Frost in the attic, advice on what I found...?

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ThaddeusV

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Hi:
I would appreciate some advice on handling my attic situation. I have a multi-level townhouse. I have attached a drawing to show what I found up there.
I get frost all the time. I have had to have the shingles replaced because of leaks that were not due to obvious holes. Snow melts along the line of the wall before the rest of the roof.

The spaces between the rafters has been blocked with 1/4" plywood and against the wall, right up to the roof sheathing, 10" yellow batts have been placed. I assume these were placed by a 'handy-man' as the rest of the house uses pink. I found that some of the yellow was frozen to the sheathing when I pulled them down.

What I need to know is should I just cut back the yellow so I doesn't reach the roof or should I also remove the 1/4" plywood? I'm thinking the plywood/yellow insulation is preventing air from moving to the vent at the roof peak and this is the major issue causing the frost.
The unit was built in 1979 during the Alberta oil-boom so who knows how good the builder was, but I can't tell if the plywood is original or placed later.

Thanks

SCN_0001.jpg
 
What is happening is warm moist air from the house migrates into the attic and freezes on the cold underside of the roof deck. To eliminate this condition you need a functional vent system. A proper vent system consists of an exhaust vent high (ridge) on the roof slope and a intake vent along the lowest part of the roof for intake (so-fit). If insulation impedes the ability of air flow, you have stagnate air and trapped moisture. This will lead to mold, compressed insulation (reducing the "R" value), higher energy costs and diminished roof life expectancy. There are ways to correct venting issues with proper use of intake and exhaust vent.


Oh, and welcome to House Repair Talk!
 
Thanks for the response!

There are vents (in diagram attached to original post) and soffit on both sides of the roof and they are clear. There is some air leakage from areas where vents come through the ceiling and I will be sealing these up in the spring. What I'm really looking for is an opinion on the plywood. Is there any conceivable reason for blocking-up the space between the roof rafters? Maybe they thought it was good to hold the loose insulation in place? Building practice change? Or just a bad idea from a mediocre builder or handy-man?
 
The vent on the lower roof seems to be to low and was probably added later. they don't put vents on both sides so if you lower the plywood a few inches you may want to plug the lower vent. You may want to check to see if you have enough venting on the high side and make sure your soffet vents are open.
 
Thanks for the reply!
Yeah, I wondered about that vent.

As for moving the plywood a few inches, well that would be removing it. The rafters are 2x4 and are almost right on the header for the wall so I'm gonna just remove them and adjust the insulation so there is a clear path up to the high vent.

While checking again today I also found that the stack is completely open to the attic for about 2 feet from the attic floor to the ceiling on the high side :confused: Looks like they didn't want to deal with the vent for the plumbing so they just stapled the vapor-barrier around it and when whoever put in the yellow insulation was at work they pushed so hard it opened up the barrier...:mad:

I guess I'll need to go through the bathroom wall to fix that and seal it.
 
If you remove the plywood, plug that lower roof vent, as it will screw up the airflow for the whole roof.
 
Plugging the vent occurred to me as well.
Thanks guys!
It's always good to have someone reinforce what I was thinking.
 
ThaddeusV said:
Plugging the vent occurred to me as well.
Thanks guys!
It's always good to have someone reinforce what I was thinking.
Click to expand...

When you introduce a vent in the middle of the roof you short cycle the natural air flow and create stagnate air spaces. Air, like many things in life, takes the path of least resistance.
 
whoa - Isn't the lower vent in the drawing the equivalent of a soffit vent? I wouldn't plug that up. I would remove the plywood in the middle to allow for air flow between the two vents. I would also check for bathroom vent fans dumping moist air into the attic and make sure they are piped outside.
 
handyguys said:
whoa - Isn't the lower vent in the drawing the equivalent of a soffit vent? I wouldn't plug that up. I would remove the plywood in the middle to allow for air flow between the two vents. I would also check for bathroom vent fans dumping moist air into the attic and make sure they are piped outside.
Click to expand...

If there are sofit vents you do not want a vent in the middle of the roof. There also needs to be unobstructed air flow from the eave to the ridge.
 
Air flow is the key. Removing the plywood is the first thing to do this will allow air to carry the moisture up to the upper vent and out of the building. As it is now the plywood is trapping moisture against the roof causing frost.
If you want to eliminate the problem for good and the areas are accessible spray foam insulation is exceptional. It would eliminate the need for vents at all. I live in an old home 1800's and cut the draftiness almost totally out by using spray foam against the roof deck and around the band board in the basement.
I realize this is a much more expensive fix but I believe it would be permanent and ultimately save you money on your heating and cooling along with making the home much more comfortable.
 
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