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hellonoko 02-14-2012 03:45 PM

floating a sub floor over foamular 150
I am insulating the slab in my daylight basement.

Is the below a solid way to do it?

The floor will need to support a wood stove. 40 gallon water heat. Tile shower. Gas furnace. Washer / dryer.

Does Foamular 150 have a strong enough PSI for this application or should I use something stronger?

I keep hearing how foam board is not 'structural' even though some of it has a high PSI for say garage floors. What does structural mean in this sense? That I can't nail/screw to it?

Can I float the sleepers over the foam board as in this display or do I need to put them between the foam board?

Can I put strips of foamular between my sleepers for even more insulation?


jklingel 02-14-2012 05:30 PM

Here is the deal w/ foam, as far as people know now, and as far as I understand it, having read the DOW web site and talked w/ a tech at an EPS plant. After 30 yrs of experience, this may change. If you want to avoid "creep" (the foam crushing), then use foam that is 3x psi what you require. If your calcs say "I have a 15 psi load", then use 45 psi foam, etc. I don't know why foam is listed at its 5% or 10% deformation rating, but it is (EPS is at 10%, XPS may be at 5%, maybe; I forget). When you compress foam about 5-7%, you get beyond its elasticity and get into the deformation range. At the 3x rule I stated, you may experience 1% compression. So, use stout foam or accept compression. In your pic, (1) Ensure that the floor is FLAT. (2) Why the PT sleepers? Moisture concerns? Make sure there are plenty of them if you use them, esp w/ a fake wood or wood floor. Flexing is not necessarily their friend. (3) Are you installing a vapor barrier under the foam? (4) Breathable flooring is usually the best on slabs, unless it is very dry for sure and always will be. If moisture comes up from the slab, a vinyl flooring may come lose.

hellonoko 02-14-2012 07:02 PM


Thanks, I will be using sleepers because I am pretty sure the City code is going to make me... The inspector didn't have a clue when I asked about PSI requirements but got super excited about the need for sleepers.

I will not put a vapor barrier under the foam. From what I have read on building sciences and the like you want your basement to breath and dry inwardly and trapping moisture that comes up from the slab or in from the walls is very bad. Also I have read that XPS blocks moisture but also lets it slowly evaporate through as it has a low permeability rating.

Good idea on lots of sleeper.

I guess what I don't understand is why the sleepers exist. If the weight of whatever is on top is going down onto the sleepers and then into the foam anyways in my diagram are the sleepers just there to mount the floor/subfloor to or do they provide weight distribution also?

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