Using EPS or Polyiso insulation in the attic and crawlspace?

5 years ago

BLUF: Is it advisable, beneficial, or even meet code to use EPS or Polyiso insulation between the ceiling and floor joists?

First off, I'm a first time home owner and my extent of insulation knowledge come from YouTube and a few articles.

My wife and I recently purchase our home just south of Atlanta, GA. 1800 sq ft built in 1967 with a vented attic and crawlspace with mediocre/poor insulation between the floor joists and laughable insulation in the attic. Periodically a guy in our area gets ahold of large amounts of surplus EPS and polyiso insulation from commercial construction jobs and puts them for sale on Craigslist and other sites for a very good price. Would there be a benefit to using this kind of insulation vs. standard batts and roles from the big box stores? What are the risks? Would using these require some extra steps to meet code?

Thanks

Featured Answer

Seabornman

5 years ago

Cutting up any kind of foam board insulation to fit in between framing is a chore. In addition, there will be obstacles: wiring, bridging, pipes. In the attic, I would recommend doing the best job you can air sealing the ceiling, installing proper baffles at the eaves (assuming there is attic venting), and then blowing in insulation. Insulation type is your choice; I prefer cellulose. The crawl space can be insulated with foam in the floor, but the better method is to insulate the foundation walls and rim joist, for which the foam does a great job. That all depends on the type of crawl space you have. Check out greenbuildingadvisors.com for good, mostly non-biased info on all of this.

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Comments (10)

shirlpp
5 years ago
Good information

https://www.ecohome.net/guides/2254/polyisocyanurate-foam-is-a-hot-new-building-product-learn-where-and-where-not-to-use-it/
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3onthetree
5 years ago
last modified: 5 years ago
Basically ^ these comments. You won't like the mess of EPS, and polyiso really should be left for top side of flat roofs. I'll throw in an additional link just to get you there, but use the link above and this one to learn about attic ventilation, air sealing, and where, what, and how to insulate. YouTube is not a good source.Building Science Corp
Tyler Arms
Original Author
5 years ago
Thanks for the feedback, guess I'll pass on the rigid insulation until I get to working on the rim joists. What are folks' thoughts on flash and bat for the insulating the ceiling? Instead of crawling around the attic with a can of Great Stuff and trying to fill small individual holes, I wonder if I might be better off just getting a FrothPack, Dap, etc. 2 part kit and do .5-1 in over the whole ceiling and then throw regular r-30 bats on top.
Seabornman
5 years ago
I don't think you're going to get enough foam out of a kit to cover that area and I'm not sure you should be creating a vapor barrier at that location in a warm climate, but it might be a good idea to use a kit to make it easier to follow wall plates and cover wiring boxes. You don't want to force foam into openings in electrical boxes
3onthetree
5 years ago
Side note, if the attic has any access like for storage of any kind, then the foam must be covered with an appropriate thermal barrier, like mineral wool batts, wood, or drywall. Not fiberglass batts.
Seabornman
5 years ago
Cellulose also qualifies as a thermal barrier. I haven't seen anything that tells at what point spray foam needs a thermal barrier. It's used as a universal filler and often left exposed without comment, yet it's generally known that covering large areas requires a barrier.
sacto_diane
5 years ago
I just want to add that you'll need to translate any online advice you get to your needs specific to your zone. Insulation in a cold northern zone will be 180° from your zone which is a hot humid zone. The already mentioned building science website is a treasure trove of information on the IECC climate zones and insulation options. One caveat with older homes is that changing the "breathablity" can sometimes lead to moisture accumulation and unforeseen water damage.

https://www.norbord.com/na/blog/whats-your-building-science-climate-zone/
3onthetree
5 years ago
Cellulose also qualifies as a thermal barrier. I haven't seen anything that tells at what point spray foam needs a thermal barrier.

Cellulose may meet flame spread and smoke developed index, but I do not believe it is an approved thermal barrier which would have to meet NFPA275.

IRC R316 Foam Plastic

- R316.4 Thermal Barrier calls for G.B. or wood cover unless:

- R316.5.3 Attics can eliminate thermal barrier if meets all 3 criteria, including foam tested for NFPA approval, or if the attic is only for repair or maintenance.acess
Seabornman
5 years ago
3onthetree, yes cellulose is not a qualified "thermal barrier" in an attic that has storage, but can be used as an "ignition barrier" in attics without storage. Makes sense, huh?