Sealed Attics in FL

[teddy bear]

A friend of mine in Southeast FL retrofitted his attic with closed vents and foam insulation on the roof deck. Data enclosed. Before sealing the attic was near 100%RH everynight with dripping ducts and high a/c cost. Unfortunately no data on before. Next step is dehumidification? TB

http://www.imagewiz.net/usr/hvac1/tb/22455_sealed_attic.jpg

[wyounger]

Once you bring the attic inside the envelope it is fairly typical to give it a little bit of conditioning, too, especially if the equipment is in the attic space. I've seen crawl spaces treated the same way. Did they remove the ceiling insulation when they foamed the roof deck?

This doesn't look bad for unconditioned space in August. I'd try letting it have 50-100 cfm from the existing system before bringing in a dehumidifier. Make sure there's a return path for whatever air you supply up there, of course.

[kevinmac]

Living down here in SE Fla, I have never heard of a sealed attic. In fact everything I was told is you want a well vented attic, with good airflow. Good eve vents with adequate ridge vents to keep the humity down, and to preserve the roof!

[cem-bsee]

did the vapor barrier get "removed /sliced" on the floor of the attic?

maybe just circulating some conditioned air for a short time thru the scuttle into the attic would remove more moisture -- say for a week --

BTW, the data looks better than in my house here in Hunsville AL -- I rock along between 60- 70%.

[pstu]

>>Living down here in SE Fla, I have never heard of a sealed attic. In fact everything I was told is you want a well vented attic, with good airflow.

That is the conventional wisdom, but if you search for "Lstiburek", he has written some persuasive arguments that the conventional wisdom is not the best. Plus, he has been able to persuade builders to go along with his experiments, and the results have mostly been positive.

>>Good eve vents with adequate ridge vents to keep the humity down, and to preserve the roof!

Do you believe that ventilation keeps humidity down? How is that? Seems to me that having the building envelope at the roofline, with insulation and vapor retarder at that point, would very probably reduce humidity.

Regards -- P.Student

[Carnak]

An attic dewpoint fluctuating between 65 and 70F is pretty good with respect to ambient dewpoints upwards to 80F. I like the looks of the unvented attic with icynene.

[wyounger]

Originally posted by kevinmac
Living down here in SE Fla, I have never heard of a sealed attic. In fact everything I was told is you want a well vented attic, with good airflow. Good eve vents with adequate ridge vents to keep the humity down, and to preserve the roof!

That's the idea for an attic that's outside the building envelope. You insulate the ceilings, and ventilate the attic to keep it from getting insanely hot. The heat is what kills the roof if you don't ventilate it adequately. You want the attic to track as closely to outdoor temperature as possible, so it's like an outdoor room. Think of it as a screened-in porch. And we all know how humid outdoor air is in Florida!

If you insulate the roof deck instead of the ceiling, though, and don't ventilate the attic, it becomes just another room inside the house. Since you foam it and don't ventilate it, there's not a source of moisture anymore. There's almost zero infiltration, and it's not like you're cooking or showering in the attic. The roof doesn't get particularly hot, because there's not a hot attic on the other side of it (I think the Florida Solar Energy Center found that the roof surface on this type of construction runs about 5 degrees hotter than with a ventilated attic? Not a big deal).

The attic at that point wouldn't be any more humid than a lot of other parts of the house, such as a closet that didn't have any supply ductwork in it. Then if you give it a little tie-in to the ventilation system, the humidity (if not the temperature) should stay in line with all the rest of the house. It lets you do attic ductwork (and HVAC equipment, too) without the energy penalty of having that stuff in unconditioned space... and it makes the attic a much cleaner and more pleasant place to work (on the equipment) and for storage. With no ceiling insulation, you can put plywood right on the ceiling joists and not worry about compressing the insulation... so you put in all the plywood you want. People aren't so afraid to go up there once you keep the temperature moderate, give them somewhere to walk, and get the fiberglass out of the equation. It becomes more like a storage room than a traditional attic.

This is one area where the conventional wisdom doesn't work nearly as well as the new way, but the building industry is very conservative and notoriously slow to change. You can see how good the results are in TB's plot of the sealed attic data- and as I understand it that one didn't even have any connection to the HVAC system. Already they've solved duct sweating and cut down on cooling cost. Of course, sweating ducts in attic is a guaranteed equation for mold and ceiling stains, so fixing that is already huge. A traditional attic in Florida (or anywhere but a desert, for that matter) doesn't have a chance of keeping RH below 65% at all times of day. I bet with 50 CFM from the AC system the humidity will come down to a 50% max level. Temperature will still oscillate a lot, but the point is to keep it fully dry, not so much to cool it as if it's living space.

I visited the Southern Living ideas house in Atlanta last weekend, and it had its attic foamed in like this. I went in there at noon while it was full sun and about 85 outside. I'd say it was about 80 degrees in the attic (maybe 5 degrees warmer than the floor below), and <40% RH. I could have brought a chair in there and read a book, it was so comfortable. Not too much headroom, I'll grant, but otherwise not much different from anywhere else in the house. I already liked the idea, but going in one on a hot late-summer/early-fall day left me hugely impressed. Plus you can put any kind of equipment you want up there, like condensing furnaces and humidifiers, in a freezing climate without any special precautions, because it's not going to get super cold, either.

[nascar389]

Where can I find more information on sealing the attic? I live in Indiana..

[kevinmac]

perpetual student,
No, but proper attic ventilation will help reduce condensation. BTW if duct work is condensing, then it isn't insulted well!

[dan sw fl]

Originally posted by nascar389
Where can I find more information on sealing the attic? I live in Indiana..

http://www.icynene.com
http://www.airtightinsulation.com

Among others ...
http://www.buildingscience.com/

[dan sw fl]

Originally posted by teddy bear
A friend of mine in Southeast FL retrofitted his attic with closed vents and foam insulation on the roof deck. Data enclosed. Before sealing the attic was near 100%RH everynight with dripping ducts and high a/c cost.

Unfortunately no data on before.
Next step is dehumidification?



Teddy Bear,

Thanks for Sharing that data !

Dan