Floating An Insulated Sub-Floor On Concrete Floor
Published 2024-02-14
Mike Hopp a RI remodeling contractor, and friend, asked us to help him frame out a large garage that was being converted into a studio apartment.
The garage was framed with 2x4 walls, a truss roof system and had an existing concrete slab floor. The trusses meant that we did not have any bearing walls inside the structure and meant that we cover the entire floor slab with no interruptions to the perimeter walls with insulating foam board.
We all agreed that this continuous foam board approach would be faster than installing pressure treated sleepers and then having to cut ridged foam to fit tightly between the sleepers, and spay foam them in.
Floating Floor Overview:
Mike’s plan called for an uninterrupted layer of 2” Owens Corning extruded polystyrene (XPS)foam boards directly on top of the concrete slab, with all the seams taped. This would be followed by two layers of 3/4-inch Advantech subflooring floating [not fastened] on top of the foam. The foam additionally allowed us to achieve a complete thermal break and obtain an R-10 insulating value above the concrete slab.
Creating a Thermal Break and Vapor Barrier
In an ideal scenario, the floor under this slab would have been insulated with rigid insulation such as XPS (extruded polystyrene) board, known for its high compressive strength. Utilizing XPS beneath the slab enables the concrete to retain heat effectively.
Our primary objective in covering the concrete floor with XPS foam board is twofold: to establish a thermal break and to implement a continuous vapor barrier.
Thermal breaks are essential for maintaining optimal heat retention within concrete slabs. Their advantages include heightened energy efficiency, enhanced comfort, and the prevention of condensation.
The absence of XPS foam board could result in cold air meeting warm, heat-generated air inside the studio apartment, potentially leading to condensation in the walls or floors and fostering mold growth and other issues.
Installing the Foam:
We sed Owens Corning FOAMULAR 400 Extruded Polystyrene (XPS) Insulation boards
designed for use in load-bearing situations, such as under slab, concrete floors, foundations, roadways and rail beds, parking decks, and cold storage installations.
We installed the 2” insulation boards on the concrete floor without glue. Installing the foam boards went quickly. The board cuts easily with a table saw lays flat on a clean, vacuumed concrete floor. Once the foam was fitted on the floor, we sealed all the seams with 3M #8067 All Weather Flashing Tape and sealed the perimeter seams at the concrete walls, and utility pipe penetrations with expanding foam.
This 3M tape is a self-adhering, waterproof flashing membrane designed for seaming damp surfaces such as polyethylene vapor barrier products, building materials and seal to around openings and penetrations in exterior walls. It has a unique acrylic pressure sensitive adhesive that aggressively sticks and stays stuck both at lower and higher application temperatures.
I will note that after installing the XPS foam boards on the floor we all noticed that the room felt warmer.
Note – we also covered the foundation walls, following the same procedure, but used 2” XPS 25 psi foam board as a cost saving solution.
Installing the Subfloor:
We then moved on to installed two layers of ¾” Advantech subfloor sheathing. The first layer of subflooring was installed perpendicular to the foam, taking care to stagger the ¾” Advantech and foam joint seams. We installed the tongue and groove plywood and ensured an 1/8-inch expansion gap at the butt ends.
Titebond subfloor adhesive was used to glue the first layer of Advantech plywood to the foam board. We used the larger 28oz tubes of glue and averaged approximately 2 sheets per tube.
Note – We recommend having a cordless caulk gun to assist in speeding up the process and save your forearm and grip strength.
The second layer or plywood was installed perpendicular to the first layer of plywood, again staggering our joints, and maintaining an1/8” expansion gap between subfloor sheets.
We then glued and screwed this second layer of plywood to the first layer, installing 1-3/4” screws every 6-inches to eliminate any squeaks between the subfloors.
After the subfloor system was installed, we installed our partition walls, and attached them directly to the sub-floor.
Conclusion:
The result was a strong and stiff subfloor without having to install sleepers that might loosen, squeak, or twist over time. The big win for us was achieving a thermal break with an R-10 insulation value and a continuous vapor barrier.
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All Comments (21)
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Awesome job, it looks fantastic Rob
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This video was awesome. Thanks for sharing this clever and unique solution.👍👍
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This would be an excellent bone and joint saver in a shop,and allow electrical to be in/under floor for tooling!
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Excellent video and information. I have a similar situation in my "this old house" basement. I appreciate the product inforomation to help guide me on my project. Thanks again.
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What a great idea. Could you also grove and layer pex for in floor heating in this at the same time?
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No worry of termites or carpenter ants getting in the foam board? Long time ago we used R-Max for wall sheathing on exterior under the siding and we had a terrible time with carpenter ants building tunnels and chewing up all the foam between the foil surfaces. From the outside I could put my hand on the vinyl siding and could just barely feel movement of them crawling around but we could easily hear them. Ended up removing the siding and ripping what was left of the R-Max. Thank God we only used it on 2 houses. A friend said he had same type problem with an ICF house they poured years ago also, but I've not heard that being a problem more recently. Just curious about putting foam on top of concrete, he have used it under slabs on a few projects, but no way really to monitor under a slab, what goes on.
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Great idea. How did you resolve doorways?
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Do you think I can get away with only one layer of 3/4 Advantech? I have a garage that was converted to studio a few yrs ago. I put down an epoxy floor at the time. But the floor is cold in the winters. A couch,chair, dresser and refrigerator will go on top. I’m thinking of a low pile rug on top of the subfloor. Cost is a consideration. I will go with the two layers if really needed for the floor to function well. Thanks for the video, appreciate it.
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This is a really helpful video. Thank you for making this. Question though. If I wanted to connect my radiant heater system to this and run pec through this. How would you suggest doing that? I was thinking of either running 5/8 plywood rips to channel around the pex pipe (as opposed to buying that crazy priced quik trak stuff) Then cement board, decoupler, then tile. That brings the total height to 4.75 inches above concrete. Is that overkill?
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I like the idea of this process. Couple question. What do you think would happen if water would get between the concrete and foam board. Do you think it would cause mold growth? Will it dry out? I would really like your thoughts on that aspect. If that was a concern could you lay down a dimple mat as a barrier to act as a drying plane. Lastly what are your thoughts of using say rockwool comfort board or rocktop instead of the pink foam insulation? Thanks Rob.
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Why wasnt a vapor barrier needed? Over the concrete / under the foam?
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this is well thought out, but I have to wonder if that Titebond is rated for the foam or if it will eat through it like most non foam board rated construction adhesive?
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Can this be done on a basement floor? I have a sump pump, but I want to make sure no mold grows under the lvp I’m planning to lay down in my basement.
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Why was Foamular NGX 400 chosen instead of Foamular NGX 1000?
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I’m looking to do this exact same project on a concrete pad under a covered patio. Was this garage pad sloped? And if so was there any concern with leveling the pad prior to installing the xps and subfloor? Thank you!
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Cordless 29oz caulk gun eh? I have a Cox 41002 gun which is pretty nice but with that amount of goop to dispense, I think you're right that a electric tool is the way to go.
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Being a garage, isn’t the concrete floor sloped for drainage? How did this method correct it? (Or is this a Washington St only code)
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That Titebond adhesive is not good for foam. It’s solvent based and will eat that foam. You need something like PL Premium! Other than that, a great install!
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Couldn't moisture migrate up through the slab and get trapped under the foam insulation unable to dry?
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Is this more cost effective?
