We bought a house last year and with energy prices rising again we would like to add extra insulation in the attic. There is blown in insulation currently up there and were wondering if we could diy and fiber glass batts over the top? We would put them perpendicular to the joists so it wouldn’t crush the blown in and add some R-value on top of the wood.
Also, does anyone have any recommendations for venting this space? The only vents are in the gable ends and needless to say we had some problems with ice dams last winter.
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Adding fiberglass batts on top, doing the work yourself, is an excellent project with very fast payback. You must use unfaced batts so you don't trap moisture in the existing insulation. Running the batts crosswise as you propose is the correct way to do it. Do not insulate over recessed light fixtures unless the fixture is rated for insulation contact. If it is rated the label will say "IC Rated".
You don't say what your location or climate it, but I presume North since you mention ice dams. To prevent ice damming you want to keep the roof surface cold all over. That's done by letting in air along the soffit and out at the ridge. The best is continuous vents along both, but if you at least get it along the soffits that might be sufficient under most conditions. Of course, you can't block the air flow with your added insulation.
The insulation you add will reduce the likelihood of ice damming all by itself, but is probably not enough on its own.
In my experience, ice damming isn't necessarily related to a lack of venting. Lack of adequate insulation is often a more likely culprit, as is the presence of heating ducts and air handlers in the attic. Interior air leaks, say, around chimneys and chases (It amazes me the odd chases that I find that suck heated interior air into attics) can also be huge sources of heat leading to ice damming. A common heat source is a bath fan ducted into an attic. Even a recessed light can generate enough heat to melt snow.
And, I've seen ice dams on unheated buildings, which tells me that some ice damming is entirely unrelated to interior heat, and probably a consequence of solar heat. Often, this happens on east or west facing roofs that get relatively intense sun for a time, and then cool down rapidly as they enter shade.
In short, fixing air leaks and insulating any heating equipment (check the manufacturer's recommendations) are usually more imporant in preventing ice dams than is venting.
Go to the main FHB site and buy Fred Lugano's article, Fixing a Cold and Drafty House. Visit http://www.buildingscience.com, http://www.affordablecomfort.org, http://www.nesea.org, or http://www.pathnet.org.
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Well said, Andy.
As our national housing agency, Canada Mortgage and Housing Corporation, says in a fact sheet titled Attic Venting, Attic Moisture and Ice Dams:
"Attic ventilation is overrated. In winter, the cold outside air cannot hold much humidity or carry moisture away from the attic. In summer, attic temperatures are more affected by the sun and shingle colour than by the amount of ventilation."
Nice site http://www.ahi-ns.ca
I'm no expert, but here's what I did a few years ago...50's ranch with few inches blown in. Pretty bad ice dams and always the first one on the block to have snow melt off the roof. With below freezing temps outside, I was getting 60 degrees in the attic. In the attic, I caulked, foamed around all penetrations to the living space. Used fire block caulk around electrical fixtures. Then added fiberglass attic insulation perpendicular to the joists. Replaced the small vents at either gable with two huge ones. I didn't add ridge vents or soffit vents.
After undertaking this project one summer, the following winter I was getting 30 degree temps in the attic and no more ice dams or icicles. And now I was one of the last on the block to melt the snow off the roof. I think that cutting off air leaks from the living space and making sure the attic is well vented is just as important as insulating.