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Community and Q&A

Flash and Batt vs. Foam at Rim Joists

artisanfarms | Posted in General Questions on

I’m considering doing a Flash and Batt style insulation on the house I am renovating, or at a minimum, using spray foam to seal in the chunks of polyiso I’m using for insulation at the rim joists and top plate to help with airsealing the house.

The house is in Zone 5 with 2×6 stud walls, and 2 1/2″ polyiso on the outside of the sheathing.

What brands of foam and applicator are well suited to a DIY approach?

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  1. Expert Member
    BILL WICHERS | | #1

    For canned foam, I usually use Great Stuff in the gun (not the cans with straws). OSI makes a product that fits the same guns that my spray foam contractor prefers (I think they call it "Quadfoam"). I use the gun for all kinds of random sealing needs, from wire and pipe penetrations up to larger gaps and cracks. The small plastic tips are great for "injecting" the foam into smaller areas. BE SURE to keep some acetone and paper towel handy -- you have to clean the tip of the gun after every few applications to avoid clogging. If you clean it periodically as you use it, you can avoid issues with clogging.

    For rim joists, it can be difficult to get the foam gun into the space you're working in because the can of foam sticks up so far on top of the gun. I prefer Loctite's "Tite foam" here. It's quite a bit more expensive than Great Stuff, but it is a lot denser and I like to use it in more critical areas like holding in foam panels between joists on the rim joist. The smaller can is easier to get into position while working, just try to keep the straw on the bottom -- if you use the can with the straw on top it will run out of blowing agent much faster and waste a lot of foam.

    Short lengths of vinyl tubing are great for extending the foam gun or the straw on the can if you need more reach.

    For two part foam kits, I have used mostly the Dow Froth Packs. They work well, but you have to keep the cans warm. In commerical buildings, I like to leave the cans on top of a transformer overnight. For a house, put them in a small bathroom or closet with a space heater running. I get the cans up to around 85-90F or so before use, and they need to be at that temperature for several hours before you use them to ensure the material inside the can is completely warmed up. If you use the materail with cold cans, you'll have issues with mixing and you won't get as much coverage as you're supposed to. With the very small 12 board foot kit, submerging the cans in a bucket of warm water helps a lot since the cans cool off too fast otherwise and then you have issues with insufficient coverage.

    Be sure to get your entire project ready for foaming before starting with a 2 part kit. You want to get everything ready so that you can start and go until you run out of foam or finish your project. Every time you stop, you have to get a new tip and flush the gun again, which wastes both time and material. If you need to hold insulation panels in place along the rim joist, I like to use scrap pieces of 12 or 10 gauge copper wire. Cut the wire a little longer than the gap between the sill plate and subfloor, then press it in so that it locks in place against the foam block. This way you can have all your foam pieces positioned and held in place before foaming, and the wire will keep the block in place as the foam cures too. I go back and remove the wires a few hours later after the foam is fully cured.

    I've heard good things about "Foam it Green" too, which makes a kit similar to the Froth Pack, but I haven't used their product myself.


  2. Expert Member
    NICK KEENAN | | #2

    For big jobs generally spray foam contractors can do it more cheaply than you can buy the materials to do it yourself. The only reason to do it yourself is if you are just doing a small area, or if you are working in stages and want to be able to do small sections at a time.

    Most closed cell foams have a lot of greenhouse gases. DAP makes a low-GWP spray kit:

    It's quite a bit more expensive than the regular spray foam.

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