# Condensation under cathedral ceiling sheathing



## giansean (Mar 4, 2015)

I should add that this is the world's leakiest house and air sealing is always pretty much best effort. It was built in the time of cheap oil and big cast iron rads (which I wish it still had )


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## Nealtw (Jun 22, 2017)

giansean said:


> I should add that this is the world's leakiest house and air sealing is always pretty much best effort. It was built in the time of cheap oil and big cast iron rads (which I wish it still had )


I think you know the answer here, with out the venting it just how long the snow sits there and the heats radiates enough to melt it. You could keep the snow away or correct the venting. Best is leave cold spaces as in insulate the floor and the back of the walls of the room beside it. 
Valleys and hips always have bays that are hard to vent. Under construction cross strapping is often used . 
Ridge vents can be used but you can not cut the sheeting like at the ridge instead you drill a few well placed holes or box vents placed over a rafter to service 2 bays at a time but the bottom of a bay in a valley is near impossible to vent.


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## giansean (Mar 4, 2015)

Nealtw said:


> Best is leave cold spaces as in insulate the floor and the back of the walls of the room beside it.


Yeah it's a real challenge in this place. Even if I stuffed insulation in the floor and air got through from below, I'd have condensation on that surface too. There really is no wiggle room w/ insulation it seems, especially in this old-a$$ house lol. I suspect it always frosted on the underside to an extent, but was open enough to dry out quickly. Now I have a bunch of crap stuffed up there that is impeding that.

Say I tried to completely air seal the area with some poly, and of course the air seal the small door, which I planned to do anyway.. would that help at all? Having an open door space there surely is allowing air into the storage space, and there are numerous gaps where surfaces meet that are not fully air-sealed. Air will still get in there from places, but hopefully it won't be warmer humid air.


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## Nealtw (Jun 22, 2017)

giansean said:


> Yeah it's a real challenge in this place. Even if I stuffed insulation in the floor and air got through from below, I'd have condensation on that surface too. There really is no wiggle room w/ insulation it seems, especially in this old-a$$ house lol. I suspect it always frosted on the underside to an extent, but was open enough to dry out quickly. Now I have a bunch of crap stuffed up there that is impeding that.
> 
> Say I tried to completely air seal the area with some poly, and of course the air seal the small door, which I planned to do anyway.. would that help at all? Having an open door space there surely is allowing air into the storage space, and there are numerous gaps where surfaces meet that are not fully air-sealed. Air will still get in there from places, but hopefully it won't be warmer humid air.


Two different things there. A cold zone wants all holes from living space sealed so to keep moist air out of the area. An insulated roof wants air to move heat away from the top of the exterior wall as that will cause ice dams on the roof and cause a water leak.
If you are going to seal the space from the warm air then you should pull the insulation from the roof and even with out venting you would have a big enough air space to create a weather condition where the heat would move around the space and dissipate.

Ice damming can be solved from the outside with products under the shingles too.


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## giansean (Mar 4, 2015)

Nealtw said:


> Two different things there. A cold zone wants all holes from living space sealed so to keep moist air out of the area. An insulated roof wants air to move heat away from the top of the exterior wall as that will cause ice dams on the roof and cause a water leak.
> If you are going to seal the space from the warm air then you should pull the insulation from the roof and even with out venting you would have a big enough air space to create a weather condition where the heat would move around the space and dissipate.
> 
> Ice damming can be solved from the outside with products under the shingles too.


Oh but it's not ice damming... there is no snow (or ice) on the outside now. What I mean is frost is forming on the underside of the sheathing on the inside of the house, underneath all this insulation. Unless you mean it's the same principle... I've thankfully never had to deal w/ ice damming, knock on wood


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## giansean (Mar 4, 2015)

Ok I read that again and that's not what you are saying... sorry

But yeah those portions are only insulated because they adjoin living space, not so much to prevent ice damming. Removing all the fluff will still stink as the rooms will be even colder than they are now (it's an older part of the house and super cold, part of the whole "being tired of improvement and making do" concept . Still better than the valley rafter rotting apart I suppose.

Again too, those areas were stuffed with old crappy insulation before, leaving three possibilities as to why it never frosted up before. First that it was so old that it had sufficient airspace to dissipate as you said, second that there wasn't as much warm air introduced to it as now, or lastly, that nobody ever noticed


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## Nealtw (Jun 22, 2017)

so the common mistake made here is not putting solid blocking between the joists under the knee wall or at the end of the floor at the line of the roof. So warm moist air and fire can reach the roof structure with out obstruction. You have uses paper faced but the paper ends at the worst possible place and fiber glass is not the best for stopping air flow.

Just a guess but I suspect that light fixtures in the ceiling are supplying moisture to the area in question. If that is the case removing some floor at the perimeter and sealing things up properly might be a solution. As well as changing that loose insulation with something like foam board with a space to the sheeting connected to the chutes that go to the attic.


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## giansean (Mar 4, 2015)

Fixtures in the ceiling below you mean? There is only one fixture in the smaller gable room and it's not a recessed can. I have to believe most of the air is coming from the little door which is in the gable room, and flowing right into the bays between the roxul and the paper faced FG like you said. 

