Use of rigid foam board below grade
This is Holly’s husband Roger. So i’m going to be renovating an older home with a spray foam thermal envelope to be quite energy efficient shortly. I recently met at the property with a local energy management/modeling/consulting company and a spray foam company. At the house i have about 1/4 basement and 3/4 crawl space. The basement and crawl areas are adjacent to each other and not separated by any physical barriers. All in all for the total area underneath the house about 50% is below grade including all of the basement. This is a brick house with a brick foundation.
The spray foam company recommended against using rigid foam board because they said the process of adhering it to the brick with the appropriate “nails” or equivalent would damage the brick. They would rather spray closed cell foam throughout the entire crawl basement (because of the below grade issue).
My questions are:
-I’m operating under the assumption that installing rigid foam board would be much less expensive than having closed cell spray foam throughout the entire crawl/basement. Is this accurate?
-Is it a legitimate concern to adhere the foam board to the brick? If so, is there a way that it can be applied with an adhesive instead?
-Searching on the GBA website it looks like there is a rigid foam board product (extruded polystyrene) that is appropriate for below grade applications. Is this what i would use?
cheers!
-roger
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Replies
I agree with the spray foam company. The xps can and should be fully adhered to the brick, but that is a lot of glue and there is potential for air flow if not done perfectly. There is still that potential with the spray foam, but it is much lower and the installed price, should also be. XPS should be installed on the floors, along with a vapor barrier(poly) running a foot or more up the walls and the spray foam over this, to ensure no soil gas or moisture enter the building.
Bill's comments are bang on. I wouldn't use screw or nails on old brick (or new for that matter). Using adhesive on XPS would work, but the spray foam method would be least invasive and more effective. The cost is higher for SF, but worth it based on the foundation you are working with.
Bill's comments are bang on. I wouldn't use screw or nails on old brick (or new for that matter). Using adhesive on XPS would work, but the spray foam method would be least invasive and more effective. The cost is higher for SF, but worth it based on the foundation you are working with.
Hello Holly and Roger
they make special plastic nails with fins for attaching foam (and Sheetrock) to the mortar joints between the bricks. you drill a hole through the foam into the mortar joint and insert the plastic "nail" with simple hand pressure. you should be able to cover the soil under the house with a heavy plastic vapor barrier (check multiple prices) and lap it up the walls and then fasten the foam over the plastic to help hold it in place. You will need to leave a termite inspection gap, generally at the top of the wall, Raleigh NC inspections seems to prefer 3" but 2" should be fine, and you will want to use great stuff foam to adhere blocks of foam to the rim joists. I would also leave a de-humidifier piped to the outside on a humidistat switch. I think this will be dramatically less expensive than the spray foam and I really don't see an issue with the small amount of air circulation you might have between the foam and the brick not do I see an issue with inserting plastic nails into the mortar joints. (assuming the mortar joints are parallel to the sub floor you will just use a tape measure or story pole to locate them through the foam) I like using two layers of foam with off-set joints to bind the insulation into a continuous layer but it can be less expensive to use a single layer and seal the vertical joints with great stuff. You won't know how the prices compare until you get quotes but sealed crawl contractors are pricing all over the ball park so you'll be well advised to get several prices on all of these options.
What is the name/company who make the plastic nails? Sounds good, but also labor intensive.