![]() ![]() Last year when I wrote about my bathroom remodel, I showed another one. I mentioned two examples of where compression is OK: around windows (assuming you already have an air barrier material there) and behind junction boxes. I think maybe I just needed to put in some kind of context. 96% compression, as in your example, is too far, I’m certain. I don’t know what the bounds are on that relation, but I’m sure it’s for relatively modest compression. When you’ve reduced a 6.25″ batt down to 0.25″, you’ve gone well beyond the ability of that formula to give you an accurate number. Yes, there are limits to simple formulas like the one NAIMA gave in their paper. Just make sure the space is completely filled.”Įxcellent point, Dan. But I would not say, “…compress if you need to and don’t worry about it. I think it is important to qualify that a small amount of compression is not too bad and we should not point and shame when we see some normal, slight compression going on. Then I am curious if the curve falls off to the point that spun glass starts to act like glass? The R40 per inch is not likely so I contend that we still do not want to compress at will.Īllison I agree with most of what you are saying but I would say it with a few more boundaries. But the chart above leads me to believe the formula may only apply to a little bit of compression. Now I know it’s not our intention to cram this product into a small space with a hammer and screwdriver. (Chart image would not send, but at 1/4 inch, the R value is still 10 which makes the R per inch = 40) Here is what happens when you apply the formula to an R19 product installed into a 6.25 to. Guess what? I have discovered the world’s best insulation product in the world coming in at an R40 per inch! All you have to do is get that R19 batt into a ¼ inch space. ‘’…it can be good to be dense, to a point!” If I were the batt industry and I had been beat up for years about poor performance of poorly installed insulation, then I too would develop a chart or formula that shows little to no performance issues when the product was not perfectly installed. I sipped on the Kool-Aid, but then decided to do the math before drinking the full glass. ![]() Your comment will not appear below until approved. ![]() Rare Grade I Fiberglass Batt Insulation Sightingģ Problems with Fiberglass Batt Insulation in Floors ![]() How to Grade the Installation Quality of Insulation That’s the real measure of a good installation. Just make sure the space is completely filled. So, compress if you need to and don’t worry about it. Just put the whole piece back there and let it be compressed. You don’t need to worry about removing some of the insulation so you can do it without compression. You can then take that little rectangular piece of insulation and put it in the space between the junction box and the exterior sheathing. If you install fiberglass correctly, you need to cut notches in the insulation where it goes around junction boxes. If you use backer rod in the gap around a window and then fill the remaining space with chinked fiberglass, “it’s damned near impossible to compress the fiberglass ‘too much,’ without using a hammer!” That’s what Dana Dorsett wrote in his GBA comment to me.Īnother is behind electrical junction boxes. One place where you’re pretty much always going to end up with compression is around windows. That means the batt labeled R-19 really gives you R-18 in a closed cavity. If you put that batt in a closed 2×6 wall, it will be compressed 0.75″ because a 2×6 is 5.5″ deep. The standard R-19 fiberglass batt is 6.25″ thick. So, you don’t get the full R-value on the label, but the insulation still works perfectly well if all you’ve done is compress it. Owens Corning also has a compression chart for R-value ( pdf), and here it is: They include a general chart for how to tell what your R-value is with different levels of compression. When you compress fiber glass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation. The North American Insulation Manufacturers Association (NAIMA) has a little two-page document about compressing fiberglass insulation ( pdf). But you can compress fiberglass insulation as much as you want. Incompletely filled cavities are a problem. Last month I wrote an article in which I said installing insulation, “cavities are filled completely with as little compression as possible.” But is compression really such a bad thing? When I posted that same article on Green Building Advisor, commenter Dana Dorsett wrote, “Compression of batts is fine (resulting in a higher R/inch due to the higher density) as long as the cavity is completely filled.” ![]()
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