Cathedral ceiling with fiberglass batts next to spray-foam unvented attic?
I’ve found some great info on this website, but I’ve got a peculiar insulation situation I’m trying to sort out, which I haven’t seen addressed here. I’ve got a cathedral ceiling (those again!) in the front of my existing house that may complicate a potential spray-foam sealing of my attic. The purpose of the spray foam job is to seal and condition the attic that contains a new a/c air handler, the ductwork, and all of our seasonal storage.
My location is hot and humid New Orleans, climate zone 2A. The main attic is an uncomplicated spray-foam job, I think (more on that below). The problematic cathedral ceiling has faced fiberglass batts between 6” rafters now, with drywall ceiling below and un-insulated roof decking above. We’ve lived here for a few years, and I don’t want to tear out the cathedral ceiling to spray that roof decking, which is otherwise inaccessible, so the cathedral ceiling has to stay as it is.
That cathedral ceiling has no exterior vents allowing outside air to enter. The only ceiling penetration is a ceiling fan in the center of the peak. The only place the gap above the cathedral ceiling might vent stray moisture is through a small tunnel along the peak — there’s a flat area along the interior ceiling peak that leaves a space above it between the drywall ceiling and roof decking (picture the top section of this capital letter A), so there’s a triangular tunnel about 15 feet long, 24” wide at the bottom, about 12” high, shaped like a giant Toblerone chocolate bar. That tunnel opens to the main attic, but you can’t really see in there to know if there’s any kind of air gap maintained between the roof decking and insulation. It’s possible that much of that tunnel is filled with batt insulation – you can’t see if the insulation follows the roof decking to the peak leaving an open tunnel, or if it flattens across that 24”-wide drywall strip to muffle airflow. But regardless, any moisture in the cathedral ceiling can probably migrate into the broiling-hot general attic space via that Toblerone-tunnel.
So if I spray-foam-seal the main attic, am I creating a problem in the existing cathedral ceiling by trapping moisture? That area would be sealed off from the main attic by the foam. So any existing passive venting of the Toblerone-tunnel at the peak – and I don’t know if there is any meaningful moisture to vent – would be sealed off. Does it matter that I’m in Zone 2A and most of the moisture is outside for most of the year?
An alternative would be to not-foam the open end of the Toblerone-tunnel and let that tunnel continue to vent moisture (if it does) into the newly spray-foamed attic space … but then the foam-sealed attic would have a hole in the envelope. Picture a triangular hole a bit smaller than your computer monitor at one gable end.
FWIW, the main attic spray-foam insulation job should be fairly simple. I have a gabled roof that runs the length of the house – about 1/3 is the cathedral ceiling described above, and the other 2/3 is attic space, currently unconditioned with hot ductwork and melting Christmas decorations. It’s got faced fiberglass batts at ceiling level (kraft paper side down to meet drywall ceilings), with most of the ceiling insulation covered by plywood attic-floor decking . The fiberglass batts are in good shape, and it’s all construction that’s only about 12 years old. Oh, would I need to remove some or all of the existing kraft-paper-faced fiberglass ceiling insulation, to allow free moisture flow in/out of the newly conditioned attic?
Many thanks for this great site!
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Replies
Your mini-attic should be either vented (ie, free air flow) to the exterior or the interior (depending on where you put insulation). Sealing it up could cause moisture problems.
Hi Nola,
It's a good idea to bring the attic with your HVAC equipment into the conditioned space of the home. Here's an article with a lot of information on how to do it right: http://www.greenbuildingadvisor.com/article/creating-a-conditioned-attic
Fiberglass insulation is not ideal for unvented, cathedral ceilings. I believe that your house would be a good candidate for a vapor diffusion port on the cathedral side of the roof. Here's some info on vapor diffusion ports: http://www.greenbuildingadvisor.com/article/vapor-diffusion-ports
Jon R and Brian,
Thanks for the responses. Between those options, I'd be inclined to just leave a hole in the foam at the gable peak in the attic, so the channel above the cathedral ceiling can vent moisture into the main attic, as it currently does. I imagine I'd lose some of the benefit of an air-sealed attic by leaving that gap in the foam, but it should still be much better than what I have now.
