Over roof detail questions
I am struggling to figure out the over roof details I need for my project and would really appreciate some input.
I will be taking the roof of a log cabin down to the t&g decking and doing a rigid foam-on-top only roof (I have a gambrel roof with low ceilings on the second floor and no space to insulate the interior). I have read all the articles on the site in relation to this and have come up with (from bottom up). Peel and stick membrane, two layers of rigid insulation with joints staggered and taped….this is is where it breaks down for me.
From reading multiple articles I have gathered that in my zone I need a vented roof vs. a compact one (articles i’ve found indicate anything over 60lb ground snow load need to be vented and i’m at 90lb ground snow load). I have also seen some indication that 2″ is recommended. My questions are:
1. how much insulation do I need? I am on a budget and concerned about needing to completely redo the 2×6 fascia to raise the roof thickness and all the other details.
2. Do I absolutely need another layer of roof decking + roofing paper inbetween the vent framing and the metal roofing as i’m seeing in a lot of the articles? I can’t just fasten the metal to the vent framing? Sheet goods are scary expensive right now so if there are any options that avoid that route + the labor involved I would really appreciate some direction on that.
Thank you!
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Replies
Jacob,
Where are you located? Most advice is climate-specific.
East Idaho in the mountains, Zone 6b
Hi Jacob,
Martin's article on exterior foam is worth reading if you haven't already (https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing). In CZ6b, you ideally would install R-49. For savings, considering using reclaimed rigid foam, which is often 1/3 the cost of new material.
OK, how about the next step after that. After a lot of reading it seems I need venting between the foam and metal to avoid ice dams - do I have to do a sheet good deck, then another underlayment like roofing felt, then fasten the metal, or can I lay down vertical 2x4 battons on side (thereby creating the vented air gap) and fasten the metal roofing to that? I can't find a lot of info on whether that is a viable way of doing it or not. If I have a full peel and stick on the first roof deck, and then polyiso (which is waterproof based off everything I can find right?), am I good to fasten roofing to the battons that are creating the vent gap?
More insulation is always better, somewhere around an R30 assembly you get most of your energy savings for a compact roof.
A number of roof panels can be installed over strapping as you suggest, but I would confirm with the manufacturer that it can support your snow load. I would guess that at 90lb, you will need them mounted onto a solid deck or very tight spacing.
Since most panels have some ribs and striations, you can get a fair bit of airflow even with horizontal strapping, most ASV setups I've seen have cross strapped roofs.
Most houses have the T&G deck extend over the top plates and into the soffit area. If you have this, when sealing up your original T&G deck, it is important to air seal the grooves between the boards above the top plates. The simplest I have found is to drill a hole from the top between the boards over the top plate and fill the gap with spray foam.
The best way to deal with the extra thickness of the foam is to picture frame the edge of the foam with beveled 2x to the match the slope of your roof and cut to the thickness of the foam. This effectively increases the height of the facia board without it having to remove it. You can a piece of trim bent up by your local sheet metal shop to cover this wood and lap over the old facia trim.
Thank you for the reply. I have log walls - the second floor deck (also t&g) extends past the log, and then the t&g log roof deck goes past the floor deck on the way down. I have been wondering how insulate this connection - I was thinking about building a small kneewall on the interior since the steep space is really unusable anyways and spray foaming that. I was planning on extending the peel and stick all the way to the bottom of the roof deck which extends past the walls like you said so that would be sealed up. Any problems with this plan? The house is old and there are some areas with soffits and some without - does adding a soffit serve a purpose other than visual?