Insulating an Attic Roof to Eliminate Ice Dams
i live in NH and my house looks very picturesque with all the icicles hanging off of it. Fortunately I haven’t had any leaks from ice dams because when the roof was redone a few years ago I had them run the water barrier up a goood ways. I have two problems that I need to address. The heat escaping into the attic and the amount of insulation in the attic. The floor of the attic is mostly covered with plywood so adding more insulation there is not a possibility. What I plan to do is add more insuation at the soffits and then insulate betwen the rafters (with venting of course). Any problem with doing this ot anything I need to be aware of?
Secondly (maybe I should do this first), I need to stop heat from getting up the attic. What’s the best way to find the source of heat leaks? I have AC so there are a number of vents in the ceiling. Tbeh vents are all wrapped pretty well with insulation but what else can I do? Sorry for the general questions but I’m not sure what I should specifically ask for.
George P.
Replies
Well, if you can move around in the attic easily enough, one way to find leaks is to go up there in cold weather and feel for leaks. Also, look at the rafters and roof sheathing for staining due to condensation -- places where the wood is significantly darker.
While some leaks are due to relatively subtle things like ceiling electrical fixtures, the majority are more obvious (and, unfortunately, often harder to treat).
Look first for the obvious ones -- things like chimney/flue chases. Where, for fire code reasons, you can't, eg, stuff these tighly with fiberglass, instead cut metal flashing to tightly fit the gap.
After that look for "features". Eg, a soffited ceiling is often framed before it's drywalled, leaving a large open area where air can draft upwards through the open stud bay. A dropped ceiling is even more likely to have this problem.
Next would probably be pipe and cable penetrations. You don't need to seal every 7/8" hole where romex was pulled through, but if a 2" hole was hogged to pass several cables, seal that, and seal any pipe penetrations that have more than maybe a 1/4" gap remaining. (You should of course try to seal them all, but go for the worst of them first. Some are simply too much effort to reach for the payback.)
The tops of walls are probably the last main concern. How big an issue this is depends on the type of construction -- if drywall is tightly nailed to top plates then usually the gaps are minor, but plaster/lath construction or "odd" features in the framing can produce significant gaps. (I treated the tops of some of my drywalled walls using thick roofing paint.)
(And it goes without saying that any HVAC ducts in the attic area need to be tightly sealed, as well as sealing tighly where they penetrate the ceiling.)
Who says you can't insulate under the plywood? If it isn't "Plum full" as they say in the woods, you can blow in cellulose. Its not really much different than blowing behind plywood on a wall except it takes more holes because you don't have gravity working in your favor.
What do you currently have under the plywood and whats the r value?
One way to locate leaks is using a smoke pencil inside to find where the smoke goes. However, you've got ductwork in your attic and it could be a BIG source of leakage/loss.
Vents "wrapped pretty well w/ insulation" means nothing in terms of leakage. If your ducts aren't sealed, the leaks can be significant. Not unusual to have duct damage that increases leakage by many times.
Leakage will occur around e.g. J boxes for ceiling lights, exhaust fans, supply registers, and holes in top wall plates for wiring and plumbing ... to name a few. Sealing both around a J box AND the openings inside it can be important areas that you might consider sealing. You need to think like air in construction ... and sleuth out all possible avenues of leakage. You should be able to make a huge dent in the leakage element of your situation. You may need to increase ventilation a bit when you're done, though ... be aware.