Old house: Insulate roof with nailbase or spray foam installed from outside?
Patrick_OSullivan
| Posted in General Questions on
I have a 100 year old colonial style home in NJ (zone 4A). We are doing some substantial renovations and will soon enough get to the roof. The current roof is asphalt over cedar over furring on 2×6 rafters (which actually are 6″). The roof is completely unvented.
Looking at the front of the house, the right side of the gable houses a fully finished room in good condition. The interior of the left side of the gable will be completely stripped (and therefore there is full interior access to that part of the roof).
Additionally, there is an air handler (of course) in the unconditioned knee wall on the right side of the gable, and ducts in most of the rest of the knee wall area. (This was done, ironically, by a state certified energy improvement contractor before I knew enough to figure out a different option.)
Goals:
1. Insulate the roof as best as practicable
2. Do not disturb existing finished space
3. Install PV on roof
Known: Roof will be fully stripped and sheathed with some airtight system (likely Zip).
Here are the two potential options I’m considering:
Nail Base + Batts
Gist: Old roofs come off, new sheathing goes on. Areas of bays that will be inaccessible after sheathing installed will have mineral wool batts installed prior to sheathing installation. Nail base installed over new sheathing; PV installed on nail base using fancy mounting bracket that Martin once happened to come across and posted about.
Concerns: Let’s just face the facts, houses aren’t getting built around here with nail base on the roof. I get nervous any time I’m asking a contractor do a detail they’re not familiar with. Additionally, I wonder how this will work cosmetically with a much thicker roof.
Spray Foam (from exterior where necessary)
Gist: After sheathing installation, one side of the gable could be easily spray foamed from the inside. The area above the finished room simply could not be. While the old roof is off, there would be an opportunity to spray foam rafter bays from the outside. While the rafter depth couldn’t get me to full code insulation for my area, we’re talking about going from zero to a heck of a lot more.
Concerns: There would presumably be some air space between the exterior face of the closed cell spray foam and the new roof sheathing. If the new sheathing is detailed appropriately and air tight, is this something to be concerned about?
Priorities
1. Installation correctness/durability
2. R-value
3. Cost (while last here, it is still a priority, so the difference in cost for the R-value difference between the two options will be considered)
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Replies
>"While the old roof is off, there would be an opportunity to spray foam rafter bays from the outside. While the rafter depth couldn’t get me to full code insulation for my area, we’re talking about going from zero to a heck of a lot more."
Filling the rafter bays with fiber blown in mesh or carefully trimmed R30 rock wool batts (nominally 7.25", easily compressable to 6.0") before the roof deck goes down would deliver R25-R26 performance. Filling them with open cell foam and trimming flush with the rafter tops would deliver R22-R23.
As long as at least 30% of the total-R as nailbase or other insulation above the roof deck the roof deck is fairly protected from interior moisture drives by keeping it's average wintertime temperture above the average wintertime indoor dew point. So even 2.5" of 2lb roofing polyiso (~R14) or a 3" polyiso nailbase panel would get you there, even with R26 rock wool in the rafter bays. With the rock wool solution and 3" of roofing polyiso (or a 3.5" polyiso nailbase panel) it would meet current code on a U-factor basis, despite being somewhat shy of R49.
Reclaimed roofing foam is usually priced about 1/4-1/3 the cost of virgin stock foam, and is fairly ubiquitous in 2" through 4" thicknesses. Doing it with 3" foam and 5/8" OSB nailer deck through-screwed to the structural roof deck with 4.5" pancake head timber screws is a bit more labor intensive than nailbase, but if using reclaimed foam it will be substantially cheaper. Using a fully adhered self-healing WRB on the structural roof deck such as Grace Ice & Water Shield using standard OSB (not ZIP) is cheaper, and mitigates against leaks from getting started around the screw penetrations.
Whatever is decided for the roof stackup, be sure to run it by solar installers first to make sure it has the structural capacity for the PV in a high wind. It may or may not require a tighter fastener spacing or thicker nailer deck that what's typical for roofing alone.
Thanks Dana. Your analysis is certainly in line with my thinking.
Do you have any thoughts on the spray foam (sprayed from the exterior where necessary) option?
Deleted
Anyone perhaps have any thoughts about the exterior installed spray foam option?
Hi Patrick -
Since the spray foam option does not involve a continuous thermal or air control layer, it's not as robust a system in my book.
You also need to make sure that your spray foam installer gets his chemistry right (see this article I wrote on getting spray foam insulation done correctly: https://www.buildinggreen.com/blog/foam-place-insulation-7-tips-getting-injection-and-spray-foam-right).
The only way you can't do the topside continuous rigid insulation to (nearly) completely hide that added depth visually, is to do a chainsaw retrofit of your eaves and rebuild your overhangs (see https://www.greenbuildingadvisor.com/article/a-real-chainsaw-retrofit).
Peter
Peter,
Thanks for your reply! For some reason I didn't get a notification about it like I did the earlier reply from Dana, so I apologize for my delay in responding. I have a few comments/questions:
> Since the spray foam option does not involve a continuous thermal or air control layer, it's not as robust a system in my book.
Agree on thermal, but the Zip sheathing would be integrated into the rest of the envelope's air control layer (that I will be enhancing).
> You also need to make sure that your spray foam installer gets his chemistry right (see this article I wrote on getting spray foam insulation done correctly:
Yes, I will certainly be finding a reputable contractor. Actually spoke with one today as a referral from Demilec (as I had become interested in their low-GWP product).
Assuming the details are done right and the air sealing is adequate, do you see any issue with spraying foam (not injecting) from the outside such that the bumpy surface of the foam results in a gap between the sheathing and foam?
> The only way you can't do the topside continuous rigid insulation to (nearly) completely hide that added depth visually, is to do a chainsaw retrofit of your eaves and rebuild your overhangs
I know it's all possible to do well with nailbase, etc. It's just that I don't see a lot (read: any) of that around here so I'm always skeptical of the details being done right the further away from the "norm" I go.