Cape Cod upstairs bathroom dormer insulation


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Old 12-02-15, 04:40 PM
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Cape Cod upstairs bathroom dormer insulation

Wow! Thats a long title

But like the title says, I have a second floor bathroom in my cape cod. Its mid reno - we had to replace the roof. Currently there is no ceiling, or insulation. Id like to insulate before it gets cold. The bathroom is about 9x8, in a dormer. The attic part is not connected to the rest of the main attic, and the majority of the 3 walls are essentially exterior.

My main and primary concern is the exposed roof/attic area. It has a ridge vent and under-shingle edge vents, so there is air circulation. Im assuming that there needs to be a vapor barrier installed, being a bathroom and all. In my area, R48-R60 is recommended for attic areas. Should i get some kraft faced batts, and install unfazed R30 on top of it? Or plastic sheeting as the vapor barrier and two layers of unfaced?

Secondary, what should be in the walls? One wall is completely exterior. The other two are half-exposed, half inside the knee walls of the attic. They currently have some insulation, but if i ever should replace it, what should I use?
 
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Old 12-02-15, 07:10 PM
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You will want baffles for air to flow below the bottom of the roof. Then your insulation, vapor barrier, and drywall. Do you have sufficient depth for the required insulation plus the baffles?

Bud
 
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Old 12-03-15, 06:36 PM
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The green arrows show air travel. The darker lines are representative of the 2x6 structure(at least i think theyre 2x6)

Ill put baffles in between each rafter.

The yellow is where the second layer of insulation would go. The red areas are where rafters intersect ceiling joists.It would be hard to get insulation in there, at least the top layer running perpendicular to the rafters. The blue area is what i had planned to insulate. First with kraft-faced batts, then unfaced rolls, again running perpendicular to the rafters. Do i score the rolls so the will slip over the rafter 2x6s? or just leave that space uninsulated?
 
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Old 12-03-15, 07:04 PM
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The baffles only need to protect the under shingle vents from where they enter to above the depth of the insulation.

The actual airflow will not follow the bottom of the roof, but the cold air will spill onto the attic floor filling that space and forcing the warm air up and out. Same results, but a better description.

"The other two are half-exposed, half inside the knee walls of the attic." If the wall insulation is exposed to those side attics it should be covered with something like house wrap.

Bud

Question: Do the under shingle vents run across all rafter bays or just a few?
 

Last edited by Bud9051; 12-03-15 at 07:06 PM. Reason: question
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Old 12-03-15, 08:42 PM
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the vents run the length of the roof.
 
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Old 12-06-15, 06:12 PM
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Wife gave me go ahead to spend some $$. My plan is a baffle for each bay, kraft faced r-30 at 16 running between the joists, and unfaced r-30 perpendicular on top of that.

I think now the only questions i have left are
1. how do i deal with the space between the rafters for the upper layer? can i cut off a length for each bay to fill the space that is missed?
2. Id like to install the vent for the exhaust fan before i close it up. Someone once mentioned a condensation loop? Is there a particular way i need to hang the exhaust tubing, and is there a preferred material? (rigid vs accordion style)?
 
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Old 12-06-15, 06:53 PM
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Kraft down against the drywall, just lay it in neatly.
Cut and fit as needed to fill the space between the rafters up against the baffles.
I saw the condensation loop mentioned and ignored it. Sounds like a bad idea to collect condensation in any duct. Rigid ducts sloped to the outside and wrapped or buried in insulation.
Is your exhaust vent going out through the roof or gable end?

A well insulated exhaust duct from a bathroom should be fine. You should also install a "delayed off" switch for that fan so it continues to run for some pre-selected time after you leave the bathroom and turn it off. That extra time will not only help dry the moisture from the bathroom but dry the inside of the vent.

Dampers for exhaust vents are designed to allow air to exit. Than can be a challenge when normal convection wants to exit as well, thus lots of insulation.

Bud
 
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Old 12-06-15, 08:11 PM
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It vents thru the roof. The run would be 3 feet vertical by 3 feet horizontal. I've never had much luck with rigid ducting and elbows. Any tips?
If I were to use plastic barrier instead of faced insulation, what thickness would I use?
 
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Old 12-06-15, 08:42 PM
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First, vapor barriers are no longer considered essential in all climates. OH is cold, but borderline for requiring a vapor barrier. Their thinking is, very little moisture actually passes through materials like the drywall but a ton of moisture can be carried into the attic through air leaks. So air seal everywhere you can, below is a guide.

