The primary purpose of a roof vent is to allow porous insulation to dry out if it gets wet from a roof leak. If the air that enters is so humid that it causes moisture problems then closing the vent would be an improvement in the short term. A long term solution might be to include the attic space in the conditioned space of the house. In other words insulate the rafters with vents at the eave and ridge (or insulate with spray-in foam with no vents) and let the attic be part of the heated and cooled house. This is recommended if HVAC equipment or ducts are located in the attic.

yes, that's where our air handler is, and that's one of the reasons we'd like to close the vent. we are thinking about rennovating within about 5 years, and would likely be revising the portion of the roof with the vent at that time. however, we are plagued with bad humidity now, and would like to close the vent from the inside with plywood to help alleviate some of the sticky air. is this a good temporary fix? do we need some sort of moisture membrane on the vent side of the plywood or is that just asking for trouble?

nomoreoak:

I recommend you repost this in the Heating and Air Conditioning Forum.

As I have always understood it, generally the purpose of an attic vent is to prevent buildup of warm, moist air from the interior, against the cold underside of the roof in winter, which would cause moisture problems because of condensation. It also alleviates somewhat the buildup of very hot air in the attic on hot summer days. I am having trouble understanding what you think is the cause of the problem, it sounds like you think the moisture is entering in the air through the vent. I suppose on the exceptional, cool humid foggy morning, you might get moist air entering and some condensation in the attic. But you have moist air entering the attic from the living space all year, even in a modern well constructed home. Closing the attic vent leaves this mosture no ready exit. Modern homes in which the attic space is heated still need a space in the rafter bays between the top of the insulation and the underside of the roof for ventilation, they make a plastic insert for this purpose, to prevent the insulation from closing it off.

air qual guy -- thanks for the suggestion, i posted similar question there which went unanswered. we live in an old home in central florida and only recently had a vent put in on the roof. in the meantime, we had to have an oak tree, which significantly shaded our home, taken down and now are suffering from a lot more hot humid air up in the attic and especially in the rooms under the vent. our "clueless homeowner research" seems to indicate that in areas of high humidity, the best new recommendation is NOT to vent the attic. we'd eventually like to move the air handler out of the attic to a new location on the second floor after our rennovation. my question is what we can do in the meantime to help alleviate this new humidity issue. can we close the vent from inside the attic with plywood? we don't really want to call the roofers back as that portion of the roof will likely be reconfigured during our rennovation.

thanks!

instead of removing the vent, install a powered vent in the attic. on with both a thermostat and a humidistat. we have about 85% humidity here most of the time. before i installed a power vent we had moisture and heat problems in the attic. after, it is comfortable up there and most importantly DRY.

make sure your have a place for air to come IN the attic from outside. if you vent the heat out, with no soffit vents for replacement air to come in, then you will suck conditioned air from inside the house. this in itself will cause moisture problems, along with waste energy from teh HVAC having to work harder. plus cooler attics prolong the life of your roof.

prior to venting mine, my shingles would actually lift and curl on hot summer days. the attic was stayinf around 145 to 150 degrees, and it was ruining the roof. now it stays closer to outside temps, and the shingles stay down. sure beats having to replace shingles every week!

Below is a link to a discussion of humidity and venting issues.

Here is a link that might be useful: Humidity and Venting

we do have a few air circulation holes near the roof on the outer walls of the house. the soffits do exist, but are not vented (i know, i know, they should be). here is the link that was posted in the hvac forum:

http://www.buildingscience.com/resources/mold/Bugs_Mold_Rot.pdf

i don't mean to be controversial; there is certainly a lot of data out there weighing in on both sides of this issue. i'm just trying to figure out the easiest way to fix our obvious mistake in having the vent installed -- how do we close it off and not cause roof damage?

Attic venting in the South is commonly misunderstood to the detriment of homeowners. Joseph Lstiburek can explain it better than I can.

Here is a link that might be useful: Joe Lstiburek

thanks mighty anvil, that was a great link. the high humidity problem did not exist prior to the installation of the vent. i really don't want to be obstinate, but, i'd still really like to know, strictly from a roofing perspective, if i can close off the vent temporarily without damaging the roof, and if so, how. thanks in advance!

Assuming no part of the vent extends lower than the roof sheathing, you can use plywood to close it off, but DO NOT drive any nails/screws upward through sheathing to secure it, you could puncture the felt and/or shingles and cause a leak.

