Need advice quick - did you use foam insulation under basement slab?

8 years ago

last modified: 8 years ago

We are getting ready to pour our basement slab. It is a walkout basement with most of the back wall not underground. We have a layer of pea stone and vinyl sheeting (6 mil). My husband is contemplating putting 1" XPS foam board (Foamular) under the sheeting. We are in Michigan. Any thoughts on whether this is a good/bad idea? Labor will be free, so it will cost about $1200. Thanks!

ETA: Basement will be finished.

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dantastic

8 years ago

Short answer - yes.

http://buildingscience.com/documents/information-sheets/basement-insulation

Make sure you have enough distance for the insulation and slab thickness.

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Comments (12)

User
8 years ago
Yes. It is absolutely required. But it needs to be integrated properly into the entire insulation plan for the whole home. Leaving gaps or improper air sealing will defeat the effort expended.
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Hi illinigirl, The standard insulation used under basement floors is rigid insulation (XPS -- extruded polystrene). It comes in 2 inch thick often blue pieces of styrofoam. Dow manufacturers it among others. It is not spray foam. We used it in our build -- we are in southern Canada. The labour to lay the rigid insulation sheets was not high. The guys had to dig away a bit more gravel so the basement ceiling level was not compromised. They used the red tuck tape to attach the sheets to each other. This allowed the concrete flat workers to pour the floor with wheel barrow loads. I am not sure yet what a difference it will make as we are partway through our build. We have hydronic in-floor heat that was just turned on for the basement level. The floor is wonderfully warm (mostly due to the in-floor coils). We were required by code to place the rigid insulation because of our planned in-floor heat. Otherwise, I understand, the heat from the coils would not just heat upwards and instead would have been wasted heating underground? We did place the rigid insulation around the perimeter of our our unheated garage. I will go and step on the parts of the garage floor that have the insulation vs the parts that don't to see if I can detect a big difference and report back. Our architect spec'd for this rigid insulation under the basement slab. At the time, we had not planned for the in-floor heat. In other words, he felt it was a good practice even without in floor. The rigid insulation sheets are not cheap. We paid $25.00 a sheet (each one is 2 feet by 8 feet). We are in Canada where everything seems pricier. You can calculate the area needing coverage and see how much your builder is charging for labour vs material to help you decide if the price is reasonable. Don't forget the tack tape is also not cheap. Best of luck. Carol
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3650 PSF is equal to 25 PSI..... Their dricore products are just massively overpriced, at least in the US they are. I think it is better to put down a foundation wrap (DMX), then 1" XPS foam (25-30 PSI). Assuming that the floor is flat, you could probably get away with putting flooring (with a membrane) directly on top of it. If your floor isn't flat, you probably will need a dedicated subfloor. I'd also be sure to use some kind of flooring that is somewhat rigid and install it perpendicular to the foam boards, so that if the foam boards do try to lift or sink, the flooring will add some rigidity to prevent it from mattering.
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rwiegand
8 years ago
Yes. Use at least 2", 4" is better. 1" is far better than none, but it's really hard to go back and add more later! The current code requirement is R-10 (which 1" will not give you) under the slab if your jurisdiction has adopted the "stretch" code (which should be an absolute minimum to use anyway if you are in control of the construction process)
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just_janni
8 years ago
VERY good idea - we're in a much more temperate climate and we're doing almost 8". tape and seal the seams...
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User
8 years ago
last modified: 8 years ago
The prescriptive IECC code requirement for climate zones 4, & 5 is R-10 for a depth of 2 ft at the perimeter for a slab at grade (no requirement for a basement slab) and 4ft for zone 6.
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PRO
Springtime Builders
8 years ago
In a walkout basement, the IECC prescriptions should apply to the portions of the slab at or near grade. These prescriptions address what should be a cost-effective level of energy use. Going under the whole basement slab will improve comfort, help prevent moisture movement from the ground, condensation during humid summer weather and should better protect any susceptible finish flooring at the slab.

From the BSC link posted by dantastic regarding subslab insulation: 'such insulations do offer a significant improvement in comfort and moisture damage resistance (including against summertime condensation)."
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User
8 years ago
last modified: 8 years ago
Obviously a walk-out slab is at grade and not considered a basement at that building face so along the exposed slab edge it must have the under slab vertical insulation required in the Prescriptive Path chart of the IECC (if that is the compliance Path selected and assuming the slab is not heated.

The IECC does not require insulation under a full basement slab because the heat lost to cold outside air is usually much greater than the heat lost to the earth below a basement slab.

The rate of temperature loss through a material is proportional to the temperature difference between the surfaces. The temperature of the earth below a basement slab is permanently a bit warner than the temperature of well water which varies in the US from about 43 to 73 degrees. If the design temperature of the basement air is 70 degrees the temperature difference would vary from about 27 to 3 degrees. Therefore under slab insulation might be cost effective in North Dakota but not so much south of Kansas and Kentucky. In Michigan the difference would be about 20 degrees so insulation under a slab would be modestly cost effective and would also decrease condensation so R-5 (1" extruded polystyrene) should be adequate but if the price is right, 2" XPS can't hurt.
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just_janni
8 years ago
Surprisingly, we did some passive house modeling and in NC - slab on grade had us putting in at least 4"....
dantastic
8 years ago
Make sure that your framer, plumber, concrete crew, etc. are involved with this activity. Since the pea gravel is already down, your slab will end up higher than currently planned. 2 inches of insulation means the slab is 2 inches higher. Stairs OK? Toilet flange OK? Any sump or sewage basins? Maybe pea gravel has to be removed? Cover these details before you install the insulation.
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worthy
8 years ago
last modified: 8 years ago
Dr. Lstiburek of Building Science Corp. recommends a minimum R10 (2 in. XPS) for sub-slab insulation in your climate. (In our cold climate, I'm using 1" as the house will likely be for sale soon after move-in and buyers pay more for what they can see than hidden improvements.)

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jannicone VERY good idea - we're in a much more temperate climate and we're doing almost 8".

That's beyond even the standards of Passivhaus enthusiasts.
just_janni
8 years ago
That's beyond even the standards of Passivhaus enthusiasts.

Yeah - DH is into gross overkill.....
dekeoboe
8 years ago
Yes. We are in central NC and have 5" - house is Passivhaus certified.

Need advice quick - did you use foam insulation under basement slab?

dantastic

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