Unvented cathedral ceiling assembly for Florida
Hi All,
I’ve read a lot on this site to learn as much as I can, but I still have some persistent questions about the particulars of my situation.
I’ve gutted a 325sqft room. It’s down to the concrete block walls, slab floor, and existing roof assembly(2×4 rafters, 1×4 decking, felt, metal roof).
After much internal debate, I’ve settled on using dense-pack cellulose to insulate the unvented cathedral ceiling, however I will only be able to achieve an R12 with the small rafters that I have AND I will still have thermal bridging with the 2x4s touching the plasterboard. I need to be somewhere between R22-38 according to specs.
So, I seem to have 2 options:
1. Furr out the 2x4s by ~3in to get me to 6.5″ which would get me to R22
2. Add 2″ of polyiso rigid foam panels to the 2×4 rafters between the cellulose and the drywall. Would get me to R24 and eliminate the thermal bridging.
From what I understand, because I live in Zone 2, I don’t need to worry about condensation in the cold months.
Does these options sound correct and viable? Which would you choose?
Thank you,
Daniel
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Replies
Daniel,
Most building codes do not allow cellulose to be installed between rafters unless you include a ventilation gap between the top of the cellulose and the underside of the roof sheathing. You need to install a ventilation baffle (thin polystyrene, thin plastic, cardboard, plywood, or some type of rigid foam) to maintain this gap.
The only exception is for roofs that have a continuous layer of rigid foam above the roof sheathing -- but your roof doesn't have that. (It's also possible to create an unvented roof assembly by installing spray polyurethane foam against the underside of the roof sheathing, followed by an air-permeable insulation like cellulose.)
More details on all of these options can be found in this article: How to Build an Insulated Cathedral Ceiling.
Dense packed cellulose in an unvented assembly is not very risky at all in US climate zone 2A. With tile roof or a high solar reflectance "cool roof" there is a slight risk, but it's a very modest risk. See the second column of Table 3 on p12 (pdf pagination) of this document:
http://www.buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems
And that's at R30 (the IRC 2009 code min for zone 2A: https://energycode.pnl.gov/EnergyCodeReqs/?state=Florida ) , which makes for an even colder / wetter roof deck than any of the solutions you are proposing.
To meet code prescriptions with exterior foam in zone 2A takes only R5 on the exterior, but it's probably not really needed with cellulose since the cellulose will share the moisture burden with the roof deck, though it might be with fiberglass which has very little hygric buffering capacity. In zone 2B under a tile roof the R5 it's not required by the IRC:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
But you can't use a low-permeance foam, or foam with foil facers on the interior side or you will have created a classic moisture trap.
To meet IRC 2012 code via a calculated ceiling U-factor in zone 2 you need to be under U0.030 (per Table N1102.1.3: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm ), which is a "whole-assembly-R" of R33. So furring out to only 3" isn't going to cut it. But if you installed a continous 3" layer of paper or fiberglass-faced roofing polyisocynaurate on the underside of the rafters you'd be close- close enough that it would be tough to measure the difference in energy use. If you have more than 3" to work with you can get all the way there with 4" of fiber-faced roofing iso.
If you are not going with the fiber-faced foam concept and are going fur it out, if have only 3" to work with run the furring perpendicular to the rafters 24" o.c. and install compressed R15 fiberglass or R15 rock wool in the furring bays, which would perform at about R13 center cavity. By running the furring perpendicular to the rafters you reduce the thermal bridging fairly dramatically, achieving higher thermal performance with the same materials. If the rafters are on 16" or 24" centers you might consider R15 batts for the rafters too, which would be higher R than dense packed cellulose, and probably cheaper when looking at such a small job.
