Venting a low slope un-vaulted roof
As always I try to avoid the use of spray foam but a client we are designing a new home for is adamant on wanting low slope roofs (for aesthetics) and I cannot dissuade them 🙂 The roof slopes are 2:12 and 1:12 respectively. I know that some consider 3:12 the minimum allowable slope for a vented roof assembly but I was curious if that applies just to vaulted assemblies or unvaulted assemblies as well? In this case I could design the roof unvaulted and insulate the flat ceiling portion leaving a generous amount of air circulation space above the insulation (say 6-8″ of free airspace overtop of the insulation). Would that be sufficient to overcome the dimished stack effect?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Is exterior insulation not an option?
mdb_az,
The problem with low sloped roofs is that a slight reduction in slope on them makes a much larger diminution in function that say going from 6/12 to 7/12. You can successfully vent the roofs, but the chances of problems are a lot higher with the 1/12 one.
With a large airspace, diligent warm side air-sealing, and lots of ventilation also located on the rakes, they should work.
A bigger question is whether it ever makes sense to consciously design risk into new building assemblies. I have a short list of things I just won't do in our climate. Being in the PNW they include no overhangs, and low-sloped or flat roofs with living space under them.
As Matthew says: Have you considered an un-vented assembly with foam above the sheathing? Assembly #3 in this link. https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
This article covers exactly that topic: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs.
Short version: with at least 6" clear space, a lot of attention on air-sealing, and with one or more central mini-cupolas, it can work.
thanks for the thoughtful reply Malcom! yes if we go vented i would definitely do warm side air sealing and lots of ventilation. The current design does feature big overhangs which is a welcome change from the trend of minimal/no overhangs.