces are tightly fit edge-to-edge and walls so they can’t slide. You can tape the seams or use a few dabs of adhesive between foam and plywood to keep it in place during the build. If you choose to mechanically fasten the plywood to the van (some prefer a couple screws to prevent any shift or to reuse factory D-ring bolt holes), be careful: compressing the foam with a screw can crumple it. A pro tip is to insert small spacers or sleeves at the bolt locations – e.g. a short piece of wooden dowel or hard plastic tube the height of the foam – so when you tighten the screw, it pinches against that and not the foam. Builders “recommend PVC lumber furring strips or blocks where cabinets and other items are through-bolted… to keep the insulation from compressing when the bolts are tightened” (Van floor tips, climate considerations? | Expedition Portal). This way, you still don’t have wood everywhere, just at bolt points. If you’re not bolting anything through the floor, you can avoid this entirely.
• Add Sound Deadening to Metal (if needed): Since an unfilled floor leaves some metal exposed to air (inside that channel), you can pre-treat the metal to kill noise. Applying some CLD sound deadening mats (like Dynamat, Noico, etc.) on the floor will greatly reduce vibration resonance. You don’t need 100% coverage; even 25% coverage with patches is enough to dampen the “drum” effect. For instance, one van owner covered the entire floor with sound deadening mats and rubber pads before his plywood (overkill for most) (Van floor tips, climate considerations? | Expedition Portal). Even sticking a strip inside a few of the longer channels or on broad metal panels between ridges will quiet things down. This is something you can do regardless of fill vs. no-fill – it’s just easier to access the bare floor before the foam goes in. It will address road noise better than foam does (foam is more for thermal insulation, whereas butyl sound mats add mass to stop vibration).
• Edge and Span Supports: As mentioned, identify any sections where your plywood might not have support for a long distance. Common areas are the perimeter edges of the floor, the ends of the van, or around any large openings (like a floor vent or seat bases if you removed them). You can cut a few strips of foam (or use scrap wood/plastic) to fill just those spots. FarOutRide’s refined approach was exactly this: they “wouldn’t fill the valleys on the floor, but [would] strategically add XPS or Minicell mini-strips where there is no support over large surfaces, especially near the edges… [like] near the sliding door, around the shower pan cutout, and near the front edge” (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). Think of it as spot-filling only where needed for structure. This keeps most of the floor simple and open, but shores up any weak points. It’s a great compromise to meet all three of your priorities.

Comparison of Approaches (Summary)

Both methods – fully filling the ribs or leaving them empty – can and do work. In fact, FarOutRide has used each approach on different builds (filled on their first van, unfilled on their second) and observed success in both (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). Here’s a quick recap of the trade-offs, as highlighted by experienced van builders:

• Thermal Insulation: Filling can add a little extra R-value, but a continuous foam layer on top is far more critical. In practice, 1″ of XPS plus 1/2″ plywood is sufficient for winter vanlife even without filling, according to FarOutRide (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). They deliberately chose not to fill in their 4-season build and stayed warm, even in freezing conditions, with insulated walls/ceiling and a heater (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). If absolute maximum R-value is needed, fill the gaps with foam – but most find it not strictly necessary for comfort.
• Sound and Vibration: A filled floor has more contact and mass on the metal, which can dampen noise slightly. An unfilled floor can be just as quiet if you add other sound treatments (mats, foam underlayment). Squeaking is more about installation quality than fill vs. not. Use foam that doesn’t squeak (minicell) or add adhesive to prevent rubbing. Many van owners report no noticeable noise difference between the two methods (Bedrug VanTred Cargo Liner: Drop-In Floor Insulation for Vans?), especially once furniture and flooring are added (which tend to suppress sound anyway).
• Moisture Handling: This is arguably the biggest functional difference. If you’re in wet climates or worried about spills (imagine a leaky cooler or plumbing leak in the van), an open-channel floor gives you a hidden “drip tray” that might save you from unseen puddles. A filled floor demands more vigilance sealing up every possible water entry and maybe periodically checking under the floor if you suspect water. Neither approach is immune to condensation under the floor – you should always have a vapor barrier or rust protection on the metal – but leaving an air gap at the lowest point means any moisture has somewhere to go. This consideration often tips people in favor of not filling in humid or variable climates (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) (Bedrug VanTred Cargo Liner: Drop-In Floor Insulation for Vans?).
• Build Complexity: If this is your first van build or you’re on a tight timeline, not filling the ribs simplifies the project. It’s one less task. On the other hand, if you enjoy detailed work and want the satisfaction of “doing everything you can,” filling with foam isn’t that hard, just tedious. Some find it an easy step for peace of mind: “You probably don’t need to, but it’s very easy, not expensive, and a proven process,” said one Redditor, who didn’t want to risk any squeaks (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife). So, consider your tolerance for extra work versus the perceived benefit.

To quote a balanced perspective from FarOutRide’s review of this topic: “Both methods are commonly found in van conversions, but we still haven’t seen any real-world, long term evidence proving for sure that one method is definitely wrong.” (Bedrug VanTred Cargo Liner: Drop-In Floor Insulation for Vans?) In other words, neither choice will make or break your build as long as it’s done thoughtfully. Many vans have hit the road with unfilled floors and many with filled floors – both can last for years without issue.

