How to insulate my roof... no attic
Hi everyone,
I live in a house built in the 60s. It is a California Modern design (or so I'm told). It has very low roof pitch with 2 simple gable end roof sections. Inside it has exposed beam / vaulted ceilings. When you look up inside the house, you see 4 X 10 beams every 6 feet running as rafters would and then you see tongue and groove 2 X 6 roof decking running between the beams.
Bottom line is: there is no attic, there is no insulation whatsoever. There is only rolled roofing, felt, and one layer of 2 X 6 between the sun/rain etc... and the living room. It's in Atlanta and when it's over 85 degrees, the AC runs continuously.
The interior is beautiful... the beams and 2 X 6s are stained and polyeurethaned. I can't insulate from the inside. It would ruin the look and the ceilings are already low.
I need to build up the roof from the outside. I'm very open to all suggestions...
Here's one suggestion from a friend.
Remove existing rolled roofing. This will expose 2 X 6 tongue and groove. Glue and screw down 1/2 inch OSB. Lay 2 X 4 parallel to ridge sleepers every 2 feet with 1 1/2 inch foam board between them. Lay another layer of 1/2 inch OSB over the 2X4s and foam board and then either put roofing down... or add another set of 2 X 4 sleepers perpendicular to ridge on 16 inch centers with no foam board to create a ventilation space from soffit to ridge.
Comments / Suggestions are appreciated.
Asked by Bryan Wilber | Jul 1
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You're in climate zone 3. The current Energy Code standard for your zone for both new construction and renovation is R-30 ceiling insulation. This would require 5" of polyisocyanurate foam board or 6" of XPS (styrofoam) board.
I would suggest considering 6" SIPs (OSB/foam sandwich panels). The materials are expensive but the labor is less than installing multiple layers.
Bryan,
If you decide to install multiple layers of foam, it isn't necessary to install OSB between each layer of foam. The first layer of foam can be installed directly against the existing 2x6 sheathing (or on top of #30 asphalt felt). Only one layer of OSB or plywood sheathing is required -- on top of your final layer of foam.
If you have more than one layer of foam, stagger your foam seams in both directions. Taping the foam seams improves the air tightness of your roof assembly.
Finally, if you decide to go the SIP route, you don't really need a conventional SIP (which usually includes OSB on both sides of the foam). The product you need is called "nailbase." It's a commercial roofing product consisting of rigid foam glued to a single layer of OSB. The OSB goes up; the foam goes down over the existing sheathing.
Nailbase is the simple solution, but it's hard to properly seal between panels. You can either take extra care to seal those joints, or do the two-layers-overlapped-and-taped which to me is actually easier to do properly. My experience is limited to doing each of these methods only once, so there may be other techniques or products I'm not aware of, but I have seen several examples of SIPS that exhibited failure at the joints.
In your hot, humid climate, vapor drive will be primarily to the inside, and any mistakes in sealing your insulation could result in condensation on that nice ceiling. Venting the roof would marginally increase the lifespan of your roofing but it causes as many problems as it cures so it's not really necessary.
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