I recently removed a brick chimney, here's what I did to seal the hole.

I built a shallow box, with a plywood bottom and 2x4 sides (2x4s standing up so it's 3.5" deep). The outside of the box was exactly as wide as the gap between two of the rafters, so that it would slide snugly up between them. And of course it was more than long enough to cover the hole.

I carried this prefab up in the attic, slid it between the rafters to cover the hole (upside down of course so the plywood was up against the roof sheathing) and naiiled it securely through the 2x4s into the rafters.

Now of course I then had to go up on the roof and reroof the patch. But you seem to plan to just leave the vent installed, so you don't have to worry about that part.

thank you so very much!!! do you think i might be wise to put a moisture membrane only on the part of the plywood that is "exposed" to the vent? i realize that if i cover the whole outside of the box with the membrane it will just track inside.

Just seal the vent with a piece of plastic tarp and duct tape first. Actually, come to think of it, since you aren't removing the vent that's really all you need to do, to stop air entering. The plywood patch is overkill.

thanks again.

one other thing. around here during the winter most folk put a garbage bag or tarp over the vents during the winter. we don't have ot worry about ice dams and such like our neighbors up north, just excessive heat/humidity. if you are going to only temporarily close it off, that is all you have to do.

Contrary to convential wisdom, venting an attic does little to reduce the temperature of the exterior roofing material and therefore has virtually no effect on the useful life of the roofing. (see "Roofing Failures" by Carl G. Cash)

You might ask why asphalt shingle manufacturers void warranties if the roof is not vented and the answer is "because they can" not because it makes any sense. If the temperature of the roofing was an issue the manufacturer would give a longer warranty for lighter colors. Shingle warranties are based solely on marketing strategies.

If you insulate the rafters with fiberglass and vent at the eaves and ridge or insulate with spray foam with no vents, you can leave the air handler and ducts in the attic and save the energy that is now leaking into the attic.

If you want your roofing to last longer in a hot sunny climate, select a lighter color or plant shade trees.

mightyanvil, i have to disagree with you there. based on MY experience of going from 3 gable vents only, to a powered vent, my attic temps dropped by 40 degrees on average and my shingles quit curling. i worked as a roofer years ago, and we used to see heat damaged shingles all teh time. warranty was void if it were the fault of no vent. we put a new roof on a house and installed a vent at the same time, 3 years later the roof failed due to heat. the owner had gone in and blocked off the vent shortly after we left. he had to pay almost 9k for a new roof due to his insistence that the vent was going to let bugs in his attic! i kid you not, he thought that would just draw bugs in.

in cooler climates this is not a big deal, but down here vents are required. plus they help cool the rest of the house.

Extensive field testing of asphalt shingled roofs has shown that although venting an attic does lower the temperature of the attic space, it only lowers the temperature of the roof shingles by a few degrees which is not enough to have any meaningful effect on the longevity of the shingles. I'm just telling you what I learned from working with Carl Cash, a leading expert on asphalt-based roofing systems, who did the testing. Buy his book if you would rather get it from the source.

Here is a link that might be useful: Roofing Failures

well, i am speaking from my own experience, so i guess we just have to agree to disagree.

in my case the shingles were hot enough to melt the tar and literally allow it to run from between them, after venting this is no longer an issue even on 100+ degree days that we get a lot of.

in milder climates, it may not have the same effect. in fact, according to Cash himself his studies were not in such extreme climates. see the link below.

Here is a link that might be useful: cash's article about ventilation.

In the continental US the only way an asphalt shingle could melt from exposure to the sun would be if it had been made from temperature unstable low-end bitumens with a low melting point. I am surprised that such a shingle would be sold in a hot sunny climate.

When the attic space is part of the conditioned space (a design that is rapidly gaining popularity all over the US), the only way to avoid heat and UV related failures is to specify lighter colors and shingles that comply with ASTM D 225 (organic) and ASTM D 3462 (fiberglass).

mine are the standard tan shingles sold in many box stores, so not my first choice but the previous owner is the one who put the roof on. i have LARGE areas facing the southern sun and it do get hot up there.

the shingle itself did not melt, the tar strips that stick them together is what melts. the wind blows, lifting the tabs, and the tar gets dirt/dust on it. then the tab won't stick back down. next thing you know, roof failure. since venting the attic, i have seen a pretty decent drop in roof temps, it is still hot, but cooler than it was before.

when i reroof in a few years, i will go with a lighter color for sure.