Attic needs more ventilation

[newhomeboston]

OK, here's another problem for all the experts out there. Another dilemma caused by my having a builder use materials he was new to. Our attic in Boston has reached as high as 130 degrees on mid 80's days. The humidity is low like 15% at that point and I haven't seen it above 70% on the wettest of days yet. It cools down nice at night. This can't be a good situation, esp for ice dams in the winter.

We insulated the attic floor of our house with icynene foam. Baffles were used to allow air flow to the soffits in this cape style house with a 3rd gable wing off the back. Ridge and soffit venting were used. Problem is I think the soffits may be blocked in many spots since a smoke pen's smoke isn't getting sucked into the attic at the soffits at several spots I tried. We have continuous ridge venting and the AC/heat ducting is is in the attic but insulated.

So if the soffits are blocked, how do I introduce more air into the attic without messing up the air flow even more causing a different problem?

1. Roof power fan
2. Gable vents
3. Vents low to the floor at the gable end
4. Gable fan
5. Other....

Thanks...

[jimbo48236]

I am not an HVAC contractor, but I am a Mechanical Engineer. I think all of what you said could work; my recommendation is a temperature controlled gable vent fan. Should only open up and run when the attic is above a set temp. This is what I am planning for my house.

By the way are the areas that don't vent small less than 4 feet or large more than 4 feet? Are you sure insulation isn't blocking the vent?

Also to reduce heat gain in the summer you can buy metal paint (yes metal paint) and spray it on the roof sheeting underside. This reduces heat gain from radiation. People with metal roofing will have cooler attics then people with asphalt shingles.

Heat transfer from your home can occur by conduction, convection and radiation. It is typically modeled in terms of conduction, although infiltration through walls and around windows can contribute a significant additional loss if they are not well sealed. Radiation loss can be minimized by using foil-backed insulation as a radiation barrier.

Hope this helped.

[546kerry]

If the soffit vents blocked un block them and then put in some kind of high vents. in the gable or on the roof.Or put in some big cable vents on each and every gable.
Another problem with no venting in the attic is the warm air will go into the attic and then frezz on the underside of your roff sheeting. when it gets a little warm it will melt and leak into your house also causing rot.

[newhomeboston]

Some of the baffled areas are maybe 10 feet long over the kneewall of the crawlspace where they blew the insulation up against the rafters. If I do some power venting of any kind, I'll need a manual override switch since winter temperatures are just as problematic for ice dams as high heat in the summer for condensation (which we haven't seen yet - knock on wood) or just shingle/plywood life deterioration.

I have not been able to check visually most of the soffits as the stupid insulation contractor blew in foam so much that walking in the attic is like a moon scape and I need to get some boards to move around up there so as not to fall through the ceiling. I have seen a few from the area I can stand and they are clear to the eave but I have my doubts on those over the kneewall where I did the smoke pen test and the smoke went up over the roof rather than into the soffit vent under the eave....not a good sign.

[dan sw fl]

Originally posted by newhomeboston

We insulated the attic floor of our house with icynene foam.

Why wasn't the foam insulation put on the Attic Ceiling! ?

[54regcab]

There is a HUGE difference in quality of ridge vents. Many designs are quite restrictive allowing minimal airflow. Personally I'm a big fan of the wind turbines, especially for Oklahoma

[newhomeboston]

Dan, the house was designed for a vented attic so when we switched to foam insulation I didn't want to try and design an unvented attic on the fly so we used it in place of batts in the attic floor.

Any idea how to figure out which ridge vent was installed easily?

[uktra]

Newhomeboston--I don't understand why you think you have a problem. What does the temperature and humidity levels in summer have to do with ice dams in the winter?

[Edited by uktra on 06-16-2005 at 11:27 PM]

[newhomeboston]

Uktra,

They don't directly, but 130 degrees is going to shorten the life of my roof, make the ac less efficient, and maybe cause condensation to develop in the summer.

If its heating up this much with solar gain in the summer, then it will do the same in the winter and the attic will be above 30 when its less than 30 outside so snow will melt and ice dams develop.

Two different problems, same cause - not enough ventilation - right?

[uktra]

Shingle life is dependant on sheathing temperature rather than air temperature. Even if you had been really smart and insulated under the sheathing to create a non-ventilated attic the difference in shingle life is only about 5-7 %. In terms of A/C effeciency, what you should be concerned about is the duct/air handler tightness and insulation level. While it probably is impracticle to reinsulate your ducts, they need to be as air tight as possible. High humididty in the attic is not a problem as long as there is no dew point. The ducts and air handler are again the problem areas. Ice dams in winter are usually caused by air leaks from the rooms below the attic that bypass insulation to melt the ice at the eves. Your foam should prevent that problem.

[dan sw fl]

Originally posted by uktra
High humididty in the attic is not a problem as long as there is no dew point.

Could you repeat that ?

[old yeller]

Originally posted by uktra
Shingle life is dependant on sheathing temperature rather than air temperature. Even if you had been really smart and insulated under the sheathing to create a non-ventilated attic the difference in shingle life is only about 5-7 %. In terms of A/C effeciency, what you should be concerned about is the duct/air handler tightness and insulation level. While it probably is impracticle to reinsulate your ducts, they need to be as air tight as possible. High humididty in the attic is not a problem as long as there is no dew point. The ducts and air handler are again the problem areas. Ice dams in winter are usually caused by air leaks from the rooms below the attic that bypass insulation to melt the ice at the eves. Your foam should prevent that problem.

I'm agreeing with what you are saying, but did you mean to say the melted water would FREEZE at the eves? The eve is the overhang, correct?

