Hot hybrid roof retrofit in zone 5?
Hi folks,
We’re finally pulling the trigger and getting started on fixing up our house that’s over 115 years old!
The current roof and attic is completely uninsulated barring some poor blown-in insulation in the ceiling of the 2nd floor. Walls are balloon framed and, of course, no insulation.
We’re going to add 6″ of exterior polyiso to the roof, and a r23 mineral wool batt in the rafter bays.
Here is the question that keeps coming up: what should we do for a VCL, and where should it sit in the assembly?
Here’s the proposed build up:
1. Original 1″ roof shearing boards
2. A new peel and stick membrane. (If highly perm is desirable, something like Slopeshield. If low/no perm, pretty much and water/ice shield across the whole roof – Grace?)
3. 3″ Hunter-S polyiso
4. 3″ Hunter-S NB polyiso with a bonded CDX sheet
5. Lower application of ice and water shield per code
6. A synthetic membrane (Vent3)
7. A mesh for air gap
8. Metal shingles
Given we will be installing mineral wool in the attic walls and roof rafters, as well as eventually be adding exterior wall insulation when residing, how should we tackle this? Given we have low moisture control (excluding dehumidifiers) should we consider a vapor barrier peel and stick instead of Slopeshield which is highly perm (30!!)
I welcome all thoughts as we need to pull the trigger soon as we’ve noticed some missing shingles 🙁
Thanks in advance!
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Replies
A roof with continuous exterior insulation performs better than a regular roof thus most codes allow compliance based on the the U value of the whole assembly. There is no operating or environmental benefit of going above code min when it comes to an expensive roof assembly like this that includes rigid insulation.
This means in most cases you need less than the R49/R38 most codes call for. I would check what your code allows complies based on U factor, you can use an on-line calculator such as this to get your assembly R value:
https://www.ekotrope.com/r-value-calculator
Generally you want a solid air barrier at the roof line, so the peel and stick is the way to go. Since the polyiso at 4" to 6" is essentially a vapor retarder, it doesn't matter what the permeability of this peel and stick is. For this types of roof, I've used regular non-granular ice and water as it tends to be the cheapest. This assembly can only dry towards the interior so make sure not install any low perm membrane such as 6mil poly above the ceiling drywall.
Make sure to figure out how to transition this air barrier down to your walls, in a retrofit situation, the simplest is blocking between the rafters and spray foam at the eaves over the top plate. One of the two part spray foam kits is great for this as you don't need a lot of thickness to air seal. If you want to avoid spray foam, you can also cut and cobble some rigid in there similar to a basement rim joist air seal.
Metal shingles are not cedar, properly installed they are liquid tight and won't need any venting. The roof assembly itself also doesn't need any venting, I would not bother with anything fancy here, no need for the mesh. I like to use permeable roofing underlayment but regular synthetic roofing underlayment is good enough.
ericwages,
Akos has given you good advice.
For clarification, as this level of work is considered reroofing, so it needs to meet the current code which, in the case of Massachusetts (zone 5), has adopted the IECC 2021 code in nearly all communities (mine included). That means, R60. Now, if I COULD have an easy way of having an R49 assembly all interior, I could make something else happen, but I don't have the joist depth.
The roofing company commonly uses Titanium PSU 30 (perm 0.03) as it's peel and stick / ice and water shield. That should be sufficient for the whole assembly. Revised assembly:
1. Original 1″ roof shearing boards
2. Titanium PSU 30 peel-and-stick, perm 0.03
3. 3″ Hunter-S polyiso
4. 3″ Hunter-S NB polyiso with a bonded CDX sheet
5. Lower application of ice and water shield per code (Titanium PSU 30)
6. A synthetic membrane (Vent3)
7. A mesh for air gap
8. Metal shingles
The mesh mentioned is to provide the ability for any condensation that occurs on the back side of the shingles to drain out easily. The particular assembly here leverages interlocking shingles, but it's not air tight.
U factor is part of all codes, look at table R402.1.2 which list the U factor of 0.024. That is the equivalent of an R42 assembly, so you won't need 6" of rigid. The only thing to watch is you want 40% of your assembly R value as exterior rigid, so around 3" with R24 batts.
6 can be any synthetic underlayment, Vent3 is fine.
7 serves no purpose, I would skip it.
The rest is fine.
Thanks for the info. It looks like looking at the LTTR of the Hunter panel, and doing some approximation of the assembly with this tool - designed for walls, but useful for the calcs (https://www.appliedbuildingtech.com/fsc/woodcalculator), it looks like the numbers need to end up with a 0.024 U-factor or better given these calcs means I still need a two-part solution, but not at 6". Assuming an R23 inside batt, it needs to be a minimum of 22.5 LRRT. The permutations look like they can be:
3" H-shield + 1.5" H-shield NB = 23.7 LTTR
2.5" H-shield + 2" H-shield NB = 23.6 LTTR
2" H-Shield + 2.5" H-shield NB = 23.4 LTTR
1.5" H-shield + 3" H-shield NB = 23.6 LTTR
1" H-shield + 3.5" H-shield NB = 23.7 LTTR
The good news is the fastener sizes are all the same! I'll have the shop price these out tomorrow, but I'm guessing the first one will be the cheapest, as the NB panels have nearly a 100% price premium over equal-thickness panels without the bonded material.
Per the rough calc website, it seems that control check 1 would pass in all situations, including "No VR" and given the description above, we WILL have barrier on the external of the existing wood slats that is beneath the exterior insulation, but innermost to the assembly.
(Not sure why I can't add attachments, but here's the calc output: https://drive.google.com/file/d/1klyd43cPxsBKHHQgO2Fghmkfw9cHGdVY/view?usp=sharing)
That is better. There is still some free R value there you can capture.
The existing roof deck adds R1 and the ceiling air film is higher than the wall air film the calculator uses so gain an additional R0.5
That would drop you down to R21 for the exterior rigid part so about 3.5" to 4" of iso depending on the R value.
I would also price out regular fiber faced roofing polyiso (sold by all commercial roofing supply places), pretty much the same stuff as the Hunter part.
As for the vapor check, that calculator is for walls which are a different beast. If you want to check, a good option is:
https://www.ubakus.de/en/r-value-calculator/
A bit of a learning curve and you have to do some imperial to metric conversions but it can give you vapor profiles across the roof. For outside design temperature you use the AVERAGE temperature of your 3 coldest months (zone 5 is probably around 25F/ -4C).
I appreciate the feedback!
I intentionally ignored the existing roof deck as it's inconsistent - it's dimensional 1" boards with noticeable gaps so it won't be a consistent thermal plane. Or rather, it won't be in my mind.
I took a look at that that website. It's certainly a units conversion nightmare, especially since so many US-focused products don't have quite the same terminology (or even test for some of these).
Thanks for your help!