Insulating an existing roof
Hi Community,
I have a home built in 1910 in Minnesota and I am replacing the roof shingles due to storm damage. When the roofing contractor came out to investigate, he noticed below the shingles were the original 2x boards used as sheathing (as oppose to sheets of sheathing) and they were spaced too far apart to meet current code. There also isn’t any insulation between the 2×8 rafters (attic space was finished to be an occupied room). There are a few existing roof vents sprinkled thru-out the top-half of each roof face, but we don’t have soffits (the rafters continue to the overhang) or air vents at the bottom of the roof.
The roofer wants to replace all of the boards and lay down new sheathing boards, add new roof vents (either the box kind or ridge vent) and add insulation (type not know yet). This leads me to many questions:
1) If there are no soffit vents, is the ridge vent doing anything for us?
2) If we add insulation between the rafters and replace the board sheathing with continuous sheet sheathing and add a new ridge vent, how will air circulate to the ridge vent?
3) Was the space between the existing 2x “sheathing” acting as vents for air to travel to the existing roof vents?
4) Should we just not add insulation and replace the sheathing and roof vents and call it a day?
Thank you very much for your assistance and guidance.
Amado
GBA Detail Library
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Replies
Hi Amado,
Your uninsulated roof assembly was healthy because it was able to get wet and just as easily dry, which was the case with many leaky, uninsulated old homes. The problem is that it is also woefully inefficient, wasting energy and your money. There are ways to vent a roof without soffit intakes--there are intakes that go in the facia or under shingles--but you may not even want a vented roof assembly. With a re-roof and sheathing retrofit happening, you have lots of options for improving the situation. I recommend that you start by reading this article: How to Build an Insulated Cathedral Ceiling
Wisconsin here, and our 1930s house has the same type of build: mushroom vents sprinkled about the mid/upper roof and NO soffit vents under the overhangs, nor a ridge vent. I'm originally from the South where homes there tend to always have both types of vents, so maybe this is a Midwest build phenomena from decades gone by?
You're in a good position to really do some nice work while the roofers are redoing it, and I'd be curious to hear what GBA experts say. After reading a lot, we decided to switch to a hot/unvented roof. Our contractor just finished removing all the 1980s weatherization stuff: PVC "vent baffles" in rafter bays, pink fully stuff that had become brown, drywalled sloped ceilings that had NO insulation, and lots of mouse poop from the soffits and overhangs having holes, aka. air leaks. Then they blew in closed cell foam on the underside. The entire underside of our roof from ridge to overhang is now sealed and insulated. We already notice the difference in warmth with the fall temps setting in.
For better or worse the previous owner had just re-shingled and there's no leaking so we're holding off on adding more layers up top to increase the total R value of the roof. Our old home rafter bays aren't deep enough to get at what experts say is needed for the ideal R value with just spray foam underneath. ... or we just squeaked by. With 4" closed cell, the #s say we got around R-25 and that just meets what's needed for Climate Zone 6. I assume you're in the same Climate Zone. Anyways, to be safe, Phase 2 of our upgrade will be to get more R value up there there with rigid foam on top of the roof deck.
If I were in your situation and needing new roofing done anyways, I'd get the air sealing and insulation all done both topside and underside of the roof, shooting for R60 so the condensation worries go away and it's plenty thick for our cold winters, and not bother trying to keep the roof vented.