Insulating Attic with Low-Slope Roof
I own a 60’s ranch, and I want to add blown in insulation to the attic. I am not sure what the best approach is at the edge. The way the house was built, the 2×8 rafters have a bird’s mouth resting on the top plate, and the rafter tails extend below the top plate of the exterior walls. The space below the rafter tails is enclosed to form a soffit. That leaves only a few inches between the top plate and roof sheathing. The rafters are 24″ on center, but the ceiling joists are 16″ on center, so the width between framing members above the top plate varies. There would be no good way to install premade baffles, and because of the low slope, 4/12, cutting anything to fit would be very difficult.
There are soffit vents, but only on two sides of the house. It is a hip roof, and there are three large vents near the top of the roof (like gable end vents but not in a gable. I’m sure there is an obscure word for that), and when the roof was last replace, a vented ridge was put in.
I’m in the front range of Colorado, zone 5, so I don’t think moisture in the attic is too much of a concern. My last house had no vents besides two small rectangular vents, and it was fine. In this house I had the ceiling thoroughly sealed with spray foam, so I’m not worried about warm moist air from the house getting into the attic.
So the way I see it my options are some combination of:
1. Install baffles in some of the rafter bays to allow air to flow from the soffit. I could probably get baffles in less than half of the spaces because of the difficulty of moving around up there. This leaves almost no insulation between the top plate and the roof sheathing.
2. Attempt to block off the gap between the top plate and roof sheathing. Most likely I would not be able to get whatever material I use all the way into the gap, so this would leave some uninsulated space above the top plate.
3. Blow in insulation all the way up to the roof sheathing and hope for the best. I am planning on using cellulose, so it will settle some and naturally form a baffle. It’s pretty dense, so wind washing is less of a concern than other kinds of insulation. Some insulation will probably fall into the soffit during installation.
I don’t want to cause an ice dam problem, but based on an article I read on here somewhere (can’t find it now), it sounds like insulation is probably more important than baffles and soffit vents.
Does anyone have advice or better suggestions?
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Replies
Just curious, what did you decide to do? I've been racking brain over this exact dilemma for a while now.
I haven't done anything yet. The spray foam is supposed to be around two inches, and I blew in enough cellulose to cover it, basically level with the tops of the joists, to reduce the fire risk of exposed spray foam. I think the foam is only supposed to be around R12, but maybe they sprayed it thicker than they said. And a few inches of cellulose is pretty loose, so it's hard to imagine it's doing much. But over the last year I've been comfortable and my energy bills have been very reasonable even though I have maybe only R20 in the attic. On the plus side, that's about twice the R value of the old vermiculite that I removed.
I had a 42,000 BTU 3 zone Mitsubishi heat pump put in, and this winter I was able to eliminate pretty much* all my natural gas usage using the heat pump and wood stove instead of the boiler. But it was a mild winter and I burned a lot of wood, so I know I need more insulation if I want to be able to rely on the heat pump alone, if I'm not home to light the wood stove.
* There's one room that's poorly insulated and closed off from the rest of the house that isn't served by the heat pump. But it has a radiator, so I had to rig up a thermostat to keep that room around 40 degrees to keep the pipes from freezing. That used some gas. Eventually I'll add a mini split, but it doesn't make sense to heat this space until I replace the single pane windows.
Back to the attic, I saw a YouTube video where they used baffles that looked like a giant version of the corrugated layer in corrugated cardboard. I don't know what that's called or where to buy it, but it would be a good solution to add baffles in spaces of variable width.
I'm still not sure what to do, but adding baffles seems like the safest approach. I am worried that if I block off ventilation, moisture from behind the brick could get trapped in the soffits with no place to go. I can't tell how much of a gap there is between the brick and the sheathing, or whether the space behind the brick is open to the soffit. I should probably remove a section of soffit and investigate.
I have some more electrical work I want to do in the attic before it's full of insulation. But I really hate working up there so it hasn't been a priority. And summer is not a good time to be in the attic anyway.
Matt and User...433.
If it were me I'd install 1" pre-made foam baffles in as many bays as I could, making sure I had the minimum free vented area for the roof, and they were fairly well distributed. I would block the remaining bays, and the space below the new baffles, by compressing fiberglass batts into them, before blowing the attic floor.