Retrofit insulation in attic slanted roof section with no soffit vents
GBA Editor
| Posted in Energy Efficiency and Durability on
I have an 1850s 1 and 1/2 story New England farm house. The upstairs bedrooms all have a partially slanted ceiling that is uninsulated. There are no soffit vents and I want to just fill the whole slanted part with insulation. Will that be a problem? Also I did lay in loose cellulose insulation on the flat part of the attic on top of plastic sheeting. Was that wrong to put the plastic down first?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Ellen,
1. What is your location or climate?
2. It is possible to insulate a sloped ceiling without providing a vent channel. This is best done with closed-cell spray polyurethane foam, however.
3. Remember to put enough insulation in your ceiling to meet minimum code requirements. This varies according to your local code and your climate zone. Some rafters aren't deep enough to accommodate enough insulation, so other approaches may be necessary.
4. The answer to your question about the polyethylene vapor barrier depends on your climate. In most cases, however, as long as you live in a climate with a significant heating season, flat attic insulation installed over poly should not be a problem. It's important, however, to address air leaks through your ceiling.
Ellen,
Most cellulose manufacturers advise against using a poly vapor barrier with cellulose and some will void their warranty if it's used.
Cellulose works best when it can breathe, since it's excellent at absorbing and releasing excess humidity. A major drawback to poly in the attic flat is that, in the event of a roof leak, the water would pool on the plastic, soaking the cellulose and framing, and delaying evidence of the leak until possible moisture damage is done.
I am in Maine, that is a good point on the water/cellulose thing. I had a new roof put on 5 years ago, tore off old wood shingle roofing (original?) with 2 layers of asphalt, I should have insulated then but didn't think of it ! Nor did the roofer. He had to put on 3/4plywood over the old roof planks because it was funky in sections, all the joists were good.
This is a good example of the problem of specialization in the building trades (as in most things). A specialist roofer may be able to replace a roof more quickly and perhaps at less cost than a generalist builder, but if the big picture isn't considered then the job becomes "penny wise but pound foolish".