Attic insulation question for low slope roof (heat, insulating, paint)
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One of the next projects I'm taking on is to re-insulate the attic. All insulation was removed last summer before we started upgrading plumbing and electric. Now that I've cut into the attic space I see that much of the roof has 2x4 joists that rest on the top of the exterior walls, and due to the low slope of the roof there isn't the requisite 15" of space for blown cellulose until you get almost 3' in - which means the effective R value is going to range from about R12 up to R49 over that stretch. This wing of the house is only 20' wide, so that would mean a significant % of the total roof will be under-insulated, part of which is a bathroom that has always been way too cold.
Currently the bathroom is 50% gutted so it's going need all new drywall anyway. Aside from significantly dropping the ceiling, is there any common work around for this issue when insulating old construction that wasn't meant to be insulated? The company that was going to do the blow in didn't mention anything special that would be done. I think their plan was to just blow in as much as possible given the limitations. However, since the ceiling is going to be replaced anyway, I may as well try to improve the situation.
In a situation like that, the first thing I would recommend is a spray on radiant barrier. Radiant barriers work both directions meaning it keeps the house cool in the summer and warm in the winter. It works great but not quite as good as the radiant decking. There are different ones around but I've used one made by Sherwin Williams that has attic temps on a 100F day cooler in the attic than it is outside. Doesn't sound right but it works. Ask your insulation company if they spray radiant barrier paint. That's who does it here. If so, get the specs on it and chat with the sales folks about the value of it.
When the insulation company blows in the glas materials they should be installing baffles between the rafters to allow ventilation. In some cases they have to install 2 per section to get enough run of depth. Make sure they install them or the attic will heave like a locomotive. I'd also suggest ventilated ridge.
The simple truth is you're limited by the structure-
So, you either except the limitations or you modify the structure. Dropping the ceiling can and will provide additional space for insulation but is the additional R-value worth the amount of work and material.
A "better" solution is the addition of 1" XPS board to the ceiling joists, then drywall over that- and the usual blown-in (XPS= about a R-5). Or double it up; get R-10 in 2" of head room.
You can also do the same thing to the walls- with the exterior wall(s) open, add "non-faced" batts (R-13), cover with 1/2" XPS, then drywall.
The XPS actually does 3 jobs-in-one. It adds R-value with minimal space/thickness, provides a vapor barrier, and an air barrier (when properly sealed). The combination of all three will raise the comfort level substantially.
Where are the soffits? You don't want to block them.
What we did was build a shield/trough for the soffits (to duct the air into the attic) and then used spray foam around them, which has a slightly better R value. It isn't the best, but unless you can rip the roof apart and put in raised heel trusses, it is the best you are going to get.
Thanks - it looks like I was on the right track. I'd tacked up 1" XPS over gaping holes temporarily to keep the room from leaking too much heat and not surprisingly, it's warmer than before. Creating an xps-envelope seems like the easiest solution . One question though - with that extra 1-2" of ceiling thickness, how are the junction boxes/can lights mounted to be flush with the slightly lower ceiling height? All of the electrical is being replaced.
Venting the attic was one concern, as the blown in could easily cover the soffit gap. This section is ridge vented (unlike the rest of the house) in addition to the soffit vents. The soffits in this section have a water damage in places so perhaps ripping them out and putting in less easily obstructed vents is in order. Thinking about this more, if I ducted directly from the soffit up above the insulation, I wouldn't have to worry about keeping the old soffit vents open, would I? IE: insulation could be piled right up to the underside of the roof in the shallow parts?
Thanks - it looks like I was on the right track. I'd tacked up 1" XPS over gaping holes temporarily to keep the room from leaking too much heat and not surprisingly, it's warmer than before. Creating an xps-envelope seems like the easiest solution . One question though - with that extra 1-2" of ceiling thickness, how are the junction boxes/can lights mounted to be flush with the slightly lower ceiling height? All of the electrical is being replaced.
Venting the attic was one concern, as the blown in could easily cover the soffit gap. This section is ridge vented (unlike the rest of the house) in addition to the soffit vents. The soffits in this section have a water damage in places so perhaps ripping them out and putting in less easily obstructed vents is in order. Thinking about this more, if I ducted directly from the soffit up above the insulation, I wouldn't have to worry about keeping the old soffit vents open, would I? IE: insulation could be piled right up to the underside of the roof in the shallow parts?
There are "extensions" for electrical boxes.
And recessed cans are also adjustable. Those are pretty easy things to work around.
The pain is having to extend jambs on doors and windows; or creating a distance/dimension problem with code.
As for the attic ventilation/soffit vents, the easiest is just using baffles. Of course they are much easier to install when there is no ceiling in place, but not impossible. Obviously you don't want to have excessive amounts of blown insulation within the soffit area- if it's not improving your insulating envelope it just a waste of money.
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