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Attic ventilation and roof valley


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I'm having trouble with frost in the attic, but only in one section.

The house is a T shape and I think that the section where the two roofs join is called a valley, but I'm not sure. There are two attic sections (the top of the T is one section and the stem of the T is another) that touch but are not open to eachother. So there is a section a few rafter bays wide that has no access to outside ventilation. We are only having problems with the top of T section.

The back story is that we bought the house in 2008. The roof had a few soffits and a few square roof vents. The roof needed to be replaced and roofers put a ridge vent on each section of the house. After the first winter I noticed rust on the nails in the top T attic. The stem T attic has no moisture problems at all.

I called the roofers and they came and looked and said ventilation is fine, but the attic needs to be air sealed and everything will be fine. So air sealing company came, air sealed the attic, added insulation, baffles, and a few more soffit vents. Attic hatch door is also insulated and weather stripping added.

The problem is we are still getting frost in the section of the attic where the two roofs join. So air sealing company came back and said it must be an air circulation/ventilation issue. So they tried to better open up the soffit areas on either side of the roof valley.

I'm not confident about any of it though. The roofers and the air sealing company don't seem to understand why this is happening. The roofers say ventilation is fine, air seal company says the attic is sealed. The living space of the house had about a 10% to 15% humidity level through out winter, which is really low. So it's not like the house has a lot of moisture pouring into the attic, plus the problem is only in one section of the attic that isn't open to direct soffit vents.

Anyone have any thoughts or suggestions?

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I think our new friend is saying that those rafter bays that terminate at a valley rafter instead of the eaves are experiencing the frost issue. There's no ventilation air that's rising along the underside of the decking in those bays.

A little closed-cell foam sprayed in the affected bays might reduce the heat loss via the decking well enough to prevent condensation/frost issues. If it's sprayed and contoured thick enough, it might also deflect rising moisture to adjacent bays that do reach to the eaves and are ventilated by eaves/ridge vents.

Marc

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I think our new friend is saying that those rafter bays that terminate at a valley rafter instead of the eaves are experiencing the frost issue. There's no ventilation air that's rising along the underside of the decking in those bays.

Yes, that is exactly what I was trying to say. Thank you.

A little closed-cell foam sprayed in the affected bays might reduce the heat loss via the decking well enough to prevent condensation/frost issues. If it's sprayed and contoured thick enough, it might also deflect rising moisture to adjacent bays that do reach to the eaves and are ventilated by eaves/ridge vents.

Marc

Would the spray foam need to completely fill the bay, or just a coating in the bay will be enough?

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It should fill the bay only at the top (next to the ridge), then become thinner as it progresses towards the valley rafter until there's only a thin film of it left about 3 or 4' from the ridge. The idea is to allow the moisture to escape to adjacent bays since the ridge vent is blocked in the bays that do have foam. If the valley rafter is just a couple feet from the ridge then I'd just fill that entire bay. Small job. Not much foam involved.

Marc

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Ok here is a picture of the trouble spot. It is mostly these 3 or 4 bays that get frost. The baffles in those bays don't actually do anything. They don't lead to the outside, just a wall.

So you think putting some spray foam insulation in these bays might help, right?

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tn_201234175419_IMG_0078.jpg

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Ok here is a picture of the trouble spot. It is mostly these 3 or 4 bays that get frost. The baffles in those bays don't actually do anything. They don't lead to the outside, just a wall.

So you think putting some spray foam insulation in these bays might help, right?

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The issue as I had understood you regarded the rafter bays that terminated at a valley rafter. Your picture tells a different story.

I'd say you've simply too much moisture in the attic. It comes from the conditioned spaces in the house and should be ventilated to the outside. Ventilation works with the sealing of the attic plane to keep the dew point below the temperature of the roof deck and framing. If there's bath exhaust fans, gas flues or other sources of humidity dumping into the attic, all the air sealing and ventilation in the world isn't going to help.

Is there anything at all discharging into the attic? At some point, you may need more attic ventilation. Or you could take an entirely different approach and situate the insulation boundary of the house at the roof plane instead of the ceiling plane but that would waste what you've invested thus far.

