Tag Archives: glass fibers

How Insulation Works

I so enjoy clients who truly care about the outcomes of their post frame buildings. In this case, I’ve been back and forth with reader Eric and today we are discussing how insulation in walls works.

Eric writes:

“Mike,

Thanks again for the input. I read those articles you mentioned on the BIBs and the white liner panels. The liner panels are very common and popular around here but the article had very good points and gave me more to think about. As for the insulation, I was not familiar with the BIBs system and was impressed. I have done some more reading and reached out to a few contractors for quotes on the BIBs system. I do prefer to do things myself whenever possible however, so the option of installing fiberglass myself is still on the table. This is where I still have a question. You mentioned to fill the entire wall cavity with unfaced fiberglass and then cover with Visqueen. Is there an issue if the cavity is not completely filled and an air space is created between the fiberglass and the Tyvek® behind the metal sheeting? I ask because my walls are roughly 5 1/2” deep and obviously R13 is only 3 1/2” thick and R19 is 6 1/2” thick. If I go with R13 (cheaper) I end up with an air space. If I go with R19, I end up compressing it and losing r value anyway. Also, I have diagonal bracing in the corners which will also make it near impossible to tightly fill with fiberglass batts. I would love to go with the BIBs system but am waiting to see if it is within budget. If I can save considerable money insulating myself with batts I would most likely do so but need to be sure I am not causing problems down the road. Sorry for being so long winded and thank you very much for all your help and information.”

Mike the Pole Barn Guru responds: How Insulation Works

Fiberglass itself has little resistance to heat flow. The actual insulator is the air trapped in the tiny spaces between glass fibers. The tiny air voids slow conductive heat movement, while the glass fibers reduce radiant losses and impede air movement to block convective heat flow.

Don’t be fooled by so-called dead air spaces. Small air voids slow heat flow, but large voids don’t. A dead air space is one in which air does not move — once a gap gets larger than 3/4 inch, convection kicks in and overrides the insulating effect. Even though they contain air, uninsulated framing cavities have little or no R-value.

Making contact.

This understanding should govern the way batts are installed in the field. They should make good contact with wall and ceiling and nestle snugly against the sub-floor at the base within wall cavities. If the batts don’t touch the inside face of the drywall and the housewrap, convection coupled with air leakage will seriously undermine their thermal performance. Use only unfaced batts in exterior walls, because we’ve found inset stapled kraft-faced batts tend to create gaps between the insulation and the drywall. (Using unfaced batts also prevents the drywallers from complaining about the presence of stapling flanges on the surface of the framing.)

In other words, even though shoving batts into a wall girt cavity may seem like a no-brainer, doing it right takes some care. If a batt is simply jammed into place, its edges tend to drag along the sides of the girts on either side, which often prevents the rear corners of the batt from coming into contact with the exterior sheathing.

Moral of the Story

If your budget does not include closed cell spray foam insulation, then BIBs is a great solution and is effective.

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