Question of the Week – Part 3.
This week, we bring you a question from an engineer in New Hampshire.
“I am thinking of using 3” “roxul comfortboard is” on the exterior of an older house. I would like a detail for installing an “outie” window, the window will be an Anderson new construction window with wide flanges to accept Anderson snap on trim surround. Typically a window box is built with ¾” plywood, but in this case the window frame needs to be at least 3” wide to accept the flange, I would prefer not to use a double 2x to accomplish this since it would create a sizeable thermal break in the system.”
“Good to talk with you about your retrofit window details. As discussed, we’d be happy to take a look at your drawings, including wall and roof assembly details. We regularly make project recommendations using the Pro Clima airtight systems.
Regarding your window question, the 3″ wide flange makes things a bit tricky, especially if you’re determined to avoid thermal bridging. As we discussed, you can still build a 3/4″ plywood box, but screw the flanges into CompaCFoam. One could make a strong argument for this approach by noting that, in addition to lowering heat loss, the CompaCFoam will prolong the life of the windows by preventing condensation damage.”
CompaCFoam is a vapor-permeable, high-density EPS foam insulation that can provide structural support and insulation for windows. For more about CompaCFoam, check it out here. We hope to update you soon with one of the earlier projects to use the material in the US – the SURE House Solar Decathalon project by the Stevens Institute of Technology in Hoboken, NJ.
Don’t forget to check out the other questions in our series:
Have a great weekend all.
**Please note that in republishing our question of the week, we sometimes edit the text of the original question and response in order to provide additional clarity to the issue.**