Huber over foam over Huber?
Howdy folks,
First off, thanks for this great resource. am building a 34’x38′ ‘addition’ onto my garage with an ADU in it to pay for it all up here in Lebanon, NH (climate zone 6).
Got a screaming deal on some used commercial 3″ polyiso and so decided to base my insulation plans around that.
Roof is 12′ rafters to be filled with mineral wool, then 5/8″ Huber zip system, then the polyiso on top. Intello membrane on the interior of the conditioned space.
The roofing contractor (standing seam, due to low roof slope and desire to DIY install solar in the spring) needs to have sheathing under the metal.
Seems crazy to me to do ANOTHER layer of zip system (god just the tape is so expensive) on top of the foam, but if I’m going to go to all the trouble of paying down 1/2 OSB, I might as well get some 7/16″ Huber and have a belt and suspenders against moisture… right? It is a very low slope… one side is about 1.1/12 (I know, I’m crazy) and there’s a valley in it where it meets the existing building (I know, I know) so to some extent I feel like all protection against moisture there is good.
Then I’m like, well, if I’m going to put more zip on top, I might as well get zip R6 and use the EPS exterior to keep my polyiso a little warmer so that it doesn’t do the cold-weather performance drop that polyiso is so famous for… it’s only a little bit more money…
Also, seems like to keep my ratios right, I would need two layers of the poly iso on the outside of my first sheathing – or would I be better off to go to the store like a jerk and buy some EPS or XPS to be the outer layer?
You see my conundrum.
Bonus question – I have some long screws to fasten it down, but how in gods name do I keep track of where the rafters are through insulation and another layer of sheathing? I will admit that my layout is not as regular as one might like.
Thaaaanks.
~Rory
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Replies
Look at some of the older metal roof videos by Matt Risinger where he uses diagonal 2x4 furring strips instead of a second layer of sheathing and attaches the roof to the furring strips instead of needing a second layer of sheathing.
The cold weather polyiso performance drop is not something you need to worry about, even with lower performance it beats most other products and only the outer most layer takes the full brunt of the reduction.
Matt Risinger did Huber- Polyiso - Huber on the entire roof of his personal home in Texas.
On his personal house, yes, but some of the older galvalume metal roof videos he was able to get away with just furring strips instead of a continuous second layer of sheathing. It also creates a nice air gap for venting.
Hi Rory, I just did a new roof over an existing 19th century barn for a retrofit. I filled the rafters with cellulose, zip on top, then two layers of 2.5” foil faced iso taped with vana. I strapped the foam down with 1”x4” rough cut pine above each rafter. Then I made a horizontal layer of 5/4”x4” rough cut for the standing seam. I put the horizontal 24” on center and did 2 clips for roof per horizontal for solar. I ended up really liking this assembly for a number of reasons:
-the space between iso and metal allowed us to use a vented ridge cap and that cavity continues to the wall so my rainscreen has a lot of air flow. In this case the foil-faced polyiso acts as wrb rainscreen, allowing for drying and cooling. Not a bulletproof screen relying on foil lamination but I figure with the airflow and little actual water getting in there it’ll stay dry. also the zip is below the poly so that will stop water and allow drying.
-the horizontal nailers made roofing easy. It acts as a ladder with no need to screw cleats into sheathing.
-the 5/4” nailers allowed us to use 1” screws for roof clips, a stronger connection especially for solar.
-I was able to use the horizontal nailers as outriggers connected to a 2”x fascia board so it doesn't look like my roof assembly is 15”+ thick. It feels rugged but if you wanted to be more secure or longer overhang you could use 2”x for horizontal, or you can have blocking there too and cover block with a soffit.
Ooh, I like that. Way easier and much cheaper. I'll check with the roofing contractor and see if they'll accept that.
brooksbend,
Looks like a nice project! A couple of minor points:
Some codes preclude connecting wall rain-screens to roof vent channels. They are seen as concealed spaces which need fire blocking at the top.
"also the zip is below the poly so that will stop water and allow drying."
It may stop water, but I don't see how it allows drying.
As for finding the rafters, I did use full sawn 2"x10" so I had another half inch to play with. Make sure your rafters line up exactly where you want them and come up with some system of marking at least a reference rafter, like the end rafter, before covering and adding anything to walls etc. If you're using sheet sheathing on top then theoretically once you place your first sheet you can measure off of that sheet to get center of rafter. I would use Headloks as they have a flat head. You will know if you miss the rafter. If you do, I would have a marker handy and mark the misses and come back to take them out and fill the hole so you're not juggling that task and the task of strapping at the same time.
If you do the system I described, you can mark the ridge and the eave for each rafter and chalk lines onto the poly. I chalked the line so that I could line it up with one end of my batten and then screw eyeballed in the center of the batten. It was surprisingly successful and I only have a couple misses on the entire roof.
Yeah, certainly easier with battens than sheets!
Roofing contractor wasn't stoked, but said I could do it as long as no more than 8" gap between horizontal nailers, which I think will save a lot of time and $. (still need continuous sheathing in the valley, that makes sense.)
I'm likely going to have to mark every rafter since my layout wasn't perfect or regular and since it's multistage, some of them are offset from each other sigggghhhhhh. But I guess we'll figure it out.
INcidentally, if I have paper faced polyiso (used commercial, I know oh well it was very cheap), should I cover it with a membrane of some sort under the metal?
Diagonal furring strips would promote better airflow and drainage (in the event water ever made it into the air space) compared to this horizontal furring strip method.
Matthew, what I’m describing are vertical battens down first for drainage and venting, then horizontal nailers on top for securing the roof. I don’t see how diagonal battens would be better than vertical battens in terms of drainage and venting but maybe I’m missing something.