Convert Florida Attic from vented to unvented
I have a house in Florida (ASHRAE 1/99 temp range, 46-92 F) that previously had a ventilated attic. I had plans to turn it into a sealed attic with spray foam and when replacing the roof and siding I choose to get rid of the soffit vents and ridge vents and the attic is now unvented but I am not sure I still want spray foam. I am leaning towards using rockwool or bibs against the roof deck but have concerns about air sealing the roof deck itself.
Reasons I wanted a sealed attic.
1. Better HVAC performance since ductwork is in the attic
2. Better hurricane performance from wind driven rain in the ridge vents and soffit collapse causing roof failure. Soffits are solid plywood with unvented vinyl over them.
Concerns I have with using spray foam
1. This is my main concern as it saves $1000$ a year on insurance. Hurricane clips and roof sheathing nailing schedule inspections that must be done each time insurance is renewed(roof deck and rafter ends access is required)
2. I’ve heard remodeling after spray foam is installed is a pain. Not sure it matters in an attic?
3. Cost of closed cell or open cell foam is expensive.
Concerns I have with air permeable insulation (rockwool, bibs, open cell spray foam)
1. I don’t have a vapor diffusion port and I’m not sure if it’s even needed in my climate as our 1% temperature is 46 degrees and code requires insulation or vapor diffusion barriers if the roof deck is below 45 degrees.
2. Air sealing of the attic. I have air sealing from the soffit to the roof deck with rigid insulation. I don’t know how to air seal the roof deck, is the synthetic underlayment (not peel and stick, not taped at seems) under asphalt shingles sufficient to air seal? Or do I need spray foam here?
3. I didn’t know about vapor diffusion ports and insulation installers and builders in the area didn’t mention it when I asked about ridge rot. They said they didn’t experience ridge rot. I do not currently have a vapor diffusion port.
This is a 4/12 roof made with trusses and dimension lumber sheathing. Walls are air sealed with zip and seams taped as well taped to the soffits. Bottom of zip is liquid flashed to the concrete block wall below. Ask windows and doors have been replaced and caulked.
TLDR; I’d like to air seal and insulate my attic to make it unventilated with my ductwork inside. I desire to leave the end of my rafters and rafters exposed for inspections and spray foam doesn’t allow for this. I’m ok with peeling back batt insulation for inspections.
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Replies
sdpo7d,
If you want to convert to an un-vented attic I would either use one of these assemblies
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Or for projects in climate zones 1,2 and 3, much easier would be to use a vapour-diffusion port. It may not be required, but using permeable insulation without a way to exhaust moisture is asking for trouble.
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
I'm in zone 2. That second project is what I am thinking of doing to avoid spray foam. My concern with that method is, what do I need.to do to the roof deck to make it air tight? Do I just assume the synthetic underlayment is acceptable. I think I can have a roofer retrofit the vapor diffusion port.
sdpo7d,
This is a good opportunity to clarify how these attics should be built - something I don't know much about as I live too far north to use vapour-diffusion ports. Hopefully others will contribute to the discussion.
Unlike most vented and un-vented roof assemblies, the ones shown in Joe L's article don't appear to have an air-barrier under the insulation to stop moist attic air from reaching the sheathing. I'd be interested in an explanation as to why that isn't necessary, and how they are detailed.
Bump.
"2. Air sealing of the attic. I have air sealing from the soffit to the roof deck with rigid insulation."
Please explain this further - do you have exterior rigid insulation on the outside of the sheathing, underneath the underlayment and shingles?
Using permeable insulation, like Malcolm mentioned, will allow interior moisture to migrate through it and possibly condense on the underside of the roof decking. The fact that you have an impermeable underlayment on TOP of the roof decking does not prevent rot on the underside since the humidity is coming from the living areas of the house, not the outdoor air in this case.
Since it would be expensive to add a vapor diffusion port, you can do something really simple like install a plug-in dehumidifier and a floor drain somewhere in your attic space to keep RH down and prevent condensation. Maybe install a humidity sensor near the roofline and monitor it if you want to be extra safe. The one plus of having the permeable insulation is that you should be able to identify moisture problems more quickly than if you have something like open-cell spray foam.
matthew,
I'm not at all familiar with Florida codes. Is there a code compliant un-vented roof with interior permeable insulation? I thought you needed a vapour diffusion port?
That's why I asked if he has exterior insulation or not. 2023 Florida Building Code, Chapter 8 Section 806.5 (https://codes.iccsafe.org/content/FLRC2023P1/chapter-8-roof-ceiling-construction):
"In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control."
but then it also gives an alternative:
"Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months."
So given his 1% design temp is already above 45 degF I see no reason why he can't use air-permeable insulation according to the code.
Insulation
I do not have exterior insulation on the roof deck. I understand there is that exception in the building code which makes me feel ok about not having a vapor port but it also adds additional cost if I need one.
Air sealing
I have walls below my roof built with zip sheathing that's been air sealed and insulated with rockwool inside. At the transition from my zip sheathing to the plywood on the underside of my soffit, it is taped with zip tape to make that air barrier continuous. From inside the attic, there is rigid foam between the soffit (that's sealed to the walls) and roof deck. I plan to seal the edges of this rigid foam to make it air tight. My roof deck has no air sealing. Is that needed anywhere other than around penetrations? I have dimensional lumber sheathing there so, large gaps covered between boards covered with synthetic underlayment.