Unvented addition roof
Looking for some helpful advice regarding a currently unvented roof assembly.
The house is about 100 years old — there was a small 1-story addition done to the rear of the house. The back porch roof was extended to create a rear bedroom and bathroom. We are currently renovating the bathroom, and noticing that this section of the roof is unvented. There are no soffit vents, and no place for a ridge vent as the roof terminates into the side of the house (just below or at the 2nd floor).
Currently there is a small amount of insulation in the ceiling below the roof. As far as I see it, I can only come up with 1 possible solution — looking for help to know if this is correct.
Since I cannot vent this roof easily, I am looking at unvented options. We just had this section re-shingled so I cannot add foam insulation above the roof deck. Does this only leave me with an option of closed-cell foam in the rafter bays followed by rigid foam underneath to help with thermal bridging? Just seems like a large $$ in closed cell foam.
Thoughts? Sorry if I did not provide a clear enough picture. This is in climate zone 5.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Jason,
It's possible to vent the roof you describe, if that's what you want to do. The soffit vent should be relatively easy; on the uphill side, the standard solution is the Roof-2-Wall vent manufactured by Cor-a-Vent. (See the illustration below.)
If you want to have an unvented roof, that's possible too. For a discussion of the many ways to create an unvented roof assembly, see How to Build an Insulated Cathedral Ceiling.
Thanks Martin -
I guess I am indecisive. One of the issues is that the first section of this roof is pitched normally (probably about 12/6) and is shingled, but then transitions to a much lower pitched (12/2?) roof covered with self adhering membrane. Indoors, I would like to vault the ceiling where the roof is 12/6, and leave it flat were it is 12/2.
If I choose unvented, I will have to spray foam the entire underside of the roof deck, then insulate to the appropriate R-value and drywall -away.
If I try to vent, I would need to fir out the rafters where I want to vault the ceiling so I have room for loose insulation + a vent channel... add more fluffy insulation above the flat ceiling, air seal the entire ceiling, and cut into the roof for the Roof-2-Wall vent and add soffit vents -- and even then, since the roof is multi-pitched, can I count on proper ventilation flow?
Am I missing a less costly solution to this problem?
Jason,
I think you have correctly listed your options. The third option is to strip off the roofing, install rigid foam on top of the roof sheathing, and re-roof.
Now it's just a question of sharpening your pencil and figuring which way is cheapest (or easiest, or more desirable).
That's what I thought -- thanks so much... it is a time consuming fix, but in the end all worth it knowing it is done correctly. Thanks again - I really enjoy reading the articles on this site.
If I could possibly steal your ear for one more question Martin -- I read your article regarding building insulated cathedral ceilings. At one point it says:
"First of all, you can’t use air-permeable insulation... to insulate an unvented roof assembly unless the roof assembly also includes a layer of air-impermeable insulation (either spray polyurethane foam or rigid foam panels) directly above or directly below the roof sheathing"
does this mean it is possible to use the rigid foam directly BELOW the roof sheathing (in lieu of spray foam)? If this is possible I would assume it would need to be virtually air tight -- cut to fit snuggly and caulked at the seams? I also presume I would have to have a thick enough amount of rigid foam to achieve an R-15 value (climate 4a)
Jason,
Your assumptions are all correct.