Thermal bridging at overhangs
flatwood
| Posted in General Questions on
I have seen this question asked, but never answered.
We are designing a house in climate zone 4A, North Carolina mountain area, and need at least 3 feet of roof overhang. This is necessary to keep the summer rains off the open windows, and to provide shading. The roof will be an unvented shed type, ceiling at the bottom of the rafters, about 4/12 pitch, with exterior foam board wrapping the whole house.
So the question is, how big a deal is the thermal bridging caused by say a 2×6, scabbed on to the interior of a larger rafter and extending out 3 feet? All well air sealed and insulated at the building envelope.
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Replies
Marla,
There are two issues here: the thermal bridging through the scabbed-on sisters, and air leakage. The air leakage is particularly difficult to address.
All decisions like this require a compromise. You have to look at your construction budget and decide how much you are willing to spend to aim for perfection.
If you want to reduce air leakage and thermal bridging to a minimum, the best approach is to frame the house without any overhangs at the rakes or eaves; then to install rigid foam on the exterior side of the wall sheathing and roof sheathing; and finally, to install "applied overhangs."
Here is an article that explains the technique: Airtight Wall and Roof Sheathing.
Thank you for your response. We are planning on using construction details as you say (ext. foam boards, taped sheathing, rain screen, etc.)
Since a large overhang will greatly contribute to the livability of the house, would it be better to create an air and thermal seal underneath the rafters? Like a giant raft of foam board and plywood dropped down on and sealed to the top plate of the walls. Maybe like a homemade SIP, but the structural members would be on the inside, sheathing and foam on the outside. More like a too big beefed up wall lying on top. Then the "raft" hanging over the outside of the walls could be clad with wood products, and the top with roofing materials. Inside would need good sized furring strips to hold the drywall.
We are the designers and laborers, so fussy non-traditional techniques would be fine.
Our concerns are saving energy (money) down the road, and moisture (vapor) in the building components. But we don't understand if rafter extensions are a small percentage of the performance, or killers. I do know that we can live without using the ac if we can leave the windows open at night and through a rain shower. Thus the need for at least three foot overhangs.
I am with you; keep those big shades and rain barrier eaves (but you need to water the grass under these eaves).
Caulk and tape (air seal) the heck out of your plates and blocks (where rafters pass through and rest on the envelope/frame); use rigid foam to insulate the soffit side of the blocking and rafter tails with rigid foam on their top and bottom edges (caulk sealed to the foam on the blocking).
Presumably you are isolating the rafter tops across the roof with foam already - for the same reason - so its a matter of carrying that past the envelope perimeter and joining at the wall plane. The bottom side can be done without any rafter cuts if you plan the soffit details. Its fussy but stronger and easier than patching on gable end type ladder boxes around the perimeter. I would not worry about the heat loss from the few square inches of the tails' ends where you attach the facia carrying 3 ft from the envelope. I don't like scabbing anything onto a roof edge; it leads to poor visual lines and structurally weak eaves. This is one of the poorest done air sealing and insulation detail areas in any house and worth the time and fuss to do properly. Instead of dimensional lumber facia, try 100% rot free all vinyl ... it takes and retains paint much better than wood.
http://www.royalbuildingproducts.com/uploadedFiles/Never-Rot-Exterior-Mouldings-Trim-Brochure.pdf?n=9741
3 foot overhangs seem like a lot of potential wind uplift to me. To fight the uplift you may have to put braces down to solid wood on the walls and that seems like a lot of trouble and visual mess to avoid a couple of square feet of thermal bridging.
In order for a structure to remain well insulated it must remain in one piece. An overhang of that size in hurricane territory seems a bit on the edge when attached through foam
Maybe I'm not understanding the whole picture, but why not
simply continue the main rafters out long enough to embody
the overhang at the eaves without scabbing sections together?
The gables might be a little more interesting but some sort of
cantilevered barge rafters to the rakes might make for strong
structure.
_H*
Another way to deal with this problem (if not too late) is to utilize a roof SIP. The SIP would provide your roof structure, sheathing, insulation and provide a 24" overhang without having to frame out a soffit and vent it. That would also address your thermal bridging.
36" overhangs can become an issue in some areas due to wind gusts and snow loads. A 100mph wind will take that overhang and turn it into an airplane wing. The overhang will literally want to take flight and rip off of your top plates.
I agree with Peter, simplest is to extend the SIPs.
Here is how its done using SIPS.
http://www.chaleffandrogers.com/templates/sip2-sips_eave_destruction.html
But I disagree with him on the risk. Slope has more to do with uplift than eaves. Unless you are in a hurricane or tornado zone, a 36" eave is not going to be an issue. If your in one of those zones, eaves are the least of your structural design/materials concerns. In additon to the aesthetics, wide eaves will help significanlty with cooling and drying.
http://www.sciencedirect.com/science/article/pii/067610594900515
Retrofitting a Roof for High Wind Uplift - Georgia-Pacific Building ...
http://www.buildgp.com/DocumentViewer.aspx?repository=bp&elementid=3195
Fitch,
The SIP manufacturers and the "SIP Prescriptive Method" by SIPA allow up to a 24" overhang. After the 24" length most manufacturers do not allow for longer spans. Some will allow a 36" overhang but require bracing. A SIP basically "cantilevers" over the wall top plate and 24" is the "safe" limit but with 36" you will most likely need to brace it with some dimensional lumber.
Peter … the issue is insulating at the eave. The SIP only needs to give a target R value which can be done by lengthening it a few inches (not the entire distance hoped for in this thread); extra overhang can be grafted onto the fascia with a structural return soffit. That simple SIP and soffit option is in the accompanying diagrams.
http://tinyurl.com/jwc2tqq
Check the engineering. There are few limits to SIP spans; they cantilever better than any other wood beam design except a flitch plate beam.
Here is a 4 ft SIP overhang:
http://www.sips.org/building-excellence-awards/2014-building-excellence-ellmann-residence