How to insulate roof without access?
janCad
| Posted in Energy, Heating & Insulation on
My 100 year old house has a converted attic with 2X4 rafters and drywall attached directly to the underside of the rafters. The fiberglass insulation is too thin and I’m having ice damming along the gutter. I have a new roof, so I don’t want to tear that up. Is there a way I can insulate from below without tearing out the drywall? Would rigid foam offer any solution? Alternatively, any ideas about stopping the ice dam from forming? Thanks!
Replies
You can drill small holes in each rafter bay and blow in cellulose insulation, but with 2 x 4 rafters it likely will not be enough R-value.
There is a company that makes slow rise foam that would give you R-21 in a 2x4 rafter bay. http://www.foampower.com/products/slow_rise.html
with FG already inplace, it will baffle and befuddle any attempt to add anything else effectively.
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Nope. same here.
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I have irriatable Vowel syndrome.
There is no magic bullet.
If you want to insulate that roof properly, you're going to have to do some demolition, or you could insulate it on the outside and then build a new roof over that. Considering the difference in cost between gyprock and a whole new roof, I'd recommend you bite the (lead) bullet and tear out the gyprock.
Once you've done that, nail 2x2s to the underside of the roof deck along both sides of each rafter, then nail Black Joe to that, creating clear ventways down to the eaves in each rafter bay. Bump up the depth of the rafter bays by lag-bolting another 2x4 (on edge) along each rafter (this will give you 5½" of insulation space) and then use the insulation of your choice.
FG batts will give you R-20; you can bump that up a good bit by using Reflectix bubble-foil VB instead of ordinary poly or foil-n-kraft. You'll have to strap that before laying new gyprock; it's too springy to put gyprock directly on top of it.
Sprayed in urethane foam will likely give you the highest R rating, but it's expensive unless the attic is very large. The foam companies have to charge a minimum to show up with the truck, and it's not cost-effective for small jobs.
The most important feature of this for preventing ice-damming is to make sure you have good, clear, air-flow under the entire roof deck, with sufficient eaves inlets and gable-wall (or ridge turbine) outlets. (Do not count on those proprietary 'ridge-vent' systems; they don't work when buried in snow.) You have to keep the roof deck cold or snow will melt, run down to the eaves, and re-freeze creating the dam.
Pay special attention to the area where plumbing vent pipes pass through the roof deck. Vent pipes are always a source of heat and a frequent cause of severe ice-damming downslope from them. A good way to solve this problem is to build a louvered coupola to cover the vent pipe(s) (if that won't clash with the style of your house). By preventing any snow from landing on the roof near the vent pipe, you eliminate the possibility of melt water running down to re-freeze on the cold eaves.
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