Confused about R value per inch
I’m looking at comparison charts online for R value on a variety of insulation types and I’m confused when comparing signer’s ranges and then the claimed r value on mineral wool or fiber batts.
Most places suggest the following per inch….
3.1-3.4 per inch for fiberglass batts
3.1-3.4 per inch for mineral wool batts
3.8-3.9 per inch for blown cellulose
4 per inch for TimberBatt
But then going to buy the actual products for a 2×4 wall…
R-15 for 3.5″ of mineral wool (4.1 per in)
R-15 for 3.5′ of fiberglass (4.1 per in)
R-14 for 3.5″ of TimberBatt (4 per in)
So what’s the real performance? I know ~R1 isn’t going to make a huge difference on a whole wall r value, and things like thermal density and air sealing might matter more. But why the huge difference in available data? Am I missing something?
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Replies
Batts can be of different densities. I would go by what the manufacturer specifies, rather than a range from some website. R15 for a 3.5" batt matches Rockwool's high density batt, whereas their standard batt is R13. Unless you're in Canada, where the only option is R14. I wonder whether that's because of a different rating method, or they actually sell a product that's in between the regular and high density batts here.
Manufacturers typically publish the highest R-values they can get from controlled, ASTM testing, and they typically round up as much as possible, which is understandable because so many of us focus on the decimal points. In real life, installations are rarely perfect, moisture may influence thermal conductivity and other variables have an effect.
As a designer, I rarely specify a particular brand, so I use generic values and err on the safe side. (Except when using insulation with high global warming potential, including all closed-cell foams, for thermal compliance with building codes, where I don't want any more than necessary.)
Honestly, the difference here is not "huge" at all. Same type of product doesn't mean they are same, just like you can't compare Ferrari to a Toyota Corolla even they are both cars.
R values changes with density, weight, age, getting wet, how many times it get wet and dried off, etc. Manufacturer tell you their test value, which is in the perfect condition. Many organization publish the measured performance of an aged material in a real project, typically 15 year average performance, that might cause a big discrepancy compared to a brand new product. The best you can do is believe the manufacturer tag, but keep in mind it is only true when it's in perfect condition, and it will not be so perfect as soon as you open the wrap. Or you can just use the lower number to leave yourself some wiggling room.