Converting a Screened Porch into Conditioned Space
Back with some questions regarding my four-season project (turning a screened porch into a conditioned room), in New Jersey, zone 5. The roof (approximately 24’x15′) is a lean-to roof, with a 15′ run and a 4′ rise; it’s unvented, with no skylights or anything else through the roof surface. There is a 1.5″ (I think) air gap between the boards that run across the rafters and the plywood that forms the roof deck; it was created (not by me) to make room for electrical wires; but there is no actual venting system; nor can one easily be created (I already discussed that option on this forum). The roof currently has shingles with ice-and-water shield for the whole surface of the roof; the whole roof, plywood included, is only two years old. I am planning to cut the rafters’ tails; install ZIP System R6 sheathing all around, “closing” the envelope with the new roof (see below); remove all the shingles; install 2″x4″ sleepers on the plywood, corresponding to each one of the rafters; install 1.5″ polyiso boards between the sleepers, flush with them; install additional layer of 2″ polyiso boards on top, offset from the ones underneath them (securing the insulation with screws and large, plastic washers); install a new layer of plywood (“intersecting” the ZIP sheathing used for the walls), new ice-and-water shield, and new shingles. In that process, I will also create overhangs on the two sides of the roof and, of course, redo the overhang where the rafters tails will have been clipped. Yet, my questions below are not about that part. In summary, I will be following the process that Martin Holladay and others have presented, and which is often discussed on this forum. (One example of what I am planning to do is summarized in this video, although my project, on the walls, will have the ZIP System R6 sheathing, and no furring strips: https://www.youtube.com/watch?v=JmsosK2My6o). Above the ceiling’s sheetrock, between the rafters, I will install R-23 mineral wool batts. I’ll appreciate receiving feedback on any of these questions: 1) Will I be fine, in terms of staying above the dew point, with R-20-21 above the deck (3.5″ of iso boards), and R-23 under the deck? Would I be fine with R-18 or so (3″ of iso boards) instead, and R23 underneath, if I were to go for a thinner layer of polyiso (maybe 2″+1″ of foil-faced, abandoning the option of using reclaimed boards, for which see below)? 2) If I use reclaimed fiberglass-faced polyiso boards, I might be able to save quite a bit of money (the R value will be a nominal 5.9 rather than 6.5, but I don’t see that as being much of a problem; in either case, I will not be able to reach R-49, but I would be above the mandated R-38 for old construction). Will I be missing the foil-faced polyiso, or should I not worry about that at all? 3) Using the 2″x4″ sleepers was my idea, to make the installation of the new plywood easier for the contractor. That way, even if using 2″ boards above those sleepers, the screws will have to be only maybe 3.5″ (penetrating into the sleepers, but not the beams––is a longer screw, which penetrates the lower layer of plywood or indeed beam as well, a better idea by the way?). My question is whether the sleepers are a good idea; should I instead just ask to lay down two layers of foam boards, and, even if the insulation thickness were to be 3.5″, drive screws through the foam layers and plywood, into the rafters? I am hoping to propose to the contractor an assembly that be as easy as possible for him, to minimize mistakes and problems. One obvious advantage of not installing sleepers is that, that way, the iso boards need not be cut, reducing possible air passages. The sleepers would also add weight to the roof. 4) If the ice-and-shield membrane does not get ruined in the process of removing the shingles, shall I ask to leave it alone, or should the membrane be removed from the plywood for some reason? Is “sandwiching” insulation boards between layers of ice-and-water shield membrane a good or bad idea, if it turns out that the membrane can be left where it is? 5) I am very worried about the airsealing not having been done right, especially since I will not be the one working on the roof. I am thinking of asking to tape all the edges, and maybe even the various boards with each other, already for the first layer of ISO boards, and then repeat the process (and certainly tape all the boards with each other) for the second layer. I could also, in addition, run tape, from below, between the rafters. However, apart from maybe the edges at the perimeter of the ceiling, I am not sure what I can reasonably tape; there are are 1″ deep wooden boards, below the plywood, that are not at all airtight, but taping each one of them, I suspect, would be very costly. (Any recommendations on the tapes I should look at, for the outside––in addition to ZIP tape where appropriate––and the inside?) Lastly, I am going to ask to apply sealant to the sheetrock prior to installing it on the ceiling. Will all of that enough? Shall I add a vapor retarder latex paint to the ceiling, and, if I were to do that, will that make it difficult for any humidity to dry towards the interior of the house? The new room, by the way, will be in direct communication with the kitchen; so, there will be some additional vapor from cooking. 6) Given the situation as described, would there be any advantage to using, for the new roof deck, ZIP roof sheathing (the brown one), or is using regular plywood going to be just fine? I would have some additional questions on how to optimize this assembly (where to stop with the ice-and-water shield, and things like that). Yet, my post is already too long as it. Thank you so much everyone!
