Need advice on the retrofit of a 1932 beach cabin — 2″ XPS foam panels on roof?
We have a classic, small (24′ X 24′) 1932 beach cabin in Ocean Park, Washington, and it’s time to replace the existing composition roof.
The second floor has a 5-in-12 gable roof, constructed with 14-ft. 2 X 4s, 24 inches on center, which are attached at their lower ends to the first-floor ceiling joists. Ceiling height is low, at just 80″ at the peak. There is no ceiling insulation on the second floor, and we would like to maintain the open beam look.
The climate is mild, often quite windy, humid and very rainy, averaging 74″ of rain per year. Temperature averages a low of 35 in January, to a high of 67 in August. We have a portable dehumidifier, which we run continuously throughout the winter.I am proposing the following retrofits, doing all of the work myself:
A) The 2 X 4 24″ OC roof joists are inadequate, and I plan to nail two additional 2 X 4’s to the side of each joist, creating 4.5″ X 3.5″ roof joists.
B) The existing roof sheathing is 1 X 6 Doug Fir boards, laid edge to edge. Sheathing is in excellent shape. There is a layer of 15/32nds OSB nailed to the 1 X sheathing, installed about 15 years ago (also in very good shape). I plan on carefully removing the OSB sheeting, then installing 2″ XPS pink rigid foam panels (Owens Corning Formular 150) over the 1 X sheathing, then reinstalling the OSB over the foam (staggering OSB and foam joints). The foam would be attached to the 1 X sheathing with 3″ screws into the roof joists. I would tape the edges of the foam. The OSB would then be screwed to the joists with 4″ screws. I would use plastic H clips on the OSB where needed. The 2″ foam will be R10, with no additional insulation between the roof joists, which should be acceptable considering the very mild climate. Certainly better than what we currently have!
Here are my questions:
1) Do I need to use furring strips between the foam panels? I was planning on furring out the edges and peak of the roof with 2 X 4’s cut to 2″, but would like to avoid furring between the 4 X 8 foam panels if possible-it would be extra work, and allow additional thermal bridging. Will just furring the roof edges be adequate? I cant imagine the OSB coming loose, even in very high winds.
2) Do I need to ventilate the new structure? I am hoping to nail the composition roof directly to the OSB, with no ventilated space (similar to a SIPS panel). I was planning on using 15 lb roofing felt between the 1 x sheathing and the foam, and either 15 Ib felt or Tyvek house wrap between the OSB and the comp roof. From what I have read, the dew point would typically be somewhere in the middle of the foam. The existing, exposed 1 X 6 sheathing will be carefully caulked and painted with multiple coats of oil based paint on the inside, creating an interior moisture barrier.
An interesting point about the house: it was built by Clara Smith and her carpenter uncle in the Summer of 1932, and is a classic “single wall” construction, with the original Cedar sidewall shingles. Nearly all of the windows are the original, site built windows. The uncle was a real “old school” master carpenter-the house was lovingly built by hand, with no evidence of any power tools. The cabin has remained water tight and rot free through 79 wet winters, with nearly no maintenance.
Any and all advice is greatly appreciated!
Thanks again!
Doug Caldwell
Ocean park Washington
[email protected]
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Replies
No reason for furring.
I am a fan of venting above foam to help shingle life, tho there is disagreement on the necessity.
pack the sheets of foam tightly together, a bit of spray foam if required to get them airtight.
What I have done is hold them down with foam screws through strapping, then screw the sheathing down to the strapping.
Can I talk you into 3 inches?
Works great with 2, 2x screwed down on the flat on the edges
Also allows for 2 sheets with staggered seams, great for airsealing
Thanks Keith!
Yes, I could go with 3"-good idea. I assume I will only need to tape the top layer of foam? Do you think I need 15 lb felt under the foam?
Thanks again!!
Doug,
OK, you've agreed to 3 inches. Can I talk you into 4 inches? Two layers of 2-inch foam with staggered seams.
As long as you are going to the trouble of fixing your roof, do it right. Install 2x4s on top of the rigid foam for venting, then OSB or plywood, then asphalt felt and shingles.
There is no need for asphalt felt under the rigid foam. And you can either tape both layers of foam (if you enjoy such details and strive for perfection) or just tape the top layer of foam (which is probably just as good).
One thing to note: Make sure you are running the 2x4 strapping from eave to peak @ 24" OC lined up with the rafter assemblies. Then you will wind up with very strong site built trusses about 10" deep.
You might even be able to skip the sister rafters, but if you do, consult a structural engineer.
Thanks for all of your great advice, Keith and Martin! After considering your posts, I have decided on two 2" layers of XPS foam (total R20) over the existing 3/4" sheathing.
My hopefully final question-do I really need the ventilation space between the foam panels and the OSB? It will be considerably easier to construct without the space and vent openings (recall that the home is in humid climate Zone 4C). I am also concerned as to how to vent the 24" wide bays at the eaves and ridge. The wind blows a lot here, and I am concerned about wind driven rain entering the vents. In short, I really want to avoid the ventilated spaces altogether.
Doug,
The short answer is that an unventilated roof will work -- so if you don't want to ventilate the roof, you don't have to.
A longer answer could be very long indeed. But I don't think you want that.
The only reasons to vent a foam roof are to theoretically protect the shingles and to theoretically reduce the thermal transfer of heat through the roof. So, no you don't need to do it.
While you would not know it on this site, there are other things to think about besides energy efficiency
[heresy]
Keeping the water out is one of those things. I chose to use a ton of ice and water and synthetic underlayment and to vent my roof and use really good shingles because I believe keeping the rain out is pretty much th top priority. and the house I bought was pretty bad at it.
I am NOT saying these things conflict[rain vs energy] except in the brain power commitment and cash flow.
[IMG]http://i99.photobucket.com/albums/l317/t92006/house/0911091239a.jpg[/IMG]
[IMG]http://i99.photobucket.com/albums/l317/t92006/house/0917091525.jpg[/IMG]
pics of my roof project.
I think you will find that if you do it in a similar fashion to what I have done, it will not cost much, and will not leak.
I was going to quibble with Martin on the 2x4, thinking it overkill compared to strapping, but I just love Kevin's plan of using the 2x4 on the roof as structural support. Brilliant. About 8 inch foam screws wound through the whole thing and that roof will last the millennium.