Condensation in Attic

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Qvist

Burning Hunk
Mar 5, 2019
194
Harpers Ferry WV
I am looking for some advice on attic condensation issues. I have a 50s house with a metal roof with no sheathing underneath it. The attic is vented in the soffits and gable ends and poorly insulated with the equivalent of R19 bats with no vapor barrier. I am losing a lot of heat in the attic I can see it rising off the roof! There is also an air handler in the attic and ducts. I have air sealed the light fixtures and fans but have not yet sealed the top plates of the walls. I am looking into having 1 inch of spray foam put down on the attic floor to vapor and air seal and then have r38 fiberglass blown on top. However it's quite expensive and I'm also not sure if it would solve the issue because I will continue to have warm air lost from the air handler and ducts. Therefore I am looking into having bubble foil insulation placed on the back of the rafters to form a vapor barrier so the air can't hit the metal and then insulating the attic with blown in only. I cannot spray foam the roof deck because of the extra weight on roof system and because of yet more expense. Does anyone know of any other solutions? I know I need air and vapor sealing and insulation but I don't want to go the route of expensive spray foam and still have the issue because of the air handler. Or alternately insulating with only blown in fiberglass and that not fix the issue therefore nesseitating removal of the new insulation. Feels like a wild goose chase. Sorry for the long explanation. Just looking for opinions or alternate ideas.
 

peakbagger

Minister of Fire
Jul 11, 2008
7,677
Northern NH
My guess is the best money should be spent sealing the air handler and ducts. Ideally get someone with duct blaster to test the system for leakage. They would temporarily seal off all the registers and then any air flow going to the system would be leakage. Duct work is infamous for leaking. Unless every joint it sealed it can add up and frequently transitions from the ducts to the outlet registers are leaky. Same thing with the air handler. Poorly fitting filters and access panels can leak. A duct blaster is not absolutely required, a blower fan and a low differential manometer can be used. The goal is get the air leakage down.

Have you had a blower door test of the house? That can identify the best bang for the buck air leak fixes.

With respect to sealing the gap above the top plates and the roof, do you have "proper" vents to assure air flow up through the soffits and against the roof? I have to ask as some folks stick insulation in that gap. Also how big are your soffit vents. Ideally they should be continuous the length of the roof. If you have the drill in type round ones, there needs to be lot of them, far more than a typical installation. Ideally multiple the length of the soffit in inches , mutiply by 2" and then divide the result by the open area of the drill in type vents. That will give you how many vents you need to drill in. My guess is vent every other roof rafter bay.

Same question on the attic vent at the ridge pole. The vent area at the top of the roof should be equal to the amount of open soffit area. A couple of whirlybird vents will not work. Ideally you need a continuous ridge vent or some very large gable end vents. It goes back to basic math, you need to calculate the free venting area of the roof and the soffits and make sure they are equal.

The other really basic question is where are the bathroom and kitchen vents vented?. Many builders route the vents to the soffits which effectively is pumping moisture in the attic. The vents need to be routed up through the roof ideally up near the ridge pole.

Even with real great venting, there can be an occasional issue on cold clear nights where the radiant cooling of the roof subcools the roof and if the outdoor air temp is near the dewpoint water can condense under the roof deck especially if it is a dark color. This is far worse with poor attic venting.

Last but not least is give the ceilings a coat of latex paint that can reduce vapor transmission into the attic.
 
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Qvist

Burning Hunk
Mar 5, 2019
194
Harpers Ferry WV
I haven't had any tests done. Seems like a good idea though. The venting is insufficient. I have the drill in vents 2 per rafter but they are only on half the house because the other half is a porch roof. The gable vents are also undersized. I have installed a power vent to force the issue and it helps if left on at all times. I have the bath fan going out the roof. The kitchen has no vent.
 

zrock

Minister of Fire
Dec 2, 2017
1,276
bc
Get some tin foil tape and seal up all the joints of your air handler. The one we had installed all the runs came pre insulated. If you spray foam the floor/ceiling properly that will stop 100% of the heat loss between your main floor and attic. it will not be 1 inch of foam it will fill up your rafters. Also you do not want to insulate your roof deck anyway as the attic in reality is suppose to be a cold zone unless your living up their. I would also have them hit the air handler piping if it is not insulated..
 
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peakbagger

Minister of Fire
Jul 11, 2008
7,677
Northern NH
Many energy programs use a thin coat of spray foam above the ceiling (which means removing what ever insulation is there). That acts to air seal and then put less expensive insulation like fiberglass or cellulose above it.
 

EbS-P

Minister of Fire
Jan 19, 2019
3,508
SE North Carolina
When we get weather swings warm and wet to cold I imagine that there will always be some condensation. Any time there is dew on your car and if you are properly vented, the outside air that is condescending on you car could be condensing on the outside or inside of the the roof. Proper ventilation should get rid of quickly once the sun comes out. Just my thoughts. I see attic insulation as separate from the condensation. Adding more won’t change the humidity in the attic or make the roof warmer.
 

stoveliker

Minister of Fire
Nov 17, 2019
6,162
Long Island NY
"the outside air that is condescending on you car"

air should only be condescending on gas-powered cars, not electric ones ;)
 

Grizzerbear

Minister of Fire
Feb 12, 2019
1,319
SW Missoura
I would insulate your ducts and pull the metal roofing and lay a bubble insulation/vapor barrier down between rafters and the roofing like you said. It's pretty common practice......around here anyways....to do this since metal will condensate with the right conditions like stated by someone else.I have a metal roof and I put a vapor barrier down before laying the metal though my ductwork is below the house. I would personally think about spray foaming the ducts. The foil faced batt insulation will still let the duct sweat and then you will have a bigger mess. Buy the do it yourself kit and the price punch for all this shouldn't be too bad. Just a lot of labor.
 

blades

Minister of Fire
Nov 23, 2008
3,784
WI, Leroy
in respect to fiberglass vs/ cellulose, fiberglass allows a lot air movement through itself, cellulose does not. fiberglass also loses r value as temps drop, again cellulose does not. fiberglass mfg been hiding this for years. do a bit of research on that. not hard to find.
 
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