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Do I need to seal the top of outer walls in the attic?


rayhogan's Avatar
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NJ

12-11-12, 09:01 AM   #1  
Do I need to seal the top of outer walls in the attic?

I'm working on sealing all of the gaps and holes in my attic before adding insulation. The house was built in 1980.

It's going well and I'm done with all interior walls.

Now I find myself in the hardest physical part ... sealing the tops of the outer walls. Specifically, I mean where the top floors' ceilings' sheetrock meets the top plate of the exterior wall.

Here's my problem:
On the closer side (inside edge) of the wall, I see the same type of sheetrock-to-wall gap as interior walls, but due to the lack of move-around space, I cannot see the exterior wall interface ... probably where the sheathing meets the wall top plate.

Does this interface need to be sealed? (I'm assuming it does)

and if so:

1) How do I reach it?
2) What should I use to seal it (cans of spray foam will probably not work as I would need to spray around the corner?
3) I'm about to also cut some holes in the soffits (one side has no vents), should I do it from there?


Thanks,

Ray

 
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Halton's Avatar
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12-11-12, 09:33 AM   #2  
.



The short answer is no.....

The whole purpose is to stop the warm moist indoor air from entering the walls and attic.

Sealing the top of the exterior sheathing to the top plate isn't necessary.

Any little moisture that does happen to make it's way into the wall cavity could then be trapped.

The air from the soffit travels right over the top plate and should carry any moisture away.



.

 
Bud9051's Avatar
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12-11-12, 10:22 AM   #3  
Hi Ray and welcome to the forum.
Here is a check list and guide to air sealing, although a quick spin through it I did not see their discussion on exterior top plates, but I'm sure it is mentioned in there somewhere.
http://www.efficiencyvermont.com/ste...ide_062507.pdf

Getting into those hard to reach places boils down to judging the effort vs the benefits. Yes, all leaks should be sealed. But if you actually got them all, you would have to cut some holes (HRV) to bring in enough fresh air. In the real world, close is good enough and as Halton said, in that location, any leakage will quickly join the flow of air into and out of the attic.

The air sealing priorities start with the largest leaks and end with what else you can get to.

As for the soffit venting, you said cut some holes. What are you planning to install for vents. All vents have a "net free area" that you must take into consideration. 50% is a ballpark number, but those small round pop in vents are as low as 19%.

Baffles to protect the bottom of the roof from insulation and provide a ventilation air path. And if you can, cover the ends of the insulation to protect it from the incoming air, called wind washing.


Bud

 
rayhogan's Avatar
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12-11-12, 12:59 PM   #4  
Thanks Guys,

The house is new to me and I'm backfilling several discovered problems (including R12 in the attic)


For the soffit vents, there is a different configuration on the front and back...

The front, I'm planning on a series (4) of touching 3 inch holes covered by one of the rectangular covers. I'm shooting for 10 total across the Front. There is currently no ventilation on the front.

In the back, the soffit is vinyl siding with some inserted vented pieces (about 5 total). There are some lame 1" holes underneath there and basically I plan, on the same configuration of holes as the front.

Someone mentioned that putting additional holes in adjacent rafter sections may also be worth it as the air would pass through them from the occasional vented vinyl inserts.

What is the recommended material for blocking the wind washing?

Also and LOL: "Ask me about my self-belay device" for working on the ladder.

 
Bud9051's Avatar
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12-11-12, 03:08 PM   #5  
You haven't mentioned what is in place for upper vents, but your total vent area should be 1 ft² of vent area for every 150 ft² of attic floor. Small holes do not add up to very much and then it all has to be discounted 50% to come up with the true net free area. A rough guess tells me you are coming close and if you have done a good job at air sealing you can get by with less. Especially if you end up with more now than you had before.

Putting holes in rafter tails sounds ok, but not in the rafters.

Just about anything that block air flow will work, as long as you maintain the air channel above the insulation, the baffle.

Bud

 
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