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Community and Q&A

Is what my contractor telling me correct?

Michael Dunwell | Posted in GBA Pro Help on

I have placed a contract for repair and insulation of flat roofs. A fibreglass finish to the roof.
The contractor stated that screws to hold the Xltratherm boards in place would not affects the insulation values also that fibreglass insulation compressed into 75mm rafters by 50mm Xltratherm boards is equivalent to 140mm of Xltratherm boards.

Is this a reasonable statement?

I am trying to discover how much if any degrading in insulating properties occurs when insulation boards are attached to roof beams by screws.

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  1. Charlie Sullivan | | #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 | | #2

    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 | | #3


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