Attic temperature/ventilation - smart ideas needed

[second opinion]

well got the cellulose...still a bit too warm. Even with a gable fan running all night it only got down to 6 degrees warmer than the outside temp when I got up at 6am today. I guess the R6 (or 8 I forget) flex duct and wrapped plenum and the Trane TWE just radiate too much heat up in my attic. How do most other new homes get by since many have similar complicated roofs with ducts up in the attic.

I would be cautious running the gable vent fan while checking temperature rise in the atic.
If you have any air leakage in the wall cavities, chases, drilled holes from plumbing vents, wiring, air handler, register boots, etc., you may be creating a negative pressure in the atic and drawing the heated air from the conditioned space and giving you the warmer atic.

There was another mention of humid atic that would point to the negative pressure.

In the summer time if all is not sealed you will loose cooling also.

[deejoe]

Newhomeboston; It would seem as if your ceiling is lacking a "vapor " barrier on the warm side(just under the insulation. Also, inadequate insulation R value can be a cause of this ice damming too. Also the soffit and the high (near the peak) ridge vents need to be sized to allow enough air flow thru-out the attic.

For every 300 sq ft of attic floor area there should be 1 sq ft of 'free' area venting, divided 50/50 between the soffit and the ridge vents.
As long as you have adequate insulation within an attic it can NEVER have too much ventilation. and even with roof snow coverage an attic will still breathe.
note; a properly vented attic, using just soffit and near the peak (continuous ridge vent is best) vents, needs no gable or other fan type vents.

[stvc]

I think I would leave that fan off and see how the house performs now that is better insulated. We have had alot of snow, rain, cold and warm patterns that also can cause ice build up, even on my shed roof.
I'm also not so sure it is completely unusual that it is slightly warmer in your attic than outside, I might experiment with that a little.

[stvc]

I was curious so I put a thermometer in my shed that has a vented roof and is mostly shaded. The outdoor temp was 31 and it was 44 in the shed.
So like I was thinking before you might be too worried and what you are seeing is quite normal.

[deejoe]

A PROPERLY vented (via soffit/ridge) and well INSULATED attic should be at least within 20% -30% of that outside temp.
in other words if it's 30 f outside, ideally it should be approx 36-39F within the attic. The closer to the outside temp the better.

[Colt Hero]

What if your attic is finished. My sister lives in New England and is considering finishing off a very large attic space atop a colonial-style house. Presumably the house has soffits and a ridge vent. What do you have to do in this case to prevent the ice dams? You still have to maintain close to outside air temp in the winter on the backside of the roof decking, right? How is this done?

[cjcargo]

What if your attic is finished. My sister lives in New England and is considering finishing off a very large attic space atop a colonial-style house. Presumably the house has soffits and a ridge vent. What do you have to do in this case to prevent the ice dams? You still have to maintain close to outside air temp in the winter on the backside of the roof decking, right? How is this done?

I forget what they are called, but panels are placed against the roof deck. Insulation is placed inside the roof joist, then dry wall as normal. The panels allow for ventilation between the insulation and roof deck with out blocking air flow to a from all the vents. This is the same practice when doing a vaulted ceilings in homes.

[SJProwler]

I'm suddenly having issues with Ice Dams since replacement of my roof. Attic is insulated to R-38. Full Soffit ventilation and previously had a Hard Plastic Ridgevent. When we replaced the roof the roofing supplier recommended Cobra Vent in place of the rigid plastic. Now I have Icicles and Ice Dams. You guys think the cobra isn't allowing as much air to flow as the rigid plastic? Previous 9 years of winters never had a single icicle. Now they're everywhere.

I'm not looking forward to climbing up there in middle of the winter to peel off the ridge cap/cobra vent and replace it.

[dash]

No ice damn issues,but we have high humidity issues with Cobra vents several times causing mold/mildew on the ceilings.

[newhomeboston]

I was curious so I put a thermometer in my shed that has a vented roof and is mostly shaded. The outdoor temp was 31 and it was 44 in the shed.
So like I was thinking before you might be too worried and what you are seeing is quite normal.

Sorry for taking a while to get back to you guys...no big snow till now and I was out of town too. We got almost a foot of snow in the Boston area and without running the fan last night the attic was 41 degrees this morning after it had reached 25 outside as the low for a while. If I recall, the last big snow before the fan/cellulose it reached 45 but I had run the fan a bit yesterday on its timer.

