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Community and Q&A

What spray-on insulation should I use under my house?

user-586120 | Posted in Green Products and Materials on

I live in a 100 year-old house on a pier and beam foundation with wood floors. The floors are rather cold in winter and I’m thinking of having foam insulation blown on the underside. While I want the best R value I can get, I’m concerned with toxicity as well as environmental damage. Also, should I be concerned about damage to the wood? Thank you.


  1. GBA Editor
    Martin Holladay | | #1

    What climate zone are you in?

    The best spray foam insulation for this application is closed-cell spray polyurethane foam.

    If you prefer a different type of insulation, you can install one or more layers of foil-faced rigid polyisocyanurate insulation on the underside of your floor joists. If you go this route, you need to make sure that all the polyiso seams are sealed with a high-quality tape, and that the perimeter of the polyiso is sealed against air leakage. If there are any penetrations through the foam, these need to be air sealed as well.

    Whatever insulation you choose, you should try to at least meet the minimum R-values for floor insulation required by your local building code.

  2. user-586120 | | #2

    I live in Fort Worth, Texas - i think that's Zone 7. I'm not interested in using batts as that seems to draw mice and rats. Reading about polyurethane foam, I'm concerned about out-gassing and toxicity. Could you speak to those concerns?

  3. wjrobinson | | #3

    Susan, my two cents worth...

    Icynene water blown done during the most favorable weather by the best company by their best crew by their best spray person. I interview the men directly. Had a overly tired gent spraying sloppy, had a guy spraying his very first time... though he did better since he was being carefully overseen... My company has thousands of job completions... Find out who is the best in your area and rely on them to sell you a great product.

    I like water blown Icynene products only.

    Also there is more work that may need to be considered when one makes a big change to add spray foam insulation. Get some bids that offer explain what else may or may not need to be considered. Ventilation, moisture concerns, insects, etc.

    Added edit; Outgassing concern, Water blown installed right like it has been for me has no smell that any of us can detect but was done in a new build and had lots of time to cure. The best way to be safe is to me to not use spray foam. Since using it on three sites and reading about all the problems a few have had with it... I say forget it is the only way to really allay any outgassing concerns any of us may have. Assemblies can be sealed with other products to be more airtight and then can be insulated with products that have less chance of outgassing.

  4. Expert Member
    Dana Dorsett | | #4

    Fort Worth is in D.O.E. US climate zone 3 (which is different from USDA plant hardiness zones.)

    In Ft. Worth installing 1- 2" of rigid foil faced polyiso (seam taped) under the joists, and filling the joist bays with blown cellulose works pretty well. The borate fire retardents in the cellulose works as a anti wood boring insect agent, has very low toxicity to humans, and doesn't outgas. Those agents are also eye irritants to rodents, and while it's not toxic to them, rodent nests in cellulose are somewhat rare. (I once had a red squirrel chew his way into a bag of cellulose after chewing his way through the siding and sheathing into the garage- he made a mess, removing about a fists-worth of cellulose but didn't nest there, taking up residence in an incompletely filled stud bay. I eventually evicted him, dense packed the wall with cellulose, and sealed his entry holes with 1-part can-foam.)

    The foil facers on polyiso are gas-tight, and manufactureing defects that would cause polyol outgassing are usually caught at the manufacturer, and would almost never be shipped in the first place, so it's a different situation than site-applied 2-part foam, where operator error becomes YOUR problem.

    IRC code min for floors in zone 3 is R19 (see:, which takes about 5.5" of cellulose, but it's worth adding the inch of foam on the floor joists to limit the amount of "temperature striping" at the joists, which have only about R1/inch of depth to the cellulose at R3.6/inch. Adding R6 of foam to a 9.5" deep 2x10 joist increases the R value at the joist to about R17 instead of R11-ish. If your joists are full dimension 2x12s or something you may want to just put up an OSB layer instead of foam, and pack it full of cellulose, sealing the OSB to the joists with acoustic sealant caulk as you go, and sealing the seams of the OSB with fiber reinforced duct mastic.

  5. user-586120 | | #5

    AJ Builder, is the spray foam you mentioned different from the icynene water blown insulation? Any I find that icynene comes in both open and closed cell. Are you using closed cell?
    Dana, I appreciate your opinion and I'm not sure where you're located but using polyiso and blown cellulose sounds like an awful lot of work (and lots of my money) fitting it around all the pipes, wires and other assorted structures under my house. Another positive for the foam to me is that is still allows things to be moved or added to without major destruction to the insulation.

  6. wjrobinson | | #6

    Susan, first let me say that Dana gave you great advice, and if you called contractors for price and asked also about doing future modifications... if it were me I would say not any worse than spray foam.

    OK.... as to Icynene... I only have used 1/2 pound open cell water blown. You can take a cured piece and shove it on your face and smell nothing bad. Not so with 2 pound spray foam. Icynene has one more product that is water blown that is more of a closed cell foam but not a 2 pound product.

    What you need to do is call contractors and start interviewing them. I would interview a contractor that does remodel work in your area and talk to them about dozens of successful insulation projects that they have done. You can start by visiting the web, lumber yards, and jobsites.

    The biggest mistake people make is to buy price from a person who is great at talking you into using them. Instead their quantity of good work and happy recent customers that you can call or visit is the person to contract with.

  7. wjrobinson | | #7

    Susan... Oops as I didn't reread your question well. Spraying foam outside your home is far safer for you than spraying an existing home inside. Get a few good insulating quotes, call their insurance agent and you may be all set.

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