Recommendations for Your Build

Given your priorities (thermal first, then sound, then minimizing height), here is a recommended path forward:

• Use a Continuous Insulation Layer: Ensure you cover the entire floor with XPS foam board (at least 1/2″, up to 1″ if you can afford the height). This addresses the bulk of your insulation needs. It also provides a decoupling layer for sound. The metal ridges will contact the foam, but that’s okay. Cut the foam panels to fit edge to edge. If possible, stagger the seams of the foam and the plywood (don’t have all seams lining up) to avoid a weak line (How to Build an Insulated Subfloor in Your Van Conversion | by Jeremiah Luke Barnett | Medium).
• Spot-Fill Key Areas Only: Rather than filling every groove, cut a few strips of foam or use scrap wood/PVC only in strategic spots: along the outer edges of the floor (where the plywood might otherwise flop into a channel), and any area you anticipate a point load that falls between ridges (for example, the landing point of a heavy partition or the base of a cabinet that isn’t over a ridge). This ensures you maintain support where it’s truly needed (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). You’ll still leave most valleys open for airflow.
• Add a Thin Noise Buffer on Ridges: For extra credit, lay strips of thin minicell foam or even duct-tape some pieces of rubber on top of the metal ridges before dropping the foam boards in. This will prevent the foam-on-metal squeak potential and add a hair more insulation over the rib peaks. It’s a low-effort step that many recommend if skipping full fills. (Alternatively, a full sheet of 1/8″ foam under the XPS as discussed will achieve the same effect.)
• Secure the Layers: Glue the XPS to the metal floor, and also glue or at least tightly screw the plywood to the XPS (screwing down into the foam, using large washers or plates to spread load, works well). By essentially laminating the floor layers together, you create one stiff assembly that won’t shift or creak. If you prefer not to glue for removability, you can skip gluing the foam to metal – but definitely glue the plywood to the foam or use a few screws through to the metal at the edges. The aim is to eliminate any slipping. A well-bonded foam-plywood floor will feel as solid as one attached directly to metal.
• Sound Deadening: Before installing the foam, stick some sound deadener patches in the middle of a few floor panels (the large flat areas between ridges) to kill resonance. Additionally, consider your finished flooring – a layer of vinyl plank, rubber coin flooring, or carpet will all help absorb noise and add a bit of thermal break on top. If you do all this, the difference between filled vs unfilled valleys in terms of road noise will be negligible.
• Monitor and Ventilate: Whichever way you build, occasionally check under the mats or edges for any signs of moisture in the first few months (especially if you travel from cold to warm climates which can cause condensation). If you ever spill a large amount of water inside, make sure to lift a corner of the flooring and let the channels dry out. You can even drill a couple small holes through the floor (in inconspicuous spots) and seal them with rubber grommets – these can act as drains or inspection ports. Many vans come with factory holes that you can utilize for this purpose. Keeping an eye on this will ensure longevity of your floor and van metal.

Bottom Line: For maximizing insulation and minimizing height – focus on a thick continuous insulation layer (and avoid wood thermal bridges). For sound – use damping materials and secure, squeak-free assembly. Fully filling every valley with foam is optional; it will give a minor boost in R-value and potentially peace and quiet, but many experienced builders feel it’s not strictly required (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers). In fact, several experts explicitly chose not to fill in order to save work and allow moisture egress, and they have “plenty warm” vans with quiet floors (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife) (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide).

Given your priorities, a hybrid approach is likely best: do not fill all valleys, but do add foam where it counts (edges or large spans) and implement sound/insulation measures as described. This approach was echoed by FarOutRide’s later recommendation and others in the community as a smart compromise (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide). You’ll achieve nearly the same thermal performance and silence as a fully filled floor, without the extra labor or risk of trapped moisture. And you won’t increase your floor height unnecessarily.

Lastly, it’s worth noting that the van conversion community has had success with both methods. As one forum member aptly put it, after seeing many builds: “Both methods are common… we haven’t seen proof that one is definitely wrong” (Bedrug VanTred Cargo Liner: Drop-In Floor Insulation for Vans?). Focus on the overall quality of your insulation install and make sure your subfloor is well secured. If you do that, you can feel confident whether or not you filled the little grooves.

Sources & Community Insights: This recommendation is based on a compilation of van builder experiences and expert advice, including FarOutRide’s floor insulation guide (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide), discussions from van forums and Reddit (r/VanLife and r/vandwellers) (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife) (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers), and professional upfitter tips (DIYVan/Hein and others) (Van floor tips, climate considerations? | Expedition Portal). These sources reflect a broad consensus that structurally, XPS foam and plywood are sufficient on their own, and that strategic support plus good installation practices will meet your goals. In summary: you do not strictly “need” to fill the Transit’s floor ribs, but doing so with foam strips is a valid method if you choose. With the hybrid approach recommended, you’ll maximize insulation and sound control while keeping the floor profile low – hitting all three of your priorities. Enjoy your van build!

(DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) (DIY Van Floor – Insulation, Subfloor & Covering Install Guide – FarOutRide) (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife) (Do I need to fill the ribs in my floor before laying XPS? : r/VanLife) (Trying to understand why people use firing strips when doing subfloor in van. Is it not just as good to use adhesive and secure rigid foam then plywood then flooring? Would love feedback, especially from those with experience. : r/vandwellers)