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Sealing vapour barrier at edges of attic.


mathx's Avatar
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10-18-15, 04:04 PM   #1  
Sealing vapour barrier at edges of attic.

Seems on the gable ends the ceiling lathe sticks out over the wall top plate:





How do I seal a vapour barrier to these ends?

I also have a smaller problem at the sloped roof sides (simple 2 sided gable roof) -the roof comes down just inches above the wall top plate. Getting in there so seal things is going to be a trick!

Looking down the roof along the rafter:


 
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Keith Weagle's Avatar
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10-18-15, 05:26 PM   #2  
In a situation like that, your best option is two coats of oil paint on the inside of the walls, and air seal as best as possible around and lights or outlets, and around trimwork. The oil paint is considered a vapour barrier, but the air sealing is the most important part.

The only other viable option in my opinion would be spray foam in the attic, which will have the same results with the benifit of a 100% air seal. (assuming they can get access to all the areas needed).

 
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10-18-15, 05:42 PM   #3  
I agree with Keith. Trying to vapor seal that from above would be difficult and yield poor results. Being a plaster ceiling below it probably has a few coats of paint already so topping it off with a good oil paint should do fine.

If you blow in cellulose insulation it will fill all of the irregular spaces and reduce any remaining leakage at the same time.

Have you addressed the attic ventilation?

Bud

 
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10-18-15, 08:49 PM   #4  
We do have a few coats of latex, plus whatever else was on there before. FWIG they're all fairly good vapour barriers. I did foam up all penetrations (light fixtures, wiring) that run down into the ceilings, but I bet there's a lot of leaks up inside the outside walls. (The outside wall wiring goes down the sides of the walls just inside the brick into a 1 inch gap, cant possibly get in there to do anything useful, except stuff batts down there to reduce airflow...)

And the situation might even be worse - the outside (and inside...) walls arent as high as the ceilings in the rooms -- the rooms have walls that curve up into the ceiling. So the wall topplate is below the attic joists, and way down the soffit wells! Annoying. "Old houses have character!" they say...

You can see here:



(and check my previous post's shots down the well)

That corner where my (temporary..) batts stop is NOT the wall topplate, thats the start of the sloped ceiling in the room. Way down the well is a crosspiece sittting on the bottom slope of the well leaving a 1" gap, and there's another 1.5' of well past that, i think that crosspiece is sitting on the topplate, and thats where the rafters are birdsmouthed in. Totally inaccessible.

I've vacuumed out the wells, but I fear Ive made the situation worse: the fill stuffed in likely previously reduced airflow, but in a bad way - any moisture couldnt dry out properly, and soffits werent venting the attic (once I got them cleaned I could feel a bit of flow). But it DID insulate, a bit, at least by reducing airflow.

Considering the image, and the spray of nails the stupid roofers put into a number of wells (look at right, sigh...), Its going to be hard to get anything down in there. Even worse, you can see the joists/crossties are inside the rafters (and most only in with 2 nails!), so I cant even get a full-width piece of insulating foam (or roxul batt) down there as it has to be narrow enough to pass the end of the crosstie, leaving a nice 1 5/8"s gap. I need something that compresses to get it by the ties then fills the well's width, but leaves 1" for air to the roof, and somehow covers all the way down to the wall topplate (which is like 2 1/2" feet diagonally down...)

Not sure what to do in there to insulate that curve/slope expressed in the rooms, and I guess Im giving up on vapour barrier since Ill never be able to seal it down in the soffit wells nor the gable ends in any useful way. Never was a vapour barrier previously - perhaps the thin insulation (~R6-8?) previously in there was thin enough to avoid issues by allowing airflow. Does that mean I can't insulate substantially without risking condenstation?

All these issues made me wish I had looked more closely before getting the shingles redone - I mighta just opted to rebuild the whole roof properly!


Last edited by mathx; 10-18-15 at 10:05 PM.
 
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10-18-15, 11:14 PM   #5  
Soffit vents?
Ridge vent?
High temp. expanding foam around the chimney, any electrical, ceiling fixtures or plumbing vents get sealed with regular expanding foam, foam baffles at the bottom of the rafters then 12" of blown in insulation.
Recommended Levels of Insulation : ENERGY STAR

 
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10-18-15, 11:52 PM   #6  
soffit are supposedly vented, but wondering if there's holes behind the gratings or just solid woof. hard to get something into the soffit wells there due to narrowness and darn roofing nails. would try to use foam board for baffling/moore venting if I could.

no ridgevent, just a regular set of 4 8x8" vents. adding gable vents too (replacing windows with them).

sealed electrical and other (bathroom drain vent) penetrations with expanding foam already. could do foam at edges of attic, but the bigger issue is how to insulate the sloped part of the ceilings of the rooms (expressed as the bottom surface of the soffit well) AND get a moore vent/baffle in there.

 
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10-19-15, 04:46 AM   #7  
I just made notes as I read so will post and add more later. Hopefully Joe or others have more experience with the construction you describe.

The gap between the brick and the house is there to allow drying of the brick. But drying into the attic doesn't seem good. I'll attach a link that talks a lot about bricks and insulation, although I don't recall how relevant it is to your situation.

I've seen the curved ceiling (I think) as you describe, but never from the attic or as part of any construction so no experience to help. We may need your best attempt at sketching a cross section of that curve and soffit.

I'm currently thinking along the lines you are suggesting, some form of (I'll steal Martin's phrase) Cut-and-cobble to piece together some rigid foam and provide the needed air path below the roof deck and keep the eventual insulation captive below. Whatever is created should also prevent the insulation from blocking the soffits. At some point you will need to get up there and determine IF there are vent holes behind those grills you see. May even need more of them.

Bud

 
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10-25-15, 08:26 AM   #8  
hey Bud thanks again -

got that link you were talking about? Yes cut and cobble to get past the butt-ends of the rafters seems required. I cant use anything soft because I need to be able to jam it into the right position. Ill use thick aluminum lined foam.

[ATTACH=CONFIG]57959[/ATTACH]

Of course the soffit well includes the space between each rafter, just not below it.

Attached Images
     

Last edited by mathx; 10-25-15 at 08:29 AM. Reason: fix image
 
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