Roof insulation plan — low slope — trusses
Alan Afsari| Posted inGBA Pro Helpon
Prior roof questions — //m.etiketa4.com/question/roof-insulation-plan#comment-146956?utm_medium=email&utm_source=notification&utm_campaign=comment_notification&utm_content=view
Climate Zone 5 – low slope roof –
most of the roof is cathedral – 12″ I-joists – flash and batt (fill- with BIBS) approach. From areas that use trusses and the interior ceiling is flat, do you do the same thing? (flash and fill – foam on undersurface of the roof deck with BIBS suspended below it). We would still use an unventilated assembly.
Thanks,
Alan
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Alan,
The image that you attempted to upload didn't work. (That problem may be due to a site glitch on GBA -- I've had the same problem recently. I hope the problem is resolved soon.)
If you use the flash-and-fill approach for your low-slope roof, that doesn't mean you have to use the flash-and-fill approach everywhere. But for the roof assembly that has a horizontal ceiling, I'm not sure what type of attic you have above the horizontal ceiling. You wrote that this assembly with a horizontal ceiling would still be "unventilated" -- and I'm not sure why, if there is an attic above.
Martin - the slope is 1.5:12. There is only about 11" for an air space and insulation where the roof meets the exterior wall. I didn't think there is enough room to ventilate it. I also thought the low slope has inadequate vertical difference to get air to move through an air/vent without a dog house on the roof - something we would like to avoid.
Is my understanding of this inaccurate?
Thanks, Alan
Alan,
For the portions of your house with a horizontal ceiling, it would be safer to keep the flash-and-fill insulation along the roof plane, not against the horizontal ceiling. Otherwise, there is a risk that you'll get moisture condensing on your cold roof sheathing.
Without any attic ventilation, this risk increases. All it takes is a few air leaks for your roof sheathing to be in trouble.