Flash-and-Batt Insulating Barndominium Walls
We’re in a seemingly never ending cycle of racing towards net zero post frame homes, shouses (shop/houses) and barndominiums (read more on net zero post frame here: https://www.hansenpolebuildings.com/2019/01/net-zero-post-frame-homes/).
One possible design solution involves what is known as “flash-and-batt” where two inches of closed cell spray foam insulation is applied to steel siding interior surface between barn style wall girts, then balance of insulation cavity is fitted with fiberglass batts.
Today’s expert opinion is rendered by Martin Holladay, former editor of the Green Building Advisor web site. You can read more about Martin at www.MartinHolladay.com.
Even though thicker is always better with any type of insulation, applying a thin layer of spray foam is a good way to get air-sealing benefits at considerable cost savings over full-thickness spray foam.
Some spray-foam contractors dismissively call the technique “flash-and-dash”; they point out that fiberglass batts may fail to remain in contact with the spray foam, creating an air space and the potential for convective air currents through the insulation. But I think this is a relatively insignificant problem, particularly if the cavity is fairly airtight. Besides, it’s easy to minimize the chance of a potential air space by simply choosing a thicker batt. In fact, batts that are compressed slightly as they are installed will yield higher R-values than ones that just fill the cavity.
Another concern is that in a heating climate, the flash-and-batt method creates a vapor retarder on the wrong side (the cold-in-winter side) of the fiberglass batt. But whether the spray foam actually becomes a vapor retarder depends on the type of foam used. Open-cell foams — that is, foams with a density of about 1/2 pound per cubic foot — are very vapor-permeable. However, since many low-density-foam manufacturers, including Icynene, recommend against the flash-and-batt method, most proponents use closed-cell foam with a density of about 2 pounds per cubic foot.
One inch of closed-cell foam has a permeance of about 2 perms, while 2 inches has a permeance of about 1.2 perms, so closed-cell foams are effective vapor retarders.
But does installing a vapor retarder on the cold-in-winter side of a wall create a problem? Actually, research has shown that exterior foam can safely be used as part of a cold-climate wall or roof — as long as the foam is thick enough. As a rule of thumb, walls with exterior foam sheathing or flash-and-batt closed-cell foam will avoid condensation problems as long as the foam is at least 1 inch thick in climate zone 5 (Pennsylvania, Iowa, Nevada) or 2 inches thick in climate zone 7 (northern Minnesota).
Since exterior foam reduces a wall’s ability to dry to the exterior, it’s important to choose an interior vapor retarder that allows drying to the interior — such as kraft-paper facing or vapor-retarder paint — instead of sheet poly.
Unfortunately, the easiest way to add insulation to just about any place in your home is to install fiberglass batts. Fiberglass batts are typically the worst insulation for any job, but they’re easy to pick up in the store and easy to roll out, so people use them. The image below shows an atrocious installation at a two-year-old home in an upscale neighborhood of an inner-ring suburb of the Twin Cities. Yep, this passed the city inspection.
Cellulose is made from recycled, ground-up paper with boric acid added for insect control and fire resistance. If you choose to install cellulose yourself, you can buy the insulation in bags from your local home improvement store. If you buy enough, they’ll probably let you use an insulation blower for free. Don’t try to buy a single bag and spread it out by hand for spot-insulation; it’s way too densely packed (ask me how I know). The DIY cellulose insulation method is very dusty, but it’ll get the job done. If you hire a pro, they’ll use wet-spray cellulose, which adds a small amount of water to the cellulose to help control the dust and to slightly increase the insulation value per inch.
This seems to be all that’s ever used in new-construction homes and has an R-value of approximately 2.5 per inch. Like cellulose, you need a big machine to blow it in. You can’t simply buy it in bags and spread it around yourself. My biggest complaint with fiberglass is that it’s itchy and it’s a lung irritant. I’ve found that older fiberglass is way worse on your skin and lungs than the newer stuff, however. I have no scientific evidence to back this up, but I don’t need any. I’m completely sure of this based on personal experience.
With closed-cell foam, you also get a moisture barrier at over 2″ thick. Because of this and the higher insulating value per inch, most foam insulation used in Minnesota is closed-cell. To tell the difference between the two, try poking it with your finger. You can easily poke a hole in open cell foam, but not closed-cell foam. That stuff is way too hard.