So, to answer the question, 4 mil would be fine. Vapor barriers are a bit strange in how they work. If you cover 80%, they are 80% effective. Having some gaps, holes, tears, does not destroy their performance unlike insulation which is far more susceptible to minor imperfections.

I haven't done a lot of duct work, but a couple of good tin snips, left and right, and it should assemble just fine. I would tape all joints with a good foil tape. Others here will add to my lack of experience on this (please).

http://www.efficiencyvermont.com/ste...ide_062507.pdf

Bud
 
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Old 12-07-15, 12:16 PM
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The only reason i though i needed a vapor barrier, or faced insulation was due to the fact that a bathroom would be underneath. Is this not the case?
 
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Old 12-07-15, 02:16 PM
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The timer switch that leaves the fan running after you turn it off will remove a lot of the moisture. And the paint you select can actually be a vapor barrier. Several discussions here about which paint to use.

Bud
 
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Old 01-08-16, 08:56 AM
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Exclamation

So I finally insulated the bathroom. I used unfaced rolls of R-30, and some 4 mil plastic sheeting to hold it up until its drywall time. I check it very few days to see how the temperature is holding. And until today, everything was fine. Weve finally hit winter temperatures so I figured today is the ultimate test. And then I notice water collecting in the plastic.
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Furious, as weve just had the roof replaced, I pull down the sheeting from the worst bay, pull out the insulation to reveal that the OSB is soaked, and condensation dripping off the air baffles. Strangely enough, its only on one side of the roof.

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When I had the roof replaced, they installed under-shingle vents and a ridge vent. So why is moisture building up? Im assuming there must be airflow going in from somewhere, but why arent the vents allowing it to escape?? Like I said previously, one wall of they bathroom is exterior, one is interior, and the two sides are a bit of both (I lied, one side is also interior - it connects to a closet which is also under this dormer roof). The red line shows the split between bathroom, on the right, and closet, on the left. On each side of the dormer is attic space, behind knee walls. There is an access door to each space, but as I now think about it, on the offending side, the access door is in a heated bedroom, while the other is inside the enclosed closet in the dormer.

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I need to:

1. Remove the moisture ASAP so not to destroy the roof decking

2. Prevent it from happening again, so I can have a happy wife.


Oh, please help!!!!
 
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Old 01-08-16, 09:10 AM
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This is the wall of the wet side of the bathroom. I pulled the insulation off one bay to see if there is any damage behind it, as I can see some water marks running down the 2x4’s Should there be a vapor barrier anywhere? Above the roofline, it is exterior wall connected to the bathroom. It looked like there was some insulation shoved up in the gap to maybe try to create an air break.

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Old 01-08-16, 09:29 AM
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You have a real mess there and from what I can see you are allowing inside air into a cold attic. Ventilation cannot handle that much moisture.

The insulation you installed with the plastic over it would need to be backed by a rigid surface (drywall) and air tight. Just plastic is never air tight and as the house breathes, opening and closing doors or wind, that plastic allows air to be pumped into the attic. Once there it cools and deposits its moisture on the first cold surface it can find.

Every access to an attic space needs to be air tight and if you are installing a vapor barrier it goes to the warm side of the insulation.

Others will comment here, but it looks like the plastic has to come down, the wet insulation removed, and everything dried out. Drying it out this time of year may be difficult as any inside air will want to deposit more moisture. Then, before it gets dripping wet again, new insulation and the drywall with at least one coat of tape to seal it. The vapor barrier is your decision, some do some don't.

Bud
 
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Old 01-08-16, 09:43 AM
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That makes sense. But why is it on just one side of the attic? This looks like what made it need replaced in the first place - the shingles were intact, but the decking was completely rotted out, just on the one side. Do I need to completely seal the one wall between the attic and the dormer? Guess I should call a pro and get some suggestions! Yikes Thanks
 
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Old 01-08-16, 10:22 AM
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The house needs a well defined thermal barrier which includes insulation, air barrier, and vapor barrier if used. They need to be continuous and all in contact with that plane. Not one here and the other 4' over there. So, yes, it sounds like that that wall between the attic and dormer needs to be sealed as well.

Careful who you bring is as a pro as few fully understand condensation but they will have the advantage of being there to look at everything up close. Unfortunately, all problems look like roofing problems to a roofer and so on.

Bud
 
 

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