Friday, June 23, 2017

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

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.


  1. For the most part, I agree with your comments however, if flash & fill (not batt) is done correctly, it works great and gets the air tightness required for high performance homes. Let's remove the chemical / sustainable / "green" side of things for now....and the "proper installation" - true of any building product.

    If a flash & fill is being specified, the "flash" needs to be closed cell foam that is at least 1/2 of the total R-Value of the insulation...that calculation gets a little fun, but...This puts a vapor barrier in the center of the wall, allowing it to dry in both directions. This also (should) put the dew point somewhere inside the spray foam - protecting the sheeting for moisture damage.

    Then fill the balance of the cavity with dense pack cellulose, a very moisture tolerant material. This gets a fully filled cavity - even on the imperfections of the spray foam - that can dry in 2 directions.

    With all that - I do like your wall I consult with seem to think it is an expensive assembly, but none have tried it, but I am working them!!

    1. Kevin, accepting your proposition to ignore chemicals, and assume proper installation to address your points.

      I will grant that loose cellulose fill has good properties for controlling moisture in this assembly because it acts as a desiccant absorbing moisture if present, and releasing it later when conditions change. But it throws the fast and easy rational of Flash & Batt out the window. Now you are back to hanging an interior membrane to retain the cellulose, and the multi-step install of blowing and handling equipment, etc. Like the spray foam you now have two systems that require a machine to install.

      I generally don’t favor cellulose as a solution. Architects have been romanced on this because of the recycled paper content, and this has been spun into all manner of more heady levels of sustainability talk - its green, its carbon neutral, its magic green building sauce. What it is is shredded newspaper. It has some good properties, but expediency is not one of them, and scalability will forever keep it on the margin. With the state of newspaper publishing in the US there will never be enough newspaper to feed the NA housing industry, and really thats a good thing.

      Your description of putting half the R Value in the foam is ideal, but in practice the material defies any effort to apply an even amount of material. Cellulose will obviously be superior to batts in conforming to the resulting uneven surface, but its the thermal bridge of the stud that is more the problem here. The dew point will never sit neatly in the middle of the foam. And as you approach the stud the dew point will always shift to the interior, and at temperature extremes very likely outside of the foam.

      And then there is still the chemicals, and still the all to likely chance of getting a poor installation. So my criticism of it stands. However I’ve never really seen it as an alternate to the higher performance versions of the USA New Wall. Its been something I’ve seen people do as an up-sell, or because it was an easy method to promote in improvement in performance. But without any thermal break its not really something I would ever consider high performance.