I’m installing extruded polystyrene rigid foam board over my exterior sheathing. I got a little behind schedule and now the temps have dropped to the 30s (Fahrenheit). Do I need to allow for the foam to expand in warmer weather? How big of gaps should I leave (if any) on the seams and the edges? I don’t want it to distort my siding when we get back into the 90s next summer.

  • Great question. In many years of construction I've never encountered the requirement (or need) to do so. I'll wait for those with more technical knowledge of EPS to weigh in. – isherwood Oct 11 '18 at 18:48

You should be safe to leave a gap of ~3/16", based on the coefficient of thermal expansion in the table here (https://insulationcorp.com/wp-content/uploads/2016/08/ICA-LITE_Spec_Sheet.pdf). With a value of 35x10e-6in/in per deg F, assuming a 100deg maximum temperature delta x 48" wide piece-> 0.000035*100*48=0.168". This would assume 100deg from your install temperature I believe... so 1/8" may be more appropriate @30deg (F).

You should look-up and find your particular mfr/model spec sheet to find a similar table for the product you plan to install, and perform the calculation for it.

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