Sounds like the insulation may or forced ventilation may have helped some but not dramatically as we were getting big ice dams with the attic at 45 during the last heavy snow.

I've got a guy coming in two weeks to do a blower door test and also use a thermographic camera to see if we have any cold spots in the walls/ceiling.

But from what this other guy was saying about Cobra mesh vent which is essentially what I have, it does sound like I need a better ridge like Shingle Vent II that some people swear by. Still not sure how the attic breaths when the ridge is covered with snow though.

Test results in two weeks. I assume enough attics have lesser insulated ductwork in the attic up North and don't have problems so this has to be fixable.

[teddy bear]

To stop the moist air from moving from the house into the attic when ventilating the attic, reverse the attic fan to blow outside air into the attic. This reduce moisture and air temperature. Regards TB

[Shophound]

Newhomeboston,

Do the upstairs portions of your home have recessed "can" lights? If so, are they sealed? Can lights are notorious leakers if not sealed/rated IC for insulation contact. Unsealed, they can be a large contributer to setting up a stack effect in the home.

Your indoor humidity levels...what do they normally run? Do you use an indoor humidifier? If you've been leaving it off, lately, what do your indoor humidity levels typically run?

[newhomeboston]

Newhomeboston,

Do the upstairs portions of your home have recessed "can" lights? If so, are they sealed? Can lights are notorious leakers if not sealed/rated IC for insulation contact. Unsealed, they can be a large contributer to setting up a stack effect in the home.

Your indoor humidity levels...what do they normally run? Do you use an indoor humidifier? If you've been leaving it off, lately, what do your indoor humidity levels typically run?

No cans, etc...ceiling upstairs is sealed with blown in foam. BUT Icynene now says that it needs a vapor retarder and my builder just primed the ceilings (cheapskate and I didn't notice the difference). So RH migrates through the ceiling into the attic where it generally should be dispersed with good ventilation but ours isn't that good. So I think I need to paint the ceiling with some latex paint which I guess has been shown to be an effective vapor retarder, not a barrier, but slows the migration down.

When the humdifier is on in the house we keep it 35% in the winter about as it adjusts based on outside temp but without it the RH is 25-30%.

[tomd7735]

I have heard of a special paint that acts as a vapor barrier in this situation, but I'm not sure of the composition. It could be sort of a rubber/latex type. But regular primer won't do it, not even KILZ.

[newhomeboston]

well the results are in and the energy audit showed there was plenty of insulation in the attic but there was some leakage in the ductwork noticed during the blower door test....

I guess the poor attic ventilation combined with slight air leakage and the R8 insulated ductwork still radiating a lot of heat just is too much to overcome. I had the duct system installed very carefully with a solid trunk line and flex runs to the rooms so I'm not sure where the leakage could be to try and fix that. And the R8 insulation was the best the contractor said was possible. So I'm not sure I can really affect those two issues much.

I guess I can try and improve ventilation which may help or the other suggestion the auditor had was to use foam and create a closed attic by insulating the underside of the roof. Does anyone have any preference?

And if I go for the closed attic, is there a problem that I might cause somehow? I still have the attic floor insulated with foam already...will insulating the roof underside create a situation where the attic doesn't have any air movement, or excess humidity/condensation somehow?

THanks.

[cem-bsee]

visit BUILDINGSCIENCE.com

[valerioac]

insatall an attic fan to work on attic temp common problen in florida attic temp 140 degrees,,,like putting duct work over a campfire,,,heat is penetrating the duct even if it is r-6.
i fix many customer problems with attic fans use the right cfm for square footage.

[beachtech]

seal it up and install a dehumidifier thats what we are doing here, but our humidity is in the summer months, so thick you can cut it with a knife

[bonafide]

insatall an attic fan to work on attic temp common problen in florida attic temp 140 degrees,,,like putting duct work over a campfire,,,heat is penetrating the duct even if it is r-6.
i fix many customer problems with attic fans use the right cfm for square footage.

And it's official, The can has been opened!

[bonafide]

And it's official, The can has been opened!

See for yourself

http://www.hvac-talk.com/vbb/showthread.php?t=133420