Replies

1. 
   charlie_sullivan | Apr 09, 2015 09:10pm | #1

   For North Americans: Xltratherm is "Factory made Rigid Polyurethane Foam (PUR)"

   Michael, your contractor's statement about equivalence of fiberglass is not reasonable. 125 mm total, of which less than half is foam, is not as good as 125 mm of foam, much less 140 mm of foam, regardless of how densely packed the fiberglass is. In addition, the moisture and air permeability is very different between fiberglass and foam.

   On the other hand, I agree that the effect of screws is small enough to ignore. Suppose you have 10 screws per square meter and each has a 5 mm diameter head. That's less than 0.025% of the area. Even if the screw has zero thermal resistance, the convection process for the heat to get from the screw into the air has an R-value that is significant, about 1/36 of the R-value of the 140 mm foam. Scaling for area, that means less than a 1% effect of the overall heat loss, even if the screws themselves were made of a very high heat conductivity material.

2. GBA Editor
   Martin Holladay | Apr 10, 2015 05:55am | #2

   Michael,
   Compressing a fiberglass batt decreases the R-value of the batt, but increases the batt's R-value per inch. The chart below shows what happens.

   Hmm. The GBA site is acting up -- won't let me post an image.

   .

3. GBA Editor
   Travis Christopher | Apr 10, 2015 10:41am | #3

   test