Sloped roof attic insulation remodel
I am in the planning stages of finishing out the 2nd floor of an expandable cape modular home in climate zone 5. The entire space is insulated but otherwise unfinished. I am wanting to update the insulation package for comfort and efficiency while everything is open. I have read numerous articles here and from other online sources and have a decent idea of the forces at play and what to consider/solve for. Like many before me that have gone round and round with all the information out there, I feel overwhelmed and would like feedback about my specific situation.
The Data:
-Built 2003
-CZ 5- far western massachusetts, rural setting, open to fields on all sides
-Lots of sun exposure on rear eave side, minimal direct sun exposure other 3
-24 gauge spectralume standing seam roofing vented with eave and ridge vents and rafter baffles. No exterior penetrations- ie. vents exit gable wall
-OSB roof seathing
-2×6 rafters
-9/12 pitch
Only vents (plumbing, dryer, radon) running through “triangle” behind kneewall
-One window per gable end, no gables or sunlights in roof planes
-Currently has r19 faced fiberglass following roof plane, walls, and flat ceiling
-Rafter bays are sealed off with 2″ rigid foam against rafters and roof decking. Sealed ti each with foam. rafter baffles inplace for ventilation.
-5′ kneewall with 2×4 construction
-Vinyl siding with unknown moisture barrier under it
-Thermally isolated from lower floor with r30 Rockwool under subfloor
-Plan is for bedroom in one half
-Other half will be dressing room and “bathroom”
-Shower and sink vanity will be open to dressing room
-Only the water closet will be isolated from the dressing room with walls/door
-Basically one big room “split” by the staircase from lower floor
-Overall footprint is 16′ x 39′
-Will be heated/cooled with Mitsubishi H2i Heat pump
There is no need to bring this up to r60 code, but I would like to get as close as I can. Exterior insulation is not a possability. Any solution must be attanable from the inside.
My thoughts are to remove the r19 fiberglass and replace with R23 unfaced rockwool then install rigid foam over the studs (2″ would be better, but I have r5 onhand) air seal it, and install drywall over that (possibly with strapping). I would add blown in over the flat ceiling to hopefully get r60 in the uppermost attic space that has nothing in it. This is quite costly and I would like to explore other options.
Another option I am thinking of is removing the insulation from the ends of the raters to the kneewall. Leaving the r30 between the floor joists, building out the 2×4 kneewall studs to get r30 or more between them, replacing the r19 glass in the sloped span between the kneewall and the flat ceiling with r23rockwool, and leaving the r19 glass in the flat ceiling and adding blown in glass up to r60 in the peak. This may or may not be less costly, I havent done the calculation for r30 and up rockwool. Rockwool areas would either get covered by rigid foam or vapor barrier primer and airsealed wall penetrations.
I would like to do a good job, make it comfortable for my wife and I for several decades, mitigate energy expenses that are surely to go up over our lifetime, and leave a quality product for our son to inherit. I do not have a limitless budget, but am willing to invest in a sound solution within reason.
Thanks in advance for your time and thoughts.
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Replies
In general, the problem with half story building is not so much the insulation and more the fact that they tend to leak so fast that insulation can’t do its job with all the air moving thru and around the insulation.
Start with a blower door test and keep air sealing and testing until you can get an ACH50 test under 2.
Note ripping out R19 and replacing it with R24 is foolish from an economic point of view as it will never save enough energy to recover its costs.
Walta
Jeffrey,
Excluding what's sometimes called the "devil's triangle" from the conditioned area of the attic is possible but very difficult. https://www.greenbuildingadvisor.com/article/insulating-behind-kneewalls
Unless it is in bad shape, I would as Walta suggested and leave the existing batt insulation in place, adding foam to the interior face of the rafters, and concentrating your efforts on air-sealing.
How is the existing system being vented? You described eave & ridge vents & rafter baffles. Are the baffles continuous from the ridge down to the vertical knee wall or all the way down to the eave intake? Are they under the OSB and over the existing R-19? You described rafter bays sealed off with ridgid foam but with an opening for the vent baffles. It is unclear what this accomplishes - are the vent baffles somehow sealed to the underside of the OSB?
What is your current air barrier layer - gyp? Have you done a blower door? If so where are your air leaks?
Thanks guys for the replies.
-022 The roof is vented by continuous baffles from the eave intakes to above the ceiling joist/collar ties- not quite all the way to the peak.
They are stapled to the underside of the osb roof sheathing but not foamed to them. When I sealed the ends of the floor joist bays with rigid foam, I cut out reliefs for them in the end caps and foamed the caps to the joists(sides), toplate(bottom), and roofsheathing/baffles(top).
The space is basically raw at this point with just kraft faced fiberglass in the stud and rafter bays, so if I understand gyp to be gypsum for my air barrier, I have nothing at this point. Depening on if I line all of the interior surfaces with rigid foam or go straight to the studs with drywall it'll be one or the other, XPS or gyp.
A concern i had about XPS over the existing kraft paper is having a double vapor barrier. Should I plan to puncture the kraft paper to prevent trapping moisture between the layers, or is that unnecessary if I ensure that i have completely air sealed all of the XPS seams, joints, junctures, and penetrations (windows and outlets).
Jeffrey,
Double vapour-barriers are only a potential problem when they occur on opposite sides of an assembly. You can have one against another without issues. Kraft paper also opens up to over 4 perms when damp, so you are fine.
I have not had a blower door test done. Probably a good place to start.
This the the low end of the rafter/floor joist bay described above. Done with the spray can foam, before I learned about the reusable guns. The guns are SO much more efficient with the material.
You don't really need to air seal vent baffles for them to work. The goal is to prevent the vent from getting blocked with loose fill insulating materials, and to limit windwashing. Canned foam is fine to secure things, but it's not nearly as critical in this application as it would be in something like a wall assembly where you want a good air barrier.
Bill