Thermal Break / Vapor Barrier for Cathedral Ceiling
potton
| Posted in Green Products and Materials on
For a new house 30’x58′ south of Quebec province (Zone 6) with a single-slope 2:12 vented cathedral roof (ceiling same slope), the slope flatt trusses with raised heel will be 30” high, with metal roofing, R60 cellulose and probably T&G as ceiling finish. We are thinking of adding some panel for thermal break under the trusses, instead of only a poly.
However we are not keen with the idea of panels (foam or fibers) foil faced because of the possible reduction on cellular reception level inside the house…
Is there any material that combines thermal break and vapor barrier (without being alu)?
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If you are only using the foil faced materials in strips as thermal breaks, you shouldn't see any reduction in cellular signals. If you completely enclose the entire room with foil faced polyiso, then you MIGHT see some reduction in signal levels due to shielding effects. I can tell you from experience designing shielded rooms at work that it is surprisingly difficult to completely block cellular signals, typically requiring several layers of shielding to get up over 30dB or so, and you need special screened gaskets on all the doors. We don't have windows in these facilities, so I've never had to shield one of them.
If you're in a weak signal area, the shielding effect can make the difference between marginal and no signal levels in some areas of the home though.
If you want to stick with polyiso, just use roofing type polyiso instead, since roofing polyiso will typically have a kraft or fiberglass matt facer instead of foil. Either of those materials will be transparent to RF, as will be the polyiso core itself.
Bill
Thanks Bill,
Having a steel roof anyway, and no metal on the walls, do you think a foil faced ceiling would make any difference overall on RF level inside...?
I'm trying to get a single material that would provide a) something to support the cellulose; b) thermal break to trusses; c) vapor barrior; d) no reduction of RF ...
No, a foil faced polyiso layer will make no difference in RF signal levels under a steel roof -- you already have a sheilding effect from the steel roof anyway in this case. Foil faced polyiso has the advantages of being fairly stiff and rigid, easily taped, and good performance here as both insulation and an air barrier.
For your walls, EPS or XPS would work, as would non-foil faced polyiso. EPS and XPS are both made of polystyrene, and polystyrene is RF transparent -- so much so that it's often used as a protective covering for antennas and waveguides to keep out air/dirt/bugs while still allowing the signal to pass through.
A common "quick and dirty" engineering test for RF compatibility of a material is to microwave it. Put a cup of water in your microwave (to absorb the signal and protect the magnetron), and put some of whatever you want to test on the tray too, away from the cup of water. Run the microwave for a little while (30 seconds or so). WATCH it while you do this, just to be safe. If the test material heats up a lot, it's probably not a good RF material. If the test material doesn't seem to heat up, it's probably OK. Microwave ovens operate around 2.4 GHz or so, which is right in the original wifi band, and also not too far from PCS cellphone (1.9-2.3 GHz or so), so it's a fairly good test. Any more thorough testing needs some real test equipment.
Bill
I wouldn't worry too much about the thermal break for the trusses. By their nature, there is very little wood poking through the cellulose, and the diagonal chords that do poke through have a very long pathway for heat to follow. The end result is a very small thermal short through the truss chords. If you still want a thermal break, look up "Bonfiglioli Strips" on this site. In essence, they are just strips of foam fastened to the bottom of the trusses as a thermal break. Upgrade to 5/8" drywall for better cellulose support. Use vapor-retarder paint on the drywall. WIth 30" trusses, there's plenty of room for both cellulose and vent space.
FWIW, I built a kitchen addition with foil-faced polyiso on the interior walls and ceiling. All seams were taped with foil tape. No noticeable reduction in cell signal, but wifi was pretty bad inside. Then again, wifi can be bad one room away from the router with no metal in between. YMMV.
I have a metal roof and the only signal it seemed to have effected is GPS, that is a very weak signal anyways so I'm not surprised. I wouldn't worry about foil faced foam bellow a metal roof.
With T&G ceiling you do need a solid warm side air barrier, I wouldn't trust poly by itself in a low slope roof. A thin layer of rigid with taped seams is definitely the way to go.
Thanks!