Planning for a roof insulation retrofit – SPF vs. Polyiso?
We are doing a gut renovation of a home that includes super insulating the walls (1.5″ polyiso wrap + 2×4 stud bays furred out to 6″). The roof is in good condition, so we are leaving it for now.
Plan is to eventually put 4″ polyiso (R23) on the roof when the shingles wear out, and use metal roofing. This is what I have seen a lot of on this site. Have not got a quote on this, but I imagine well over 10k, given the new roofing, adding bigger overhangs, etc.
Recently I learned about SPF top coats. A local company quoted $3.70 per inch per sq ft, for R7. ($3700 for 1 inch, R7, or $6,952 for 2 inches, R14, on our roof (1,350 sq ft) to spray foam on top of the existing shingles. Our roof would be white, but this is fine because we’d otherwise put a coat of white elastomeric paint up there (we’re in the desert, and require more cooling than heating energy).
Now I’m questioning the original plan. SPF would give us less insulation, but we could do it sooner. Wondered if anyone with experience could weigh in.
One of the decisions I have to make right now is whether to invest in closing in the soffits and some aesthetic fascia improvements. If the longterm plan is polyiso and new roofing, I imagine we’d be cutting the existing overhangs and fascia off anyway, yes? So not worth fussy improvements. If the plan is to use SPF sooner than later, the fascia & soffit investments make sense.
Thanks.
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Replies
Reanna,
Spray polyurethane foam makes excellent roofing. The main disadvantage is aesthetic. Because the surface of the roofing is slightly bumpy or uneven, this type of roofing is usually installed on homes with low-slope (flat) roofs or roofs that aren't visible from the ground.
If you can accept the aesthetics, it's an approach that works well.
Here are links to two articles on the topic:
Spraying Polyurethane Foam Over an Existing Roof
Roofing with Foam
One final point: closed-cell spray foam requires a blowing agent with a high global warming potential, so green builders often avoid the product. If you care about our planet's atmosphere, you should know that polyiso is a much greener choice.
Thanks Martin. Do you know anything about installing solar panels on an SPF roof, vs. a polyiso roof?
We have high wind loads here, and have wondered whether the long screws needed to secure roofing to framing through polyiso would be able to support the panels.
In the case of spray foam, I assume the brackets for the panels would need to be installed before the foam.
Where is this house located?
A wall stackup with1.5" polyiso on the exterior of 2x4 furred out to 6" filled with would come in barely above code-min in US climate zone 7, and would not have sufficient exterior-R for dew point control at the sheathing (it would need an air-tight smart-vapor retarder). But it might be considered "superinsulated" in US climate zone 4 or lower.
Both the polymer content per R and the HFC245fa high global warming (~1000x CO2) blowing agent make SPF roofs a fairly yellow-ish going on orange shade of "green".
D.
We are in climate zone 3, for the record, so have no dew point sheathing concerns. (I did mention that we are in the desert, and have more cooling days than heating days per year. I don't think that describes any climate zones above 3).
I'm aware of the blowing agent concerns with SPF. The challenge is to get a sense of proportion on these things. EG: If we were able to do SPF 5 or 10 years earlier, for financial reasons, would those extra years of improved insulation "make up" for the ozone-damaging blowing agent? Or not? These are things I would like to know.
Reanna,
Even in Climate Zone 3, building codes include important requirements for this type of insulated roof assembly. For example: (a) The 2012 IRC requires that the assembly have a minimum R-value of R-38. (b) If you want to use foam insulation on the exterior side of the roof sheathing -- your suggested approach -- then the foam insulation layer needs to have a minimum R-value of R-5. (For more information on this type of roof assembly, see How to Build an Insulated Cathedral Ceiling.)
Once you have installed either a layer of polyiso or a layer of spray polyurethane foam on the exterior side of your roof sheathing, you still need to install the remainder of the R-38 insulation on the interior side of the roof sheathing. The best type of insulation to use on the interior side of the roof sheathing is a vapor-permeable insulation. The vapor-permeable insulation should be in direct contact with the underside of the roof sheathing.
As a cost-saving measure, also consider looking for used (reclaimed) polyiso insulation. There is a well developed market for it in some areas--not sure about your area.
As for whether doing it now outweighs the ozone depletion effect, the ozone depleting blowing agents have already been phased out, so that's a non-issue. It's the global warming effect that is now the concern. Just guessing from what I've seen of other analyses, not actually doing a calculation for your situation, I would guess that the global warming impact of the blowing agent would be a little worse than the global warming impact of your increased energy use for 7.5 years while you wait to do it right. But I don't know for sure. But then there's also your energy use over the following 30 years, which would be better with the 4" of polyiso than with the smaller amount of SPF. That I know for sure. So to me, that tips the balance to thinking it's better to wait.
Check the math-
1 inch-$3.70 x 1350 =$4995, not $3700. In any event, get a firm price for the entire job, so you can really evaluate the proposal.
Planning to re-roof your Michigan home? Consider checking with a structural engineer to ensure your roof can handle the extra weight of more insulation. If the existing poly-iso is in good shape, adding more could boost your home's warmth. Think about using an ice and water shield for added protection against winter weather. When deciding between metal and membrane roofing, balance cost, benefits, and any structural requirements like sheathing. Also, make sure your project aligns with local building codes, focusing on insulation and moisture control. Need more advice? check this for more: https://restonroof.com/sterling-va.php
Planning a roofing project in Michigan? Before diving in, consider consulting a structural engineer to ensure your roof can handle added insulation. Boosting insulation with existing poly-iso can enhance warmth, but don't forget an ice and water shield for extra winter protection. When choosing between metal and membrane roofing, weigh costs, benefits, and structural needs. Always align your project with local building codes.