Replies

1. 
   Patrick_OSullivan | Aug 01, 2022 02:26am | #1

   You likely need an "ignition barrier." See IRC 316.5.4.

   One of:
   11/2-inch-thick (38 mm) mineral fiber insulation;
   1/4-inch-thick (6.4 mm) wood structural panels;
   3/8-inch (9.5 mm) particleboard;
   1/4-inch (6.4 mm) hardboard;
   3/8-inch (9.5 mm) gypsum board; or
   Corrosion-resistant steel having a base metal thickness of 0.016 inch (0.406 mm).

   Note that if you're using the crawlspace for storage, you will trigger the thermal barrier requirements, which are different.

2. 
   caliberger | Aug 01, 2022 02:58am | #2

   Thanks insaneirish
   What would you do to achieve R-20?

1. 
   user-2310254 | Aug 01, 2022 09:25am | #3

   The 2021 IRC/IECC requires R-5 of continuous insulation in climate zone 3 in a conditioned crawlspace. R-20 is the requirement for above grade wood framed walls. CZ4C is R-10, and Marine CZ4C is R-15. But I suspect you are in CZ3C since the energy guy mentioned R-20 for walls.

   See https://basc.pnnl.gov/information/2009-2021-iecc-and-irc-minimum-insulation-requirements-new-homes.

3. Expert Member
   Dana Dorsett | Aug 01, 2022 11:16am | #4

   >"What’s the cheapest and easiest way to get R-20 on my walls?"

   Cheapest would be to find a source of used/reclaimed roofing polyiso, and glue 2 layers of 2" foam (seams overlapped) to the foundation with foam board construction adhesive, then gluing the wallboard to the interior side.

   Run this search on your local craigslist every week or so to find the cheaper-local sources of used foam:

   https://sacramento.craigslist.org/search/sss?query=rigid+insulation

   You might also try contacting local commercial roofing contractors who might sell you damaged/used goods from re-roofing project for VERY low money. In a crawlspace you almost never need full 4x8' sheets- indeed you might need to rip them down to 2-3' widths to even get them in.

   Easiest would be to spray 3" of HFO-blown closed cell polyurethane to the foundation wall, followed by a sprayed coat of intumescent paint (for fire resistance.) Not all foam + intumescent paint combinations have been fully tested & certified for code purposes, but would probably work as long as the paint had been tested & passed with at least ONE vendor's foam.

1. 
   caliberger | Aug 01, 2022 02:50pm | #8

   Thanks for your helpful response Dana. What are your favorite foam board adhesives?

4. 
   caliberger | Aug 01, 2022 02:09pm | #5

   Thank you to everyone that has commented
   I see that Thermax comes in 3.5 in thickness which would satisfy my R-value requirement
   I see it mentioned as an option in most articles on the topic but rarely mentioned in the comment section as the option that was selected
   Can anyone comment on their experience using it in a crawlspace application including cost and where purchased?

1. Expert Member
   BILL WICHERS | Aug 01, 2022 02:36pm | #6

   It's a good product, but it's expensive. If you need 3.5" thick polyiso, I'd layer two sheets instead of trying for one really thick sheet. With layered sheets, you can overlap seams which helps with air sealing too. I would use a sheet of 1.5" on the inside, and a sheet of 2" on the outside here ("outside" in this case means on the interior side of the assembly). The reason for putting the thicker sheet on the outside face is because that puts your strongest sheet on the outside which helps make for a flatter wall.

   Note that you only really need Thermax, which is rated to be left exposed, on the outside face (towards the living space or interior of the crawl space), the inner sheet doesn't need any special rating can be made using cheaper material.

   Bill

5. 
   caliberger | Aug 01, 2022 02:43pm | #7

   Thank you Bill. Very helpful. It's just a crawlspace so if Thermax is expensive, I'll probably go with the cheapest (prob thinnest) Thermax I can find on the "outside" and perhaps look for something used, as Dana suggested, on the "inside" next to the concrete