1.5 story insulation
I live in a 1.5 story cape cod in the Philadelphia suburbs (Zone 4A) built in the 1950s. As expected the second floor has no air sealing and basically no insulation. I am leaning towards using dense pack cellulose to fill the 2×6 rafters as this will not require a total tear down of the drywall. I know without using a vent channel this would require me to use foam on top of the roof deck with at least R-15. I have read all the articles on the site that have to do with my situation but I still have a few questions about this before I get the work started.
Would it be ok for me to wait a few months between getting the cellulose installed and adding the foam to the top of the roof deck (I need to get a new roof this year as well so the timing works)?
If I add 3″ of foam on top of my roof, would I still be able to add solar panels in the future or would the connection be compromised?
The drywall on the ceiling does not continue along the rafters to the ridge but flattens out in the middle. Should I have the cellulose installers fill this entire triangular area (and of course seal up the existing gable vents)?
Thanks in advance. Love the site.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Andrew,
Q. "Would it be OK for me to wait a few months between getting the cellulose installed and adding the rigid foam to the top of the roof deck?"
A. Yes. But if I were you, I wouldn't go through a winter without the rigid foam.
Q. "If I add 3 inches of foam on top of my roof, would I still be able to add solar panels in the future?"
A. Yes. Aluminum racks for PV modules come in many configurations with many different attachment methods, and there are many ways to attach racks through this type of roof assembly. Just be sure to tell your solar contractor about the rigid foam so that the correct bolts can be used.
Q. "The drywall on the ceiling does not continue along the rafters to the ridge but flattens out in the middle. Should I have the cellulose installers fill this entire triangular area (and of course seal up the existing gable vents)?"
A. That depends on the width and height of the triangular attic. There isn't much sense in making the cellulose much deeper than 24 inches (or at most 30 inches).
If the roof decking is 1x planking, not OSB or plywood it'll take a number of winters to create an issue without the exterior insulation, especially if the roof is dark and not very shaded, which raises the mean temperature of the wood via direct solar gain. (That solar reduces the number of hours it dwells below the dew point of the interior air, and starts the drying season sooner with peak temps well above the outdoor ambient.) Portions of the roof that are in shade during the day (especially on the north side) would be at higher risk, but if you're contemplating adding PV your shading factors are probably minimal. If shaded, even plywood can take it for a single winter, but OSB might be risky.
If you'll be re-roofing and installing the exterior foam before January it doesn't matter at all.
The R15 prescriptive for dew point control presumes a total R of R49 (IRC 2012 code minimum for zone 4), and it's the ratio of exterior R to the total R that matter, so you need at least 30% of the total R to be exterior to the roof deck everywhere. If you install more than R34 on the interior side you need more than R15 on the exterior. Since you can't get more than about R20-R22 into 2x6 rafters with dense packed cellulose you'll have quite a bit of margin for most of the roof.
The iffy-section will be the gable vented triangle at the top, as you'd blow by R34 at about 9" of thickness with 3.3-3.5lb cellulose. Go ahead and install the exterior foam over that section, but if it's going to be more than 9" of cellulose in the triangle keep the roof deck vented (including keeping at least an inch of clearance between roof deck & the cellulose.
Alternatively you could raise the R-value of the foam layer (a bit expensive) to accommodate whatever R-value the cellulose in the triangle ends up.
Or, you can remove the roof decking in that section, replacing the wood decking with rigid foam of the same thickness as the decking. With cellulose insulation if there is some moisture accumulation at the cellulose/foam boundary over the winter the cellulose might settle a bit, but the moisture gets safely redistributed in the cellulose, protecting the rafters & ridge beams (unless you're at R100 or something on the cellulose.) With other fiber insulation this is potentially problematic for the ridge & rafters, but pretty safe with cellulose due to the high moisture buffering capacity.
Dana, you are correct that the rood deck is 1x planking and that there is basically no shading on the house. Just wanted to make sure i am reading your first option on how to deal with the top triangle section correctly. Would be be correct to have the R-15 foam cover the entire roof, fill the top triangle area up with as much cellulose as i can fit minus an inch under the vent and leave the gable vents open due to the fact that I will not have at least 30% insulation over the roof?
Thanks again for the great info.
One more question. Before I have the rafters insulated, should i air seal between the wall top plate and the roof rafters? I did the cut and cobble method on my basement sill plate with rigid foam board, would the same idea apply?
Andrew,
This video shows a good method of addressing this area: How to Ventilate Rafter Bays When Adding Insulation.