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In zone 5 you need 40% of your assembly R value above the roof deck. With R23 bellow anything above R15 is good enough.
When doing roofs with expensive insulation such as rigid, there is no ROI for going above code. Even with reclaimed, around code min is good enough.
The embedded sleeper does make install easier but you also don't have to hit rafters with screws. You can use long length screws and faster to the roof deck instead. You can look at the fastener scheduel for something like Hunter panels as reference:
https://installpolyiso.com/docman-root/roof-polyiso-installation/low-slope/fasteners-and-fastening-requirements/h-shield-nb-in-low-slope-application/361-h-shield-nb-fm-i-90-fastening-pattern-for-low-slope-applications-16-or-32-oc/file
Some metal roofs can be installed directly over rigid but they require metal bearing plates. These are typically used for commercial systems, so won't be cheap.
Some can be installed over horizontal 1x4 strapping but most ask for a solid roof deck (OSB/CDX). This tends to be simplest for a smaller roof area. In another thread you mention the Junior HF, which is a decent budget snap lock but it is very light gauge. This means it must be installed onto solid deck and not onto strapping.
The ice and water over the main roof deck can stay, this will be your air and vapor barrier. I don't know how all those the nail holes will be once the shingles are pulled, you might need an underlayment to cover it. This can be taped at the seams and edges as the air barrier.
You'll need a 2nd underlayment under the metal roof. This doesn't need to be peel and stick, standard synthetic ones will work just as well. I prefer a permeable one as it allows for some drying of the roof deck bellow.
If you are going to embed 2x4 into the foam, I would look at using 2' wide foam so no cutting is required. There is nothing wrong with using reclaimed foam, you don't want foil faced insulation in this case anyways.
Instead of rigid, this might also be an option for you if you have the height:
https://www.greenbuildingadvisor.com/question/risk-of-rot-in-warm-roof-outlooks
Thank you, Akos. This is all very helpful indeed. Some quick follow-up questions and thoughts:
- In my area (Central NJ, zone 5), these days, code requires R-49 for new constructions; I am allowed R-38 because it's an existing house. Given the small size of the roof, and the fact that I found 1.5" (and 1") at a pretty low cost ($15/sheet), every additional layer would not add much, dollar-wise, to the investment (in fact, it would add between $165 and $180). So, it was tempting to go for three layers rather than two, and get a bit closer to that R-49 mark. However, I imagine and fear that everything might risk becoming more complicated for the contractor, especially since most folks I have been talking to are not very well versed on these approaches: longer screws and maybe other complications I am not foreseeing right now. I am not a pro at all, but on the grounds of what I have been learning –– largely thanks for this forum –– I am hoping to provide the contractor with guidances and even some fairly specific details; yet, it would still be very wife to keep the assembly as little as demanding as possible. So, one option that seems appealing is that of using 3" total of insulation, and stay happy with the resulting R-38. Yet, again, for me the main driving force in the decision would be that of making the job simpler for the contractor, as, for this project especially, the additional $$ investment to add more insulation might be quite contained.