Marc

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Sorry if I used the wrong roofing lingo. I'm not sure what you call it where two sections of roof intersect. There's to much snow on the roof right now, otherwise I would get up there and take a picture of what it looks like from the outside.

Those particular rafter bays terminate at a wall, where the other part of the house intersects. Those rafter bays have no outside soffit ventilation, even though there are some baffles. Those baffles were put in by mistake.

So what you are saying is what the roofing company told me, they said it was a moisture problem and I needed to do air sealing to seal the air leaks so no humid air from the house could get in.

So the air seal company came and sealed everything. But the frost was still appearing at only this particular spot in the attic where there is no access to soffit air to ventilate those bays. So the air sealing company told me it was a ventilation issue.

So if everything is air sealed, how do you ventilate an area of the attic that doesn't have access to soffit vents?

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Sorry if I used the wrong roofing lingo. I'm not sure what you call it where two sections of roof intersect. There's to much snow on the roof right now, otherwise I would get up there and take a picture of what it looks like from the outside.

Those particular rafter bays terminate at a wall, where the other part of the house intersects. Those rafter bays have no outside soffit ventilation, even though there are some baffles. Those baffles were put in by mistake.

So what you are saying is what the roofing company told me, they said it was a moisture problem and I needed to do air sealing to seal the air leaks so no humid air from the house could get in.

So the air seal company came and sealed everything. But the frost was still appearing at only this particular spot in the attic where there is no access to soffit air to ventilate those bays. So the air sealing company told me it was a ventilation issue.

So if everything is air sealed, how do you ventilate an area of the attic that doesn't have access to soffit vents?

I can understand how you might be frustrated. This issue, in a Wisconsin climate, CAN be frustrating. I agree with what Bill and Brandon are hinting at...there's a moisture source that's dumping into the attic in the vicinity of the frost areas, in addition to a general need for more ventilation i.e. more soffit venting and something less restrictive than the baffles.

Does TIJ have any members within reach of Waukesha?

Marc

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With only 10% to 15% humidity in the home, I think there's probably something else putting moisture in that attic. With what you've described so far (ventilation, air sealing), I would focus on finding the source of the excessive moisture.

Well, so far I've had two separate air sealing companies in my attic and both have said that everything is sealed. They are saying that nothing provides a "perfect" seal and the reason for frost in this area is because of a lack of air flow.

OP-- looks like you're talking about a "ridge", which is the peak of the roof.

Is this area above a bathroom or the master bedroom by chance?

Yes if I had room to frame the picture better, the top would show a ridge vent and the picture is looking down the slope, but the slope doesn't end in soffit venting, just a wall. The baffles you see don't actually do anything since they have no access to outside air.

This space is about 10 feet away from a bathroom. The space over the bathroom is dry.

Does anyone have any ideas on how to get air flow into rafter bays that don't have access to soffit vents?

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Does anyone have any ideas on how to get air flow into rafter bays that don't have access to soffit vents?

Yeah, but I'm at a loss to understand why two separate companies that are supposed to be professionals didn't suggest it; put edge vents in just above the gutters and install hip ridge vents in on the hip.

ONE TEAM - ONE FIGHT!!!

Mike

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Sorry for the bad quality pic and I can't get on the roof right now to show a better picture, but this will give you a better idea of the roof situation.

Where these two roofs meet is where the problem is. There is no open air flow between them. Where these two roofs meet matches the other picture that shows the 3 or 4 rafter bays that get the frost.

I don't think I can use the Edge vents you mentioned because there is no gutter where these two sections meet.

The roofing company says the ridge and soffit vents are enough. The air sealing company says that there isn't enough air flow in these particular rafter bays.

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I thought you were describing a hip roof; not a gable roof.

You don't need to have a gutter to install edge vents; just a roof edge. Aren't those open overhangs with frieze blocking between rafter tails? Why didn't they just drill holes in the frieze blocking so air could get in from beneath the eaves? That's the way it's done here. A ridge vent along the top and it'll pull so much air you'll need to wear weighted shoes when you go up in there.[;)]

ONE TEAM - ONE FIGHT!!!

