40 year old slab-on-grade: insulation under new floor
We are replacing the floor in one room of a 40-year old slab-on-grade house. Right now, the slab is fully exposed. The distance from the slab surface to the finished ceiling is exactly 8′. There is no vapor barrier or insulation. The heating climate zone is 5B, and the moisture regime is B. Two of the four walls in the room are exterior walls. We are not planning to do anything to the walls.
We are looking at engineered hardwood for the new floor. What is the best way to get some insulation under the new floor while minimizing the floor-to-ceiling height loss? I’ve seen the insulation question asked before, but some of the solutions would probably leave us with a 7’6″ ceiling, and others are not for the slab-on-grade situation.
Given materials and technologies available in 2025, what do you suggest?
Thank you.
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There are a couple things to think about in your situation. First, if you are only replacing the flooring in one room, you will have a step up into that room from any other adjacent rooms due to the added insulation and subfloor. Second, any door entering the room will need to be shortened. If the original door was 6-8, the new opening would be 6-2. This is awkward for taller people.
If you decide to add insulation on top of your existing slab, I suggest adding it to the entire floor, not just one room. Pull all the interior doors, cabinets, bathroom fixtures and any exterior door leading to this level of the home. Any windows that are at floor level will also need to be pulled. Raise all the exterior headers to accommodate the new floor elevation. You'll also have to adjust plumbing and maybe electrical outlets behind cabinets...As you can tell, the process to have it done correctly is tough.
There are some engineered flooring systems that are designed to have a pad underneath. Some of these pads can add a little R-value, increasing comfort without the work of adding larger amounts of insulation to the existing floor. Depending on thickness, you may have to trim the interior doors, much easier to do than removing and adjusting interior framing.
Hope this helps,
Randy
Another thing to look out for is stairs. If you raise the height of the floor the height of the first step is reduced, code requires all the stairs in a set to have the same rise. You'd need to either rework the whole staircase, or take out the bottom step and replace it with a landing.
Thanks for the reply. There are no stairs.
Thanks for the detailed reply. Most of your concerns do not apply to this situation given the design of the house. Just looking for a simple answer to the question as to what materials or technologies might help the situation. Looks like it might be an old-fashioned pad.
From a dollars and cents point of view floor insulation has a very long payback maybe never.
From a comfort point of view if you select a 1/8-inch-thick foam or felt underlayment the surface temp of the wood will be virtually the same.
I agree with Randy avoid any trip hazards and keep the floor as flat as possible.
Walta
Thanks for a straightforward answer to the question.
If the existing flooring in the rest of the house is 3/4" you can get a bit of insulation. You can get 3/8" engineered click hardwood flooring and install it over 3/8" cork. This gets you a bit of R value for no height increase and not much extra cost.
This one is behind the paywall, but *if* you were okay with ~1-2" height loss, it could be a reasonable option. But I agree with everyone about all of the difficulties of a step, door thresholds, and stairs.
Installing Rigid Foam Above a Concrete Slab
You can sandwich rigid foam between an existing concrete slab and new plywood or OSB subflooring
https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab
And also BSC's topside slab insulation article:
BSI-059: Slab Happy
https://buildingscience.com/documents/insights/bsi-059-slab-happy
And in case anybody mentions it, don't let anyone suggest bubble wrap as a miracle cure. :D
Stay Away from Foil-Faced Bubble Wrap
This R-1 product can be used to make Halloween costumes, but should never be used as insulation
https://www.greenbuildingadvisor.com/article/stay-away-from-foil-faced-bubble-wrap
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