Finishing out an attic. This wall faces south east, vinyl siding on the outside, 2×4 studs, dark asphalt shingles, ridge vent. The end of the room will be 12 ft high and 12′-6″ wide. We’re concerned about heat gain and will use fiberglass batt insulation in the walls between the finished space and the rest of the attic, but we’re wondering if something else should be used on this wall. One option is to furr it out to more than 4″ thick. Would rigid foam insulation be a good choice here?
I’m sorry, I thought you wanted it done the right way.
Replies
Ed
A couple of things you need to know about fiberglas, First Fiberglas is rated at 70 degrees, however when it's cold out the outside tempurature can cause what is called thermal cycling.. That is the cold air in the outside wall will settle and the warm inside air will tend to rise.. Fiberglas as you know doesn't have any properties that naturally obstructs air flow.. (they use it in furnace filters) That washing air doesn't do much to keep your heating (or cooling ) costs down.. In freezing weather the R rating can easily be reduced by half or more!
Second fiberglas like cellulose is prone to settleing, over time it can sag and leave some of the top of each stud bay devoid of insulation Not a good thing!
third fiberglas and celluliose loses any R value if it's wet or damp.. If you are unable to completely control vapor or moisture then the insulation is doing nothing..
Ridgid foam while it has great insulational properties is not without it's weaknesses.
Installing it in stud bay is problematic and difficult if not impossible to do properly, studs bow and there always seem to be a lot of obstructions in each bay. In addition rigid foam comes in 48 inch wide shapes.. the inside demension of stud bays doesn't work out well for full usage of each sheet.. Spray in foam solves all of those probelms plus it gains several other advantages. a more rigid structure and a near perfect vapor barrier among them.
small aras are expensive to do compared to jamming some batts of fiberglas into a stud bay, however whole houses can be done for only a modest up charge over professionally installed fiberglas..
Us do-it- yourselfers are forced to decide between the low installation cost of fiberglas or the low heating costs of foam. Sort of a Pay me now or pay the utility company later deal..
Good points. Since it's only one wall (the other walls are knee walls not exposed to the outside) I think I'll put on the foam and fill small gaps with expanding foam. The size inefficiency will just have to happen.
I'm sorry, I thought you wanted it done the right way.
why not try the best of both, insulate the existing cavity with bats, and put sheet goods on top of the studs (no cutting to fit cavity) then drywall on that ..
Wane,
Because fiberglas is a lousy insulation.. please read my post .
Been there, done that, .. I don't like fibber glass iether, I like mineral wool, ROXUL flexbatt is great stuff, it is allot firmer, better R value, easier to install, cuts great, won't sag, can get wet and dry out .. and I don't see the difference (other than thickening the wall and cost) between sprayed in foam in the stud bay, versus a full 4 X 8 sheet of shiplap foam board over top of the studs, actually I do, it is definetly a higher R value .. there was, is, ?, an excellent ariticle at the Building Research Branch of the National Research Council of Canada's site (http://irc.nrc-cnrc.gc.ca/publications.html) on what you term thermal cycling. I think it concluded it wasn't really thermal cycling, it was convection by air cycling, and in order for that to happen there had to be gaps in the insulation at the top and bottom of the stud cavity ... now if I could only find it again .. any ways this is an absolutely fantastic free site .. any US gov't sites like this??
Edited 5/19/2005 11:15 am ET by wane
I'm going to use 1/2" foil faced foam against the sheathing, then two layers of 1-1/2" blue foam, sealing it with expanding foam. Total R-value should be 3.2 + 7.5 + 7.5 = 18.2 which is better than the R-13 of the fiberglass batts.
I'm sorry, I thought you wanted it done the right way.
Probably a good idea, and then covering the ends of the rafters. More complicated, getting that thermal break off of the heated-with-the-roof-deck rafters--but worth it.
Going to run into any venting issues filling the rafter bays? (The roof insulation thread is some on my mind--go figure<g>.)Occupational hazard of my occupation not being around (sorry Bubba)
ED
Make sure you use sprayed in foam to seal the edges of the insulation,, if you don't and the house racks evan a tiny bit in a high wind the resulting squeeks will drive you nuts!