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Why "Flash & Batt" is a crappy wall system in a heating climate

by LiveModern Webmaster last modified Jun 24, 2017 01:06 AM
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by lavardera ( last modified Jun 23, 2017



I get asked this all the time - specifically "How about Flash & Batt", and I seem to write this explanation out at least once a month. So I thought it would be worthwhile just putting it in a blog post.

So "Flash & Batt", what is that anyway? You want to make a better performing wall, so the builder, or the builder's insulation contractor may suggest "Flash & Batt". Or sadly it may be coming from your architect because most know little about the actual science of putting a building together. On top of this all of the above may be bragging about how green this make them. What they are suggesting is this:

- Before they insulate your walls they will spray a relatively thin coat of polyurethane foam insulation on the interior of your sheathing between the studs.

- This will adhere to the wood and make your house very air-tight.

- And the polyurethane which has a very high R value per inch will replace some of the batts which has an average R value per inch, so you come out with more insulation value.

- And then they will insulate the rest of the cavity in the usual way with batts.

I’m not a fan of flash and batt. It goes against all the fundamentals I’ve learned about how to design a wall assembly. I don’t know where it started, or who ever thought it was a good idea. It is a relatively fast thing to do if you are interested in making claim to doing something that helps efficiency, but its not particularly efficient, and its a messy imprecise process fraught with opportunity to make mistakes.

- I don’t like putting the foam and whatever chemicals they are made of inside a house.

- The spray foam is supposed to make the exterior air tight, but if the formula mix is not just right over time it can shrink and tear away from the studs which spoils the airtightness, and allows moist interior air to reach the exterior surface of the sheathing where condensation can occur.

- Flash and Batt requires you to use a reverse vapor profile for the wall, meaning you are putting your air-tight barrier and vapor control line at the exterior/cold of the assembly in a heating climate. This means you must have enough insulation to prevent the dew point from moving out of the insulation into the wall cavity at all temperatures experienced, otherwise you risk seasonal condensation inside the cavity. This is easier to do with continuous exterior insulation, because it creates a thermal break of the studs. It is almost impossible to do well with spray foam between studs where each stud is a thermal bridge and the exposed sides of the stud greatly defeat the sprayed in insulation.

- This configuration sets up a double vapor barrier when installed with conventional batt insulation. I see people follow up the spray foam with a conventional kraft faced batt. They use the kraft faced batt because its easy to staple up. But realize that now you have a wall cavity with a vapor retarder on both sides - the foam, and likely OSB on the outside, and the asphalt coated Kraft on the inside. That makes it very hard for that wall assembly to dry, to the inside or the outside. This is the worst possible condition. If moisture finds it way into that wall - say the HVAC system is pressurizing the building, and interior air is finding its way through the stud cavity via an electrical box or vent or hose bib. Moist interior air flowing through the cavity will shed water as cools, and that liquid water will not readily dry to the outside or inside.

So in a heating climate, this is almost never a good idea. Rather air-tightness and vapor control should be at the same plane, on the warm side of the wall - the interior. That sets up challenges with penetrations for electrical boxes - they all have to be taped and sealed, which is why I like keeping the membrane at the studs, and overlaying an 1.5” wiring cavity so that your wires and boxes never have to penetrate the vapor control sheet. That cavity can get insulated as well which introduces a good thermal break from the studs.

Anybody I’ve met who advocates for flash and batt walls typically does not understand how the building science works. They are more likely posturing that they are green, or up-selling something that they just want to make money from. Flash and Batt is just bad practice.

If you want to learn how to make a better performing wall the right way then read our series on the USA New Wall. Start here, go to the bottom and read up.



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