- A possibility emerged: I could after all create some ventilation for the roof, under the new sheathing, by using Huber Cool-Vent panels (the ones with a 1" of polyiso), which I also found at a very affordable price. Given my concerns for not complicating the assembly (and have contractors run away from my ideas), I wonder whether I should pursue that. If using the Cool-Vent panels, the approach would be that of simply adding 3" of polyso to the existing sheathing and ice-and-water shield, then the Cool-Vent panels, then the underlayment and the shingles or metal roof. There would be a vent at the soffit and a Smart-Vent at the ridge.
One problem this assembly would create, it seems to me, has to do with the plan of adding overhangs. With the rigid insulation only, I could have continuous panels of plywood span, from the existing roof, for an additional foot to the sides; the ZIP R6 sheathing on the sides could be cut flush with the roof insulation and be perpendicular to the new sheathing,. With the Cool-Vent panels, I suspect that the panels at each of the side ends should be modified to make attaching the additional sheathing needed for the overhangs; the result might be a more complicated and maybe less solid assembly. Do you see what I mean? So, maybe I already answered my own question, and should not use the Cool-Vent's. The attraction of using them, to be honest, comes from those who –– on this forum, too –– insist that some ventilation gives them a "belt and suspender" insurance policy. Do you have any thoughts on this?
- Any synthetic underlayment, which has some breathability, you'd like to recommend? (In an older post, by the way, I read Martin stating that, if one is "set on using a permeable roofing underlayment, the obvious choice is asphalt felt, which has been used successfully for that purpose for decades." I actually have a roll of that; yet, I imagine I should get some more contemporary, synthetic product, right?)
Thanks!
The simplest for a contractor would be the hunter panels with sufficient thickness for condensation control. No need to complicate your assembly by adding rigid into the mix. Rigid over-roof is so far out of comfort for most contractors that the simpler you can make it the better. If your overhangs are not too large (under 1'), they can be bolted on and the insulation panel extended over it.
Central NJ doesn't sound like high snow load area, top venting is really only need in areas with 60lb+ snow load. Nothing wrong with using a vented panel but I would not spend extra on it.
You can't use felt under metal roof panels as it will melt and stick to the roof. There are many companies making breathable underlayments: Hal BreathX, Gaf Deckarmor, Dorkin Delta. I would find which one your local roofing places stock and use that.
Thanks, Akos! When you say "Hunter panels," are you referring to the Cool-Vent ones, or ZIP System R? (Probably the former, as the R panels are not approved for use on roofs, sadly.)
The Hunter Cool-Vent's I found only have 1" of polyiso. So, I should look at thicker ones. (That would be too bad, as I found those panels, brand new, for $20 a piece -- hence my idea of adding more polyiso under them.)
Very interesting that you would extend the insulation to the overhangs. I thought the insulation envelope to be best closed at the corner, and the overhangs be... fake so to say.
Hunter panels are available as simple nail base as well without the vent channel of the cool-vent version. If you don't mind DIY, nothing is stopping you from gluing a layer of thicker foam onto the 1" panel to bring it up to the R value you want.
As for the overhang, you need something there to attach the roofing, if you are using nail base panels might as well extend that all the way to the edge. Interrupting the roofing at the walls and putting just OSB/CDX there is more work than the additional bit of cost of the insulated panels plus you get a much stronger overhangs this way.
Akos, Thanks again! From what you say, it seems that you don't fall in the camp of those who find unvented roofs always dangerous, and opt for the belt-and-suspenders insurance policy of some venting. I'll look up Hunter's nail base. Your suggestions give me a range of options, and maybe I can discuss a couple of alternatives with the contractors.
I totally see your point about extending the whole panel, insulation and all, over the overhang. I guess the panel then sits in top of the wood pieces that, ladder-style, will secure it to the wall.
I
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