Basement insulation
Ot_82
| Posted in Energy Efficiency and Durability on
Hello. I am redoing my basement and thinking of doing spray foam insulation. I was also thinking of increasing the R-value by adding a secondary insulation material.
Can put a 2″ rigid foam first and then 2″ of spray foam? Open for any advise and suggestions. 😁
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What type of foundation (fieldstone? Brick? Poured concrete?)
Where are you located (or just DOE climate zone)?
I have concrete block as my foundation. I am in the bough of NYC
IRC code minimum for zone 4A (NYC, Westechester & Long Island) would be R10 continuous insulation. With a reasonably flat CMU wall that can be done very cheaply using 2" reclaimed roofing polyiso (~R11.4) strapped to the wall with 1x4 furring through-screwed with 3.5" masonry screws, or trapped to the wall with a 2x4 wall. Tape the seams with housewrap tape, and seal the seam to the foundation at the top & bottom with can-foam. Install 1.5-2" of EPS under the cut bottom edge of the polyiso to prevent wicking of ground moisture from the slab. (If studwall, extend the EPS under the bottom plate of the studwall as well as a capillary & thermal break.) The cheap Type-I EPS with facers found at box stores is good enough. (Adding R13 to batts to the 2x4 wall would roughly double the performance.)
Foam reclaimers often advertise here:
https://newyork.craigslist.org/search/sss?query=rigid+insulation
Don’t bother with spray foam, just use two layers of rigid foam. You’ll get the same performance for a lot less cost (especially if you use reclaimed foam), and a lot less mess (spray foam goes a lot more places than just where you want it. Trust me here :)
Tape everything and be careful of the lower edge getting wet as Dana suggests. I’d look into Hilti’s IDP fasteners for rigid foam though instead of using furring strips. The IDP fasteners are made for this application and install very quickly.
Bill
Hey guys thank you for the detailed advice.
Ok - so my basement is already framed out with 2x3 studs 3" away from the foundation wall. I did already purchased the spray foam and plan on using it because uts already here. If I install the ridgid foam to the the foundation Would you guys recommend spraying the foam over the ridgid board ?
Before installing the already on-site spray foam, rip some 1.5" thick EPS (not polyiso) into 2.5" wide strips and wedge them between the already installed 2x3s & foundation as stud extenders. Then go ahead and use the spray foam directly against the foundation from the slab all the way up to the subfloor. Start by "picture framing" around the perimeter of each stud bay to ensure that the EPS "studs" are sealed well. If you have enough spray foam for a full inch or more, that will be sufficient for limiting ground moisture incursion, and the rest of the space can be filled with fiber insulation.
Have a spray bottle full of water when spraying the band joist and foundation sill sections- give the wood a light misting, particularly at the seams which will improve curing rates and adhesion.
If there is enough spray foam for more than 2", don't apply more than that in a single pass- give it a couple of hours of cooling before adding more. Use up the whole kit- it doesn't store well for longer periods time when partially used.
If there is more than 3.5" of depth left after the spray foam, fill it in by compressing mid density or low density unfaced batts designed for 2x6 framing. R19s perform at R13 when compressed fully t 3.5", R20s perform at about R14 at that thickness. If the cavity depth is closer to 4.5" higher density batts like R21 fiberglass or R23 rock wool will work. If it's less than 3.5", R13-R15 works.
It's not usually "worth it" to cut'n'cobble rigid foam between the studs (even reclaimed foam) due to the amount of labor involved, and won't fit tightly to a spray foamed surface, whereas fiber insulation will conform to uneven surfaces just fine.
I’d use the spray foam to seal the rim joist area and tie that into the top of the rigid foam on the walls. Anything leftover after that you can try applying to the rigid foam since you already have everything, but you’re likely to be surprised at how difficult it is to get an even layer of spray foam applied.
I’m assuming you’re using one of the spray foam kits like the Dow Frothpack or similar? If so, make sure you warm the cans up for a few hours before applying. It makes a big difference in how well the foam applies and how many board feet you actually get. Don’t go crazy heating the cans, you just want the nice and warm. At work, I have my crew set the cans on top of a transformer overnight. In residential spaces, put the cans in a bathroom and run a space heater in the bathroom with the doors shut. When the cans are warm to the touch (temperature in the 80s F), you’re good to go.
Bill
Ok ok both you guys have some great pointers. I am pleased. Now it is time to time to put things in action. Thank you guys!
You'll get a better overall seal if you apply the spray foam directly to the foundation, not over a layer of rigid foam. If the rigid foam is going up first, use EPS, not polyiso, since the polyiso still needs some drying capacity toward the interior.