Attic Insulation Above Dormers - venting

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Old 01-26-05, 07:58 AM
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Exclamation Attic Insulation Above Dormers - venting

Clarification of original question below:

1) On a 2nd floor dormer with sloped ceiling sides and no exterior gutters is an airpace required ABOVE THE INSULATION in the unconditioned space above it - -i.e. do I need to install "ProVent" vent baffles between the roof rafters from the soffet area to just above the final layer of attic insulation?

I was told this was not required because there are no gutters at the bottom of that roof section, but I feel vent baffles are required for roof ventilation and request your opinions. (Note: no soffett venting - yet.)

2) Can a plastic vapor barrier placed over the unfinished attic floor cover each of the joists without harm. Heard differing expert opinions on this.

I'd much appreciate a response because this job is in progress now, but currently is on hold waiting for some good advice.

Original post below...
I read almost every post, but still have a couple attic insualtion questions: My 1930-built bungalow style boyhood home in WI is mine now & I'm re-doing all the attic insulation. A roofer checked the old fiberglass stuff and felt it was damaged with moisture. (When I first got the house we added 5 new roof vents. There is no soffett venting yet.)

My main question concerns the area above a south facing dormer (with no gutters). A roofer I know says that since there are no gutters I won't have ice damming problems above the dormer & I do not need to add vent tubes from the soffets of the dormer to the top of the insulation.

I plan on adding new fiberglass batt insulation to the horizontal area above the dormer's ceiling to the R-49 level recommended for WI. The roofer says I can have the insulation touch under the roof boards between the rafters. I am uncomfortable with this. WHat doi you think about this?

Inside the soffet area, as I lay on my belly above the room and look down into this space which is the 45 degree angled areas on the East and West sides of the dormer (roof above / room angled ceiling below) it has no openings, just a horizontal and verticle board that terminates the end of the rafters. Outside there is unvented aluminum white soffet panels.

If it needs ventilation, can I drill through it with my 3/4' self-feeding bit a few times and in spring cut out some 2" round vents on the exterior side? If so, do I drill the interior verticle board or the horizontal board at the end of the rafters to get into the exterior soffet space to get airflow???

Also, different question on same job. I prefer 4 mil plastic as a vapor barrier and put it down in such a way as to rise over each joist then drop into the next 24" space in a continous sheet (duct taping the seams). I felt that would be a tighter vapor barrier seal. One insulation company said that it's just fine to cover the horizontal joists with plastic. A different insulation company said it would trap moisture and cause rot/mold. I am at a point where I could reverse it if necessary. Next step is to add up to R-49 unfaced fiberglass batts. Please help! Question #1 is most important to me right now....

Please forgive any incorrect terminology used here. I'm no expert at roof design & insulation terms.

Last edited by gbpkrs; 01-27-05 at 05:50 AM. Reason: simplification
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Old 01-27-05, 07:59 PM
Join Date: Jan 2005
Posts: 197
attic venting

Let me see how many questions I can remember, and I'll try to come up with

the answers. You need about 2" of airspace between the insul. and the roof

boards, above those sloped dormer "walls". The actual thickness of the insul.

will depend on the dimension of your roof framing. If you have 2" by 8" rafters

in this area, you will probably be limited to R-19 insul. I would also use the

baffles to keep this airspace "honest". The use of gutters or not, doesn't

dictate how you insulate or ventilate the attic. Gutters can make an existing

ice problem worse, but contrary to popular opinion, they don't cause ice

problems. You mentioned in your original post about drilling 2" holes in the

soffits. That might help the venting problem, but expect to find lots of

"boarders" in the eaves, come spring.(small birds , bats , hornets, etc.)

Let me know if I forgot to cover any part of your questions.
Old 01-28-05, 08:34 AM
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Vapor Barrier Over Joists?

Thanks MI-Guy. I think I really only had 2 questions, I just get long-winded in trying to fully explain the situation. The curse of being a former instructor. Sat down with that roofer over breakfast today and determined exactly what you said. I need to add venting baffles above the dormer too.

The other question I had was would it hurt anything to put my 4 mil plastic vapor barrier over the joists. In other words, lay the plastic flat in the 24" space which is directly against the floor of the attic (or ceiling of the room), staple the plastic in the corners and then rise up and over the joist and down into the next 24" attic floor space, then up and over the next joist.

There are no air gaps or insulation trapped under the plastic when I do this. Will that trap moisture, cause rot or harm the joists in any way??? To put it simply, can a plastic vapor barrier go over the attic floor joists before insulation is laid down?
Old 01-28-05, 08:47 AM
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Soffett "holes"

Oh, in other areas I had been covering the holes made in the soffet boards with screening to keep the critters out , but not sure how how I can do it here because it's too hard to reach.

When I say "hole in the soffet" I am not exactly seeing daylight here. It is a vertical 3/4" thick board that had been closing off the airflow. When I drill these holes there I have provided air flow access to the triangular space outside which is the sofett area.

There is a unvented aluminum soffet panel and probably the original wood soffetts that I plan on getting round vents cut into when spring rolls around. I am not penetrating completely to the outside at this time.

Although I don't have a direct path from soffet to roof vent for ventilation at this time, the "leak-by" from the soffets to the attic space is producing a wind flow that I can feel. When I complete the job and the attic is properly vented from below, that will increase significantly airflow, I hope.
Old 01-28-05, 07:03 PM
Join Date: Jan 2005
Posts: 197
vapor barrier

I think you would be alright with the vapor barrier done in this manner.

If moisture suseptible material under a vapor barrier was a problem, then

we would see a lot of rot damaged drywall ceilings under vapor barriers.

To be on the safe side of this issue, put insul. above the plastic covered

joists, if it works between the joists, it ought to work here too.

I know ceiling vapor barriers are used in cold climates like mine, but I see

very few installed in this area. I don't see enough houses with ceiling vapor

barriers around here to come up with any good evidence that they actually

work better than houses without them. Maybe wer'e all in the dark, here

in Mi. In any case, I can't see where it could hurt, in a problem house at least

I would expect they would help.
Old 01-29-05, 06:43 AM
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Thumbs up Thanks!

Muchous Gracious MI-Guy!

Once finished, there will be more than 12" of insulation on top of the plastic that is on top of the joists (and attic floor as well). I figured a mostly continous vapor barrier might be better than one chopped at each 24" to allow for the studs. DOE says WI should have R-49, or about 16" total.

I have 2 upstairs rooms with these angled ceiling areas. It's no fun getting the plastic & insualtion down in each one of these section there since each one is a "pocket" of it's own, so I must cut plastic strips there - and bend/stretch quite a bit. Feel alot older than the 39 yrs I am.

If the woman sleeps as late as usual on Saturdays I'll go get a start on this right now.
Old 05-16-13, 09:32 PM
Join Date: May 2013
Location: USA
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Update on how it went...

I was reading your questions about the dormer insulation and I was thinking exactly the same with the baffles. I am currently in the same situation where the dormer was too tight with no airflow and started growing mold everywhere. It is obviously the hottest place in the house and most humid so i wasnt surprised... I was surprised to see no vents
Old 05-17-13, 06:51 AM
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8yr old thread and OP is not even a member anymore. If there is a question there, best to start a new thread.
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