If his icynene was applied correctly in this fashion, which I agree was not the best way, ice dams shouldn't be too much of a problem. Air leaking from the HVAC is still an issue, though- I agree with that completely. If that were leaking anought it could melt some ice and cause a dam.

Just my opinion- your fine- don't do anything.

[newhomeboston]

The ducts are well sealed to my knowledge...mastic and wrapped with foil tape. Have a little leakage that needs to be address around the air handler/acquacoil unit still.

Old Yeller, still not sure why I wouldn't get the attic heating up in the winter from solar gain and melting snow that will freeze over the eaves and cause an ice dam. Its a low slope roof. I've been thinking roof heaters might alleviate this possibly though.

[old yeller]

Originally posted by newhomeboston
The ducts are well sealed to my knowledge...mastic and wrapped with foil tape. Have a little leakage that needs to be address around the air handler/acquacoil unit still.

Old Yeller, still not sure why I wouldn't get the attic heating up in the winter from solar gain and melting snow that will freeze over the eaves and cause an ice dam. Its a low slope roof. I've been thinking roof heaters might alleviate this possibly though.

I don't have real-world dam experience in Texas, so I am only speaking from knowledge of what causes them, not experience.

It would seem to me that if you had enough solar gain to go through the snow and warm your attic, it would surely warm the eaves, also. In theory, ice dams are caused by warm roof and cold roof eaves, with the roof being warmed by leaked air from the house AND solar gain. That is why good ventilation is needed- to keep the attic cold in winter. BUT- Your attic should stay cold enough because of the icynene. Your solar gain should warm the WHOLE roof, not just the non-eave part.

[uktra]

The reason the code wants vented attics is for ice dam prevention in winter. The code was not written for hot air removal in summer. When you ventilate in winter the idea is that by letting cold air in, one will prevent the thawing of the snow on top of the roof by having the same temperature on both sides of the roof plane. This , of course is most critcal at the roof/sidewall junction where the heat from inside the home is the closest to the roof plane. With the foam both insulating and air sealing well at this location, the freeze/thaw cycle should not be a problem.

[old yeller]

uktra- I can hear MLK now.. "We can all just get along!"

sorry newboston- "inside joke"

[newhomeboston]

Maybe I'm just thick, but why won't the solar gain warm up the attic air hotter on the inside causing the snow to melt even if its just below 30 outside where the melt will freeze down?

Also, isn't my ac going to work a lot harder than it should in the summer as I can envision 150 degree days once it hits 100 around here if it gets to 130 on sunny 80 degree days. That's OK???

Thanks guys.

[pstu]

I am in Houston and have never lived north of St. Louis, so take this with a grain of salt. It seems to me when your roof is covered with snow, there will be little radiation reaching your shingles and that will make for a minor difference in winter attic temperature. It seems to me that leakage of house air into the attic would be a far greater threat for creating an ice dam.

I do think your attic temperatures are way too high and some way should be found to reduce them. I am near Houston and days typically get to 95 degrees outside this week, and my attic temperature measures a little over 110. My attic is the ventilated type, I have added some soffit ventilation but still think I may want more ventilation at the ridge line.

Another thread tells us at least one or two people have benefitted from power ventilation attic fans. It seems to me one should approach this very cautiously but it may apply in your case. However the general conventional wisdom is these things suck up energy without delivering good results -- for most people. Any other method for moderating your attic temperature ought to be considered first, IMO.

Hope this helps -- P.Student

[uktra]

Heat is transfered in three ways--conductive, convective, and radiant transfer. Hot air has little radiant transfer, but transferes heat by conductive (heat flow from one material to another by contact) and by convective (hot air going through holes from hot to cold). Since you put foam in the attic as insulation, there will be little heat transfer from convection. Attic temps in properly vented attics don't rise much more than + 30 degrees higher than the outside temp. with shingles on the roof. Lower with metal and tiles roofs. The big transfer to the drywall ceilings in rooms below attic is from radiant tranfer. This comes from the sheathing radiating down through the insulation. Different types of insulation are better or worse at slowing radiant transfer. Loose fiberglass is not as good as cellulos. Foam is excellent. Foam also does not lose R value when the temperature drops, like fiberglass does. The roof sheathing temperature can vary more than the temperature of the air in the attic. For example, when the sheathing temps climb from 100 degrees in the morning to 180 degrees in the hot afernoon, the radiant heat transfer increases over 5 times (from 28btu/h-per sq.ft. to 147 btu/h-per sq. ft. This is the dominat transfer of heat to the rooms below. Hope this helps.

[Edited by uktra on 06-17-2005 at 10:32 PM]

[pstu]

Uktra, do you have any knowledge of the conditions which can raise roof sheathing temperatures as high as 180 degrees? Is this in the Arizona desert by any chance? While mine have not approached that temperature, other people live in rather more intense climates than me.

I emphatically agree with the things you have said about radiant barrier(RB). Every new house of any quality in S. Texas seems to have RB now. Such a product would basically nullify the problem you allude to, of heating from radiation via the plywood or OSB roof decking. While I don't have handy access to research with numbers, I can feel the difference in my own attic, partially shielded by foil type RB (work in progress). And using a digital thermometer can easily measure higher sheathing temperature behind the RB vs. the unshielded portion. It tells me the RB must be bouncing away a considerable number of BTUs. The best research I have seen tells me a typical result from RB is lowered cooling needs by 8-12% as measured by electric usage. That is modest rather than spectacular, but well worth going after IMO.

I have heard there is research showing that in many summer attics, after a long day a major radiant heat source for attic air is the top layer of insulation itself. Presumably the insulation has spent all day absorbing BTUs radiated from the roof decking. Seems strange but maybe plausible. Can you confirm this?

Best wishes -- P.Student

[Edited by perpetual_student on 06-18-2005 at 12:09 AM]