I'd be very interested in the vapor diffusion ports -- what a great idea! But I'm nervous about a roofer botching the job on a relatively new (and watertight) roof. It's a new process, and local contractors aren't much into innovation. I'll definitely consider a vapor diffusion port when it's time to re-roof in a few years; maybe it'll be more common then.
Would I need to remove the kraft-paper backed insulation on the attic floor to avoid a moisture problem when I spray-foam the roof decking? I want to be sure the moisture in the attic can get into the living area for assimilation by the a/c.
I am in the process of ripping out the T&G cathedral of my house. Not too fun. But the only thing worse than doing it, at least to me, is not doing it. What a mess I have found. No air sealing and mildew. Yuck.
If you condition the attic, you can't let the chocolate bar vent into it. Total waste of time to let that air in after spending that money to insulate.
In New Orleans putting a minimum of 3/4" of polyiso (R5) above the roof deck is sufficient dew point control for rafter bays full of fluff. That would meet the IRC prescriptives for protecting the roof deck If the fiberglass in the ceiling is only R11-R13 rather than R19 (or even if it is R19) it may be worth blowing it full of cellulose or fiberglass insulation drilling from above before putting down the R5 foam to increase the air retardency as well as the R-value. (R19 batts perform at only R18 in a 2x6 cavity, but filling it cellulose and compressing the low density batt would bump that to R20.)
If you don't want to re-roof right away (probably not, given the fairly new roof", in a Gulf Coast climate it's pretty safe to just pack the rafter bays full of cellulose. Which can also be done from the inside without needing to gut the ceiling, just patching a few holes per bay. If drilled with a hole saw and saving the discs the repair is fairly trivial.
For the relative safety of an unvented cellulose cathedralized ceiling in your climate see Table 3 of this document, in particular the "2A | Houston" row, and "cellulose" column :
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
Even with a highly solar reflective light metal roof the risk isn't extreme even with R30 cellulose in the cavities, and with the extra heating/drying of dark asphalt shingles the risk is essentially zero. These simulations were somewhat worse-casing it by using only north facing pitches, which get less sun. If the cathedral ceiling roof in question is facing any direction other than north the risk is even lower.
Also take a look at the kraft faced batt column in that table. It's also fine to use a kraft facer as a "smart" vapor retarder on the conditioned space side of the unvented assembly in your climate, though a more air-tight membrane type variable permeance type vapor retarder might be even better.
Thanks very much to everyone for the responses, which I’ve been digesting for a week. Following up, assume that I’m at least a decade away from re-roofing (to add exterior insulation board) and that my wife will kill me if I cut any holes in the bedroom cathedral ceiling to blow in more insulation:
1) Would it really be a waste of the money of the spray-foam project (as one commenter says) to leave the existing breathing hole at the peak of the cathedral ceiling, to vent any stray moisture from the cathedral ceiling into the newly conditioned attic space? I know it would make the attic warmer, like leaving a small window open, but there should be minimal airflow through that gap. The gap is roughly the size of a soccer ball, and it’s the only existing vent from the cathedral ceiling.
2) Brian Pontolilo: Vapor diffusion ports will have to wait until I need to re-roof. Would it make sense to consider a version of a vapor diffusion port by using a highly vapor-permeable barrier at that gap at the peak of the cathedral ceiling, to let moisture form the peak continue to vent into the attic, but without the loss of air sealing that the foam otherwise provides?
3) Dana Dorsett: If the cellulose-packed unvented cathedral ceiling is not a huge moisture issue in Zone 2A, your referenced chart seems to indicate that fiberglass batts with kraft paper are even less of a moisture risk. But the text of the article says that an unvented cathedral ceiling with batts would only work in Zones 1 and 2B. I don’t follow the disconnect between the article’s statement and the Houston data in the table.