For cheap insurance on roof deck moisture accumulation, an 800 square foot roll of MemBrain smart vapor retarder is only about $100, and putting it between the furring and your finish ceiling dramatically reduces the rate at which moisture gets into the assembly from interior moisture drives, without appreciably slowing down the drying rate. In the spring when the sun and warmer is cooking the moisture out of the roof deck the relative humidity of the air in the insulation layer rises dramatically, making the MemBrain very vapor open. It dries much faster than it takes on moisture. Smart vapor retarders aren't exactly meeting the precise letter of code, but they can provide quite a bit of resilience. If you look at the first column in Table 3 of that BA-1001 document you'll see that in zone 2A a non-buffering fiber insulation such as 1.8lb blown fiberglass (or rock wool) the moisture risk is higher- too high to rely on standard latex paint as the only interior vapor retarder.
The smart vapor retarder only helps in the all-fiber insulation case, not the rigid foam case. Fiber faced polyiso is still under 1-perm no matter what, but still WAY more vapor permeable than a foil facers- it's still a drying path, unlike foil faced goods.
Thank you Martin, I understand the codes, but frankly I'm not concerned with them and I don't intend to get my building department involved. I am concerned about performance and functionality. From what I understand, the code is in place to allow for condensation on the underside of the roof deck to have a channel to escape. I've also learned from various sources, that condensation is not a problem in this southern hot climate. So, I'm a bit at odds with the code here.
For various reasons, venting my roof is something I don't want to do. There are portions of the roof that simply can't be vented(hip valleys). I'm also looking for an option that doesn't involve spray foam for health and environmental reasons.
Would the 2 options I listed above not work long term, or do the concerns that drive the code apply to my situation(condensation on the underside of the roof deck)?
Daniel,
Dana has provided you an assessment. He states that your approach is "not very risky." I think he's right -- but it is a code violation.
I strongly urge you to do your best to increase the R-value of your roof assembly. The 2012 IRC calls for a minimum of R-38 in Climate Zone 2.
This was so helpful. I think I'll go with the perpendicular furring and the smart vapor membrane. Seems to be the most practical, least amount of risk, and lowest cost.
To both of you, Thank you for the support
With compressed R15s in 3" furring and uncompressed R15s in the rafters with the horizontal mode the center-cavity R will be about R28, but the performance will be better than the R30 center-cavity specified under IRC 2009.
I don't know which revision of the IRC Florida codes are based on, but even though that assembly still a bit shy of R38, but it's not terrible, and better than a code-min between joists solution under IRC 2009. From a U-factor point of view IRC 2009 required U-0.035, or a "whole assembly- R" of R28.5. With the reduction in thermal bridging gained running 24" o.c. furring perpendicular to the rafters and allowing R1 for the gypsum and sheathing, U0.035 is about where it lives. It may be a bit more depending on the exact framing geometries, but it's well under U0.040.
If you can't find smart vapor retarders in local distribution (it's usually a cold-climate sort of product), you can buy it online. The midwestern regional box-store Menards sells it online in limited sizes- there may be others. MemBrain is manufactured by Certainteed- you may be able to find it at distributors handling Certainteed insulation products who cater to the building trades, but I'd probably faint if I saw it in a big blue or orange box store in Florida. :-) There is also 475 Building Supply which sells Intello online, which is a more rugged and tighter smart vapor retarder, but it's also more expensive. Given the somewhat modest risk level MemBrain should be good enough. Just keeping the interior relative humidity under 50% in winter ( the maximum of the "healthy" range recommended by health professionals), and it will be fine. Winter air is drier than summer air anyway- it's usually going to be under 50% RH except in very tight and under-ventilated houses.
Great, thank you. Looks like I can get the smart vapor retarder from Home Depot with free shipping. I haven't done any price comparisons though. http://www.homedepot.com/p/CertainTeed-MemBrain-100-in-x-50-ft-Air-Barrier-with-Smart-Vapor-Retarder-902018/205920791
Any recommendation on the carpentry for the perpendicular furring? Are you suggesting putting the 2x4 furring on edge to make the center cavity 3.5" or on it's side to get 1.5"?
The carpentry for laying it flat is easy, but I'm wondering about it on edge. Should I use 5" screws or use some simpson strong ties like maybe this one?
http://www.strongtie.com/graphics/products/large/196a-2015.gif