Should I add insulation to 7 inch space over drywall in a flat roof sun room?
I’m currently remodeling a finished sun room attached to my home.
A quick description:
-Florida climate
-Room dimensions – 36 x 12
-Flat roof
-2×7 rafters spaced 16 in. apart
-finished gypsum ceiling
-7 in space between drywall ceiling and plywood roof – currently no insulation
-The flat roof is connected to the main roof with no barrier between the main attic space and the flat roof 7 inch attic space.
– Looks like there is ventilation into the attic but no other ventilation like soffits or vents are present
My original plan was to cover over the ceiling with T&G planks using 1×2 battens attached to each rafter over the drywall and nailing the planks to that. The room gets quite hot during the summer so I figured it would be a good idea to add foiled polyiso board insulation between the battens covering the drywall and finishing over that with the T@G plank ceiling. The polysio would be between the planks and drywall ceiling. I figured there had to be some sort of insulation above the gypsum but when I went to cut out holes for electric where new lights are being installed I see there is no insulation whatsoever in the 7 in. space.
Q1: Should I add any sort of insulation between the roof and the gypsum?
Q2: I spoke with a buddy of mine who is a contractor and he recommended that I take the drywall ceiling down and add roll-out fiberglass insulation covering over that with 1/4 in. osb plywood and then add my wood T&G planks over that. Is this a good idea?
Q3: If adding insulation is a good idea but I choose to keep the drywall ceiling in place, would blown-in insulation be a good option? My thought is that I could cut holes into the ceiling between each rafter for access and patch them after. My contractor buddy didn’t think that would be a good idea.
Q4: If I kept the drywall ceiling, would adding the polyiso between the drywall and plank ceiling as I mentioned above be a good idea whether I added insulation to the space above or not?
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Replies
Jon,
Here is a link to an article that describes the best ways to insulate a flat roof: "Insulating Low-Slope Residential Roofs."
If you decide to install rigid foam (poyiso) between the existing ceiling and a new ceiling, you don't want to cut the polyiso into narrow rectangles and install the polyiso between the battens (strapping). Instead, you want to install a contiunous layer of polyiso on the interior side of the existing ceiling, followed by strapping (1x3s or 1x4s are better than 1x2s) and your new ceiling.
Blown-in insulation could work really well with no issues if you were planning to re-roof, and add an inch or more of polyiso above the roof deck, under the new roofing. The IRC calls out R5 minimum above the roof deck for unvented assemblies, with the insulation snugged up to the under side of the roof deck. Otherwise there needs to be a vent space between the fluff & roof deck per the IRC, but venting doesn't always work well with low slope roofs either.
But if you're going to cheat that, in US climate zones 1A and 2A (this covers all of FL) it's still fairly safe to blow it full of cellulose (instead of fiberglass or rock wool.) Cellulose will wick and safely store any condensation moisture, protecting the structural wood. The down side to cellulose-only solution (no topside foam) in a flat roof is that roof leaks won't be detected until the cellulose is saturated, and repairing it will involve replacing some cellulose and ceiling.
In climate zone 1A (south Florida only) you could fill it with fiberglass and it would still be pretty safe even without exterior foam. See Table 3 on page 13 of this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
See the first two columns, and first two rows. Note that in the zone 2A /Houston row even under a dark asphalt roof the roof deck has at least some modest moisture risk with fiberglass, but not with cellulose, but in zone 1A /Miami it's safe with fiberglass even under light colored "cool roof" metal roofing.
Adding foil faced polyiso under the rafters of an unvented assembly creates a moisture trap- don't do it. The roofing is vapor impermeable, so are the foil facers. Installing UNfaced 3/4" EPS (the white bead-board stuff) it will have a vapor permeance comparable to latex paint on sheet rock, which would be fine. But if it has a plastic or foil facer it's risky.
Cutting a 3" hole in the ceiling gypsum with a hole saw every 6-8 feet per rafter bay works fine for blowing insulation. It's often do-able with a single hole in the middle of the stud bay with long bays, or a single hole at one end for bays shorter than 10'. When filling a bay with multiple holes, plug the other holes with rags to keep from filling the room with insulation. With the blower on "air only", insert the ~2-1/2" blower hose to the far end of the bay. The air flow makes it somewhat stiffer, and helps direct the hose around any obstacles rather than hanging up and turning around. When you're pretty sure it's at the end, pull it back out a foot or so then turn on the mixer too. With a single stage blower, fill until you hear the blower begin to stall, the pull back a foot or so (it may take a bit of in/out to get the flow going if the end of the hose was plugged during the stall), repeat, until the end of the hose is within a foot of the hole (put a piece of tape on the hose a foot from the end before you start. Then turn the hose around to the other end, or move to the next hole.