4) I see that the kraft paper is okay to keep as a smart vapor barrier on the attic floor. Would it still make sense to remove some of that insulation to improve the cooling of the attic? I’m wondering how well-conditioned the attic would be if there’s insulation between the living area and the attic. Ideally, I wouldn’t have to install an a/c vent and return air in the attic.
Many thanks!
>"1) Would it really be a waste of the money of the spray-foam project (as one commenter says) to leave the existing breathing hole at the peak of the cathedral ceiling, to vent any stray moisture from the cathedral ceiling into the newly conditioned attic space?
It's worse than a waste of money- it's a waste of PERFORMANCE to let humid outdoor air to enter the conditioned space, or to let dry conditioned air out.
>"Would it make sense to consider a version of a vapor diffusion port by using a highly vapor-permeable barrier at that gap at the peak of the cathedral ceiling, to let moisture form the peak continue to vent into the attic, but without the loss of air sealing that the foam otherwise provides?"
No, it wouldn't make sense unless the roofing was also highly vapor permeable.
>" If the cellulose-packed unvented cathedral ceiling is not a huge moisture issue in Zone 2A, your referenced chart seems to indicate that fiberglass batts with kraft paper are even less of a moisture risk. But the text of the article says that an unvented cathedral ceiling with batts would only work in Zones 1 and 2B. I don’t follow the disconnect between the article’s statement and the Houston data in the table."
You left out the conditional parts of the text which reads:
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8. Spray fiberglass and batt insulation systems in unvented roofs will only work in Zones 1 and
2B if wintertime humidities are kept low. Any airflow into or within these roofs, even with
kraft-facing, will risk condensation in Zones 2A, 3 and higher. These results are also backed
by years of forensic experience.
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The all important conditions are "...wintertime humidities are kept low...", and controlling "...airflow into or within these roofs...".
This means you can't be...
...taking daily 2 hour long showers with no bath fan running or boiling gumbo on the stove all winter without running the kitchen exhaust and....
...you have to take care to air seal the ceiling to keep indoor air from getting in during the winter, and air seal each rafter bay at both ends (and I'd even caulk each rafter to roof deck) and seal any seams in the roof deck.
Blown cellulose and 1.8lb blown fiberglass fills perfectly & completely, and is fairly air retardent, controlling the air flow within the roof, which makes blown insulation (particularly dense-packed) a better solution than batts.
>"Would it still make sense to remove some of that insulation to improve the cooling of the attic?"
Not really. You don't really care if the attic air temps peak 5-10F warmer than the conditioned space ceiling below. That's WAY better than attic temps peaking 35-40F warmer or more. The merely somewhat warmer attic isn't a very big parasitic load, and removing the floor insulation just moves that load directly to the conditioned space below, making the ceiling warmer. There is no net efficiency to be gained by removing it (to the contrary- it cuts efficiency!), and the impact on comfort is negative.
That last point is fascinating. I get the thermodynamics of it, that it's just a matter of where a finite amount of heat is located within the shell of the house. Is there any point at which the attic (with air handler and ducts) would be too hot with a sealed attic, given enough insulation between the living space and the attic?
Clarification: the proposed hole in the foam (or hole covered with a very permeable vapor barrier) would not be for humid outside air to get into the sealed attic, it would be to allow the humidity from indoor air that has accumulated under the peak of the cathedral ceiling to be dried out by communicating with more recently conditioned indoor air. Does that change anything?
As for the other answers ... bummer. It sounds like I'm not sealed-attic-eligible unless I'm willing to do a substantial re-working of my roof to add insulation and/or ventilation to the cathedral ceiling. Are there concerns that a ridge vent or vapor diffusion vents in the cathedral ceiling would cause problems by letting moisture in during the hot, humid majority of the year? I have no soffit ventilation and no other intentional air supply into the cathedral ceiling, just one ceiling fan that I can seal.