Some is also probably coming from open electrical boxes on the now almost finished wall which are still not covered. I can take some newer pics if it helps get a better sense - that one of just the framing is older, just to give an idea of what is up under there. These pictures aren't the best.

I could definitely slip some rigid in those rafter bays/chutes as long as I don't have to go too tight to the sheathing, where two million frost covered nails exist to rip up anything that gets in their path (like the backs of my hands for instance ) However the warm air will still come through the little door into that area.

Here's a crappy-looking sketch if that helps give you a better idea of the layout. The bedroom and gable closet are living space (though the closet is horribly insulated) The small door opening is wide open, and the storage space is as in the pics.


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## Nealtw (Jun 22, 2017)

giansean said:


> Fixtures in the ceiling below you mean? There is only one fixture in the smaller gable room and it's not a recessed can. I have to believe most of the air is coming from the little door which is in the gable room, and flowing right into the bays between the roxul and the paper faced FG like you said.
> 
> Some is also probably coming from open electrical boxes on the now almost finished wall which are still not covered. I can take some newer pics if it helps get a better sense - that one of just the framing is older, just to give an idea of what is up under there. These pictures aren't the best.
> 
> ...


That is close to what I imagined. You have two different things going on fighting with each other.
Insulated roof sheeting says warm room.
Insulated walls say cold room. 

Pick one and go with it. with out sealing the door some moist air will enter but not enough to heat the room causing condensation. 

I would pull the insulation off the roof sheeting. Make sure there is no air leak from the ceiling below into this space. Add air chutes from the sofet into this space. If you do not have air chutes to the attic now slip some lengths of pvc pipe above that insulation to make the air connection.
Insulate the floor and door and seal the door.


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## giansean (Mar 4, 2015)

Thanks again Neal. The reason I went with both insulated roof AND walls is because this storage area, while warmer, was still going to be colder than the rooms it was facing. There is a lot of disagreement between folks as to whether it is acceptable to "double" insulate. Had asked on the GBA Q&A and their guru had said that there can not be "too much" insulation... but here we are eh?

The room under the space is one corner of the kitchen, and should be pretty tight (taped edges and actually has two layers of sheetrock on the ceiling). There is plenty of colder air infiltration through the block gable walls and that corner soffit at the bottom of the valley (hence the coldness)

If I do remove the fluff can I cut and cobble some polyiso to fit between the rafters, if I leave a gap for air to flow, and airseal with foam? At least it will help keep some of the cold out. If that will make the space too warm, then forget it.

That brings me to the other issue... I definitely want to be able to insulate the small section of cathedral roof adjoining the bedroom part. I care less about the gable closet side as it's always way too cold from the two other poorly insulated or uninsulated walls - even the stuff I shoved up there is only about halfway up - as far up as I could reach). 

Finally, I'd love to insulate the floor of that space as the kitchen does have a problem with keeping in heat. However it's pretty new and would require ripping up and refinishing new hardwood, which would suck ass. Is there a way to sprayfoam it maybe from the bottom (kitchen) through the ceiling perhaps? I'd actually love to do the whole kitchen, but now I'm afraid of doing anything where warm meets cold lol. The best laid plans...

Thank you again!


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## Nealtw (Jun 22, 2017)

If you use an air chute or make one for over the wall you can hold it in place with a chunk of the fluff.
Not sure what the door looks like but I would just insulate over the floor in that area and the wall and door area and make the door so it closes and seals from the room side.

I am in a suite in a basement with a closet that has never had the door closed because some fool forgot about heat in there.


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## Yodaman (Mar 9, 2015)

So many things going wrong here. No venting, no air sealing, and lack of insulation to start with.
Think of the living space in your house as a envelope. Everything inside the envelope stays warm and moist. Everything outside the envelope should be as close to outside temps as possible. This includes attics, crawl spaces, and the vents above your cathedral ceiling. You can achieve this by careful use of strategically placed upper and lower vents, air ceiling the transition between warm to cold areas. And lastly providing adequate insulation.

All are important to achieve a dry roof deck. With no low vents your rafter bays are mostly dead air space. This would be ok if you have your roof deck spray foamed. Aside from that you are left with venting, sealing and insulating.


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## giansean (Mar 4, 2015)

That's the trick... we want this small area to be part of the living space, it is just hard to make it so. We did the best we could with what we had.

Venting is great (the rafters on the bedroom side had them, before I stupidly pulled them out) but the majority of these rafters all end at the valley rafter or a gable end, not at a soffit, so I can never have full venting. 

Air sealing is easy enough. I can pull off the kraft paper and install a smart vapor barrier (i have a lot of Membrain left) or just regular poly, and tape all the seams. I am only assuming that no warm/moist interior air = no frost. if sealing is enough to keep out the moisture or at least minimize it.

As I mentioned, this space has existed for 65 years, with walls and ceilings adjoining living space insulated the same way (though with MUCH crappier insulation). The only thing I added was the polyiso on the block gables, and the fiberglass in the rafters immediately over the top of the space. That was done mostly to try minimize the cold coming into the gable closet room.The big difference is that there's an opening there now (soon to be closed), which previously held built in drawers.

Not sure if there is a happy medium, but I'll take whatever I can at this point, at least until I'm ready to gut it... again


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