Are the Solstice based spray foams actually as good as reported? Also, exterior insulation on the roof
I tried to be brief below but I guess polite and nice is all I got :). Long time reader, first-time poster.
This will be the first in a series of posts about my home that I will be asking here – thank you in advance to everyone who will be helping me understand how to proceed.
I am trying to take my old home in Dublin, CA (i believe the climate zone is 3C) which has limited insulation at the ceiling (<6″ of blown in loose cellulose) and no insulation in the walls to a near net-zero/passive like home. As part of this process I am also adding on to the home – it is currently 1350sqft and when I am done it will still be a relatively modest 1840 sqft. I am not trying to get certified/accredited – the outcome is the only thing that matters to me.
I have a solution for all my new exterior walls and roofs. New walls will be 2×6 with 2″ of mineral wool on the exterior of sheathing and blown-in/batt on the interior of sheathing. The new roofs will be 2×12 rafters – insulation type is TBD but given its depth, I am sure I can get to at least an R-40 even without exterior insulation. I would like to add exterior mineral wool insulation on the new roofs but I am being advised that in my climate zone, this is overkill; is this true? If this is not true, how can I go about cost-effectively installing this to my roof for use with a shingle roofing? I contacted Roxul and they said they do not have a detail for this and recommended I go with standing seam if I want to use mineral wool boards on the roof.
The bigger issue is with the existing walls and roofs. They are all framed with 2×4 lumber and I honestly cannot afford to reframe them all with 2*6 walls and 2×12 roof rafters. Additionally, the current roofs are over vented attic spaces. A solution I found for the existing walls on GBA and Fine homebuilding was to add furring strips on the interior walls to increase cavity depth to approximately 2×6 using rigid foam and 1x3s that I can then insulate with blown-in/batts/spray foam. I can’t afford to add exterior insulation now to the existing exterior walls, but I am going to add it in 2-4 years when I have to replace my 60-yr old stucco and siding. Does anyone have thoughts on this approach or have a more effective solution (both with respect to insulation and cost)?
The only solution I can find for the existing roofs was to maximize my insulation by using spray foam but the GWP of the foam really bothers me and is holding me back from going ahead with it. Over the weekend I read about Solstice blowing agent based foams but haven’t really found much on it yet – is it as good as being stated? Are there any downsides to them that I need to take into account such as off-gassing?
Thanks again in advance!
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Replies
Solstice is just the HFO1234ze blowing agent. The foam chemistry of the polyurethane and any associated outgassing is the same as other 2lb polyurethane, with the same isocyanate outgassing risks as when using other blowing agents. The competence and diligence of the installer is everything.
The new 2x12 raftered roof can hit about R45 even in a vented application using rock wool or 2.2lb density fiberglass. Unvented with an inch of polyiso or 1.5" of rigid rock wool above the roof deck or with an inch of closed cell polyurethane on the underside it can hit north of R50.
On the existing roof and walls it's probably not "worth it" to go with the high expense of closed cell foam in the cavities, especially if it's 16" o.c. rafter spacing and full-dimension rough sawn lumber. The thermal bridging of the R1.2/inch rafters rob the R6-7/inch foam of it's potential performance, adding very little to the overall performance of the assembly. See:
https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste
Roofs don't usually have framing fractions as high as walls so the hit isn't quite as bad, but it's still substantial. Using open cell foam or rock wool on the underside of the roof deck and applying the difference in cost between that and closed cell foam to exterior rock wool or polyiso has better bang/buck on the performance of the assembly, center-cavity R notwithstanding.
>" I would like to add exterior mineral wool insulation on the new roofs but I am being advised that in my climate zone, this is overkill; is this true?"
Whether it's "overkill" is a judgement call largely based on your values and the costs. The analysis behind BA-1005 suggests that a "compact roof" design wouldn't necessarily be overkill financially at a "whole-assembly" R of about R45, depending on actual assembly and energy costs, and at typical rafter framing fractions R45 "whole roof" is about what you get out of R49 cavity-fill + R6 continuous insulation above the roof deck:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Mind you, R6 rigid rock wool is usually quite a bit more expensive than R6 polyiso, but in fire zones it has the additional fire safety factor to consider- it is more than just R-value for heating/cooling performance sake.
Thank you very much Dana. I will review the information provided and get back with any questions on them.
As for the use of SPF on existing 2x4 roof and walls - yes, it is 16" OC for walls and I believe 20" on center for the roof; I will try to confirm this. If SPF is not a good route here for the existing roofs, should I just focus on insulating at the ceiling? I will not have any HVAC in the attic - I am replacing existing HVAC with a ductless heat pump system and so only real items up there would be the ceiling cassettes for the heat pumps, some ducting for the ERV, and some plumbing pipes.
Additionally, if I were to just insulate at the ceiling ( say to R-50 or so), do I need to focus on making my roof airtight, or just my ceiling? I would assume the latter since the attic is still vented but wanted to be sure.
Dana, thank you for your detailed response - I had a few follow-up questions.
- generally speaking, does the SPF off-gassing and other issues persist for very long durations after initial install or just for the install period and then for 1-2 days? I was getting conflicting info on my searches.
- for the existing roof area, can 2x4 framing sustain the added weight of the rigid rock wool on the exterior of sheathing? Also, do I need another layer of sheathing to then install the shingles or can they nail through the rigid insulation? If the 2nd layer of sheathing is needed, do I need an air gap above the rigid rock wool similar to walls w/ rain-screens?
- how do I get accurate info on r-value of specific materials; all I can find are ranges online. Should I be looking it up in the spec manuals for the different types of insulations?
- would adding exterior insulation require me to convert my existing roof to an unvented assembly or can it be maintained as a vented roof? my new roofs are all intended as unvented assemblies by the architect - I am unsure which is better for efficiency and also for fire safety
as for polyiso vs rigid Rockwool, I would prefer to use Rockwool if at all possible on the exterior as we are in/near fire country here. My area hasn't burnt in living memory but places close by (<150 miles) have in the last 30 years. The only exemption to this may be my crawl space as I can't think of an effective way to insulate it since it is an open crawl space.