Mike

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What year is the home?

Any renovations done?

is that an addition?

Just to clarify we looking at the attic of the lower roof and it is unvented where it runs in the addition (on the right hand side of the lower roof)

Watch carefully where the snow melts odd your house first. It is possible there is a lot of heat coming up the wall cavity and finding its way to the attic , condensing on the cold surface and freezing. I have seen it where the main power runs into a home these spots are hard to seal up, standing from the road you can see less snow here from the melting.

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Studied it some more and studied the Air Vent Inc. literature. I think you could still use a series of stepped edge vents just above the valley flashing between those rafters and that would put air along the underside of the roof.

ONE TEAM - ONE FIGHT!!!

Mike

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I thought you were describing a hip roof; not a gable roof.

You don't need to have a gutter to install edge vents; just a roof edge. Aren't those open overhangs with frieze blocking between rafter tails? Why didn't they just drill holes in the frieze blocking so air could get in from beneath the eaves? That's the way it's done here. A ridge vent along the top and it'll pull so much air you'll need to wear weighted shoes when you go up in there.[;)]

There isn't a roof edge where these two sections meet. It looks more like an upside down V

The overhangs have soffit vents and that area on either side of the upside down V is fine, no frost in those areas. It's just the area of the upside down V where the two sections meet. There is a ridge vent along the top that seems to pull air fine in the areas that have soffit vents.

What year is the home?

Any renovations done?

is that an addition?

Just to clarify we looking at the attic of the lower roof and it is unvented where it runs in the addition (on the right hand side of the lower roof)

Watch carefully where the snow melts odd your house first. It is possible there is a lot of heat coming up the wall cavity and finding its way to the attic , condensing on the cold surface and freezing. I have seen it where the main power runs into a home these spots are hard to seal up, standing from the road you can see less snow here from the melting.

The house was built around 1980, no renovations. The problem is in the upper attic. The lower attic has no problems. It's possible air is getting in somehow, it's just that the air seal company is saying it's all sealed.

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-28 oC or colder - keep humidity below 15%

-23 to -27 oC - keep humidity below 20%

-17 to -23 oC - keep humidity below 25%

-12 to -16 oC - keep humidity below 30%

-6 to -11 oC - keep humidity below 35%

+4 to -5 oC - keep humidity below 40%

found this online ^

air leakage? or moisture issue?

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-28 oC or colder - keep humidity below 15%

-23 to -27 oC - keep humidity below 20%

-17 to -23 oC - keep humidity below 25%

-12 to -16 oC - keep humidity below 30%

-6 to -11 oC - keep humidity below 35%

+4 to -5 oC - keep humidity below 40%

found this online ^

air leakage? or moisture issue?

The humidity level in the home all winter was around 15% or lower. So I don't think there is an unusual amount of moisture in the air.

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You didn't follow the link and study them or you'd understand.

Follow the link I gave you and look at the edge vents and study how they work. They go under the shingles. A narrow slit is cut in the decking beneath the vents. They could be placed anywhere on a roof and they'd work. The only question is whether they'd work in snow country.

What are you dinking around here for; the tech people at Air Vent Inc. can probably provide you a better advice. Those guys deal with roof ventilation issues day in and day out. They'd have to be better than a bunch of broken down old home inspectors spread out all over the country.

ONE TEAM - ONE FIGHT!!!

Mike

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I actually did follow the link and I just got off the phone with Air Vent people. They said they aren't sure if it will work for this situation or not. They want me to send them a picture when I can get on top of the roof so they can see exactly what it looks like where these two sections meet.

They suggested a product called Rafter Vent. It allows air to move between rafter bays and is designed for trouble areas that are blocked for some reason to outside venting. The problem though is that it is meant to be installed during construction because it goes between the roof deck and the rafter. So roof decking would have to be pulled up to install. They said sometimes people will just drill holes in the rafters, but they don't recommend that.

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