Electric radiant ceiling heat change
dfmcintyre
| Posted in Energy, Heating & Insulation on
Greetings –
We have electric radiant ceiling heat in our early 60’s built home. Would the following modifications increase the efficiency enough to make it worth the hassle of doing:
1 – Move the 11″ of blown in and fiberglass bat insulation temporarly from between the joists.
2 – Cut and install rigid foam with the foil side down.
3 – Foam the edges and openings for wiring, lights, etc.
4 – Move the bat and blown in back over the joists.
Thoughts?
Thanks – Don
Replies
Where are you located? Climate is everything. If you are in Mississippi, probably not worth doing. If you are in the frozen north, you could do that and probably a lot more to the house. If you are doing the work yourself, probably yes anywhere else.
Your step 3 alone (foam sealing) likely will make a big difference, by cutting down leakage of heated air into the attic space. Just make sure you get all the holes and the edges where ceiling meets walls; that joint can leak a lot of air as well. Be careful around any can (recessed) lights; they need to be IC rated to have insulation next to them. If they are 60's vintage, likely they are neither IC nor AT (air-tight). You can build a box from rigid foam to cover them before piling insulation around that. This has been detailed elsewhere.
The foil will be a vapor barrier, likely a good thing if you are in a heating climate. As to its effectiveness as a radiant barrier, beyond the R value of the foam itself, you'll need to space it 3/4 of an inch or so above the ceiling. An air gap is needed for the reflectivity of the foil surface to matter.
If the foil-surfaced foam is polyisocyanurate, you'll add about R7 per inch of thickness. If you are going to the trouble, add a two-inch layer rather than just one inch.
When you reinstall the removed blown-in and batt, make sure the blown-in is above the batt if you are in a heating climate.
Dick - I'm in Michigan, so depending on where your located, it might be considered the great white north. Good point about a space between the ceiling and the foam. The ceiling we're planning on working is the second floor, with attic above, about 900sq ft. There is only one can, over the original toilet which I may change out. The ceiling was constructed by nailing rockwall to the joists, stapling the heating element to it then adding wet plaster then a sheet of plaster and a final coat. It's about 1 1/4" thick. Do they make polyisocyanurate foam 2" thick or would we just stack two one inch layers?Don
Polyiso should be available in 2" thickness. Use of multiple layers of that is common in roof assemblies with the board above the framing, then plywood, then roofing. Your proposed use isn't quite like that. The labor of cutting to fit sections between ceiling joists and foam sealing edges strikes me as, well, a lot of work. Why can't you just foam seal the wire holes and other leaks, then add more loose fill on top of what is there already? Do you have a space problem up there? The R14 of 2" polyiso can be provided by about 4" of cellulose.Edit: With a heat source next to the framing, I would think that priority should be given to addition of insulation that covers the tops of the joists up there with as much loose fill as you can. The thermal bridging of the joists must be considerable at present.
Edited 6/30/2009 8:04 am ET by DickRussell
Dick -
The main question I had, was what the effect of foil sided polyiso would have, i.e. how much would it assist in radiating the heat down instead of the way the current install is operating. Right now, there is one sheet of fiberglass batting, stapled to the joists and about 8 to 10 inches of cellulose blown in on top. In the middle of the winter, I can go up into the attic (no space problem, aside from the central a/c unit taking up a few sq ft), peal back the batting and loose and feel the warm ceiling.
To properly seal all the wire holes and other leaks, temporarly moving the cellulose and removing the batting is neccessary. I was thinking that since that job would have to be done in order to foam seal the leaks, does going the extra step of cutting and laying in foiled polyiso make sense.
Don
Well, we all agree on the need to seal the leaks into the attic, so that you will do anyway. After that it's a matter of the cost and labor to cut and install pieces of foam, supported off the sheetrock with spacers and foam-sealed into place, vs. just adding more cellulose over the existing stuff. You might be able to have a contractor come in some morning and blow the additional layer for less cost than your buying it yourself.The reflective foil does radiate some heat back down, the equivalent of some amount of R worth of insulation. However, the equivalent of that can be provided by that much additional cellulose far more easily. There also is the matter of the conduction of heat through the ceiling framing, bypassing the foil and insulation. Think of it as 1.5" thick wooden fins conducting heat well up into the insulation layer. That's why I would think that you would benefit from having a lot more insulation over the tops of the joists than between them down low.Of course you could add the foam, replace the existing insulation, and then add a layer of new cellulose to boot. I'd be more inclined to spend the cost of the R-14 polyiso on additional cellulose, adding R-20 or more (6"+) to the maybe R40 of the existing batt+cellulose. Cost might be a wash, but a lot less labor on your part. Price it out both ways.
No. No substantial benefit ... Sounds like a HUGE effort and the gain will be negligible. The radiant/reflective surface will not stay reflective (i.e. very shiny) for long ... enclosed or not. My reaction to your description is WOW ... a LOT of work. Find something else to work on to save energy. You are MUCH better off.