Use a cut off sleeve from an old sweatshirt as a field gasket around the hose to keep the blow-back at the the hole at reasonable levels.
With a 1 stage box store rental blower using only the fat hose (not a narrowed down "dense packing" hose) you should be able to hit north of 2.5lbs per cubic foot density, which is fine.
At that density it's fairly air-retardent and won't settle from the modest humidity cycling creepage in your climate. ( In cold climates you'd need/want at least 3lbs, 3.5-4 lbs is even better).
I'm not sure what densities you would hit with fiberglass with that method- it'll be more than 1lb, probably not more than 1.5lb. (just a WAG.)
Thank you for the quick reply and the willingness to help. I'm very appreciative.
I read through all of your suggestions and went back to take a log hard look and decide on a course of action. I don't know what I was thinking other than I was doing it from memory but the space IS actually vented to the outside through soffits. In my mind I pictured it to be a sqaure box with a flat roof but it does in fact have an overhang as does the rest of the house. From the looks of it, it is venting into the attic space of the main house.
With that said, does it change any of the recommendations. I was leaning towards doing both the blow-in and covering over the ceiling with the whole sheets of the unfaced 3/4 EPS strapped with 1x3s as recommended. But now that I am seeing the ventilation I am thinking that I need some space above the insulation for airflow.
Baffles and fiberglass perhaps? If I went this route I would rip down the current ceiling and replace with new drywall making sure that it is as sealed and airtight as possible.
Question is do I need to even go through that expense or should I go with another solution? I'm in Orlando specifically.
Jon,
Flat roofs are tough to vent. Again, I'd like to refer you to a relevant article: "Insulating Low-Slope Residential Roofs."
If you don't want to install a layer of rigid foam on the exterior side of your roof sheathing -- always the best solution, although rarely done in a residential situation because of the need to install new roofing, which is expensive -- you should leave the rafter bays empty, and install a continuous layer of rigid foam -- as thick as you are willing to purchase -- on the underside of the rafters. Install this rigid foam in an airtight manner, and then install your ceiling.
Alternatively, you can install closed-cell spray polyurethane foam on the underside of the roof sheathing.
For reference, Orlando is smack in the middle of US zone 2A.
Venting at the soffits-0nly isn't good enough to be protective. For convective purging of air there needs to be a path to the far end of the cavity to a space that vents out at a higher elevation. Be sure that there is sufficient ventilation connection between the flat roof & main attic first, and sufficient vertical distance between the top-most vent on the main attic and soffit inlets on the flat roof for at least a 3: 12 pitch (4:12 would be better), or it won't necessarily draft very well. Ideally the main attic would be soffit to ridge vented, not just couple of gable vents, since a ridge vent offers maximal vertical gain.
If the roof deck is reasonably vented it's fine to use a full layer of foil faced polyiso on the underside, and fiberglass batts in the cavities. High density R21s are 5.5" nominal thickness, which would give you better than an inch of vent space. R23 rock wool would be a somewhat better choice if available. (Both the blue & orange box stores in my area are starting to stock rock wool batts. YMMV.) The air retardency of HD fiberglass or rock wool means they don't suffer much performance loss to wind washing. Cheap low density R19s have too much loft, but could still work with baffles, but only perform at R18 when compressed to 5.5". Similarly R22 fiberglass only performs at R19 when compressed to 5.5".
If the ventilation path is blocked to the main attic or the total run to rise of soffit to exit vent is less than 3:12 an unvented approach is more reliable. A full fill of cellulose is still a pretty good option, but runs against the IRC code prescriptives, which call out R5 minimum of air-impermeable insulation (foam) tight to the roof deck, with the fiber insulation snugged up tight to the foam. With a 36' x 12 space and 16" o.c. rafters you're looking at less than 400 square feet of roof deck to worry about. A 600 board-foot DIY closed cell foam kit runs about $600, and would be able to deliver better than an inch of coverage (at ~R6/inch of depth), which is enough to protect the roof deck. (If building construction is slow rather than booming you might be able to find a foam contractor willing to do it for about the same money as a DIY, despite the small size of the job. They would probably want to sell you on 2" though, since that's the maximum lift they can do in one pass for HFC blown foam.) At that point a full cavity fill of any fiber, any density is just fine, as long as it's a full fill. Any sheet foam behind the finish sheathing would have to be unfaced (to provide a drying path) with that approach.