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Old 09-24-2012, 01:56 PM   #1
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How would you Insulate THIS?


What do I use to insulate this (see pic). House located in VT. Continuous soffit vent and ridge vent. The rafters are 2x12. Help me with the cathedral and crawl space attic. I am thinking R-38C up to and over the crawl space, finished with 2" foam board taped.

Ceiling will be finished with T&G pine (I know I know).

Any thoughts?
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Old 09-24-2012, 03:37 PM   #2
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How would you Insulate THIS?


I love pictures!

1. How high are the stair risers?

2. Are the floor trusses bearing on another (perpendicular) header truss rather than the missing solid door header in the end wall at stairs?

3. Were the rafter ties/ceiling joists moved up higher than 1/3 the total rise (above 2nd story) or were they engineered to cantilever/bear that way?

4. Are you trying to hit R-49, for Code minimum or is there foamboard on the roof sheathing?

You are in Zone 6;http://publicecodes.cyberregs.com/ic..._11_par002.htm

Requirements; http://publicecodes.cyberregs.com/ic..._11_sec002.htm

Gary

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Old 09-24-2012, 03:58 PM   #3
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The height of the second floor is about 9.5 feet. The trusses are 18". There are only a couple trusses that are short to allow for stairs and they are headered with a header truss (see pic below). They were engineered to have a wall under. There is also a header truss under the basement stairs supported by posts. It's directly in line with the second floor header above that door.

I realize the code is R49. Fat chance on that. Good thing there are no inspections up in VT! I might get 48 with 2" foam board. Spray is not going to make the budget.

Framers added the rafter ties for strength. They are about 8"2 from floor if I recall.

Also sent a pic of the cathedral above the first floor, before they nailed up the rafter ties.
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Old 09-24-2012, 05:05 PM   #4
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How would you Insulate THIS?


Thanks! That answers #2, and 4...

Cellulose needs air above it to rid the moisture, as long as you keep that; http://www.buildingscience.com/docum...+in+flat+attic

Be sure to fire-block the ceiling plane in the back (end) wall, cavity should not be open the gable top plates. A good write-up; How to fireblock framing Not a big fan of f.g., though the higher densities are OK, if you have to, I guess:The "biggest loser" in fiberglass insulation.... I really should update that info.

Are you using foamboard exterior walls?

Gary
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Old 09-24-2012, 09:30 PM   #5
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If you are going to use foam board on the interior, why not foam to the exterior and just treat it as unvented assembly?

2" Iso to the exterior and 12" batt will get you the R-49.

Be sure to have a completely air tight drywall layer and you shouldn't have any moisture or condensation issues either.
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Old 09-25-2012, 07:41 AM   #6
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Originally Posted by Windows on Wash View Post
If you are going to use foam board on the interior, why not foam to the exterior and just treat it as unvented assembly?

2" Iso to the exterior and 12" batt will get you the R-49.

Be sure to have a completely air tight drywall layer and you shouldn't have any moisture or condensation issues either.
It's already roofed. I am not using drywall on ceilings. It's going to be pine T&G.

Are there any issues with the following plan:

1" baffles up into the crawl space/attic.
R-38C faced batts in the rafters, and in the attic crawl space
1" or 2" (depending on budget) foam board up to and across (under) the attic crawl space.
Seams calked/taped with tyvek tape
Pine T&G finish on ceiling.
R-21 in vertical walls
6mm poly sheathing over it
Finish vertical walls with drywall.

Anything I am missing?
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Old 09-25-2012, 09:06 AM   #7
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Originally Posted by aquasport17 View Post
It's already roofed. I am not using drywall on ceilings. It's going to be pine T&G.

Sorry about that. I was looking at the latest pictures and thinking it was still open. Doh!

Are there any issues with the following plan:

1" baffles up into the crawl space/attic.
R-38C faced batts in the rafters, and in the attic crawl space
1" or 2" (depending on budget) foam board up to and across (under) the attic crawl space.
Seams calked/taped with tyvek tape
Pine T&G finish on ceiling.
R-21 in vertical walls
6mm poly sheathing over it
Finish vertical walls with drywall.

Here is what I don't like. Faced batts, foam board, and a vapor barrier to the interior will created and effective vapor sandwich. They are duplicative and likely counterproductive in this case.

I would personally skip the baffles and use rigid foam ripped down into 1-2" strips to create the channel for the venting. Your roof is pretty steep so it is going to breath real well via convection even with a 1" vent.

I would then cover that vent space with the rigid foam between the rafters. If you were worried about summertime heat gain, you could use a foil faced ISO and face the foil toward the cold side but I would be hesitant here because of the lack of vapor permeability in the foil facing and the potential for the vapor trap as we discussed.

If you then put in your R-38 batt (slightly compressed and less subject to convection and settling), unfaced, and 1" foam to the inside of the framing, you would be about R-50 in a vented assembly at that point.

Might be the best of both worlds.

Be sure to seal up all the top plates at the outside walls and the attic.


Anything I am missing?


http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting/view
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Old 09-25-2012, 09:19 AM   #8
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Ok, I'm following.

That's double the rigid foam though. Not sure it's going to be cost effective.

I was not going to use plastic on the ceiling. Just the vertical walls.

What about using the baffles with unfaced R38C and then 1" rigid over the rafters? That would give me R42 with a vapor barrier from the rigid foam.

Maybe not BEST, but better?
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Old 09-25-2012, 02:47 PM   #9
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That will certainly work and R-42 that is vented is still really good.

I was just angling for the rigid foam in lieu of the vent chute because you mentioned 1-2" of foam to the interior and if you were going with 2", you could just split it and save on the cost of the baffles.

The rigid foam as a vent also provides more insulation value at that level than does the baffle and is less likely to get manipulated and blocked.

if you seal up the top plates where the drywall meets the top of the wall, you will largely seal up any moisture movement via air loss.

If code requires a vapor retarder (Class I), you have to use it.
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Old 09-25-2012, 03:01 PM   #10
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That will certainly work and R-42 that is vented is still really good.

I was just angling for the rigid foam in lieu of the vent chute because you mentioned 1-2" of foam to the interior and if you were going with 2", you could just split it and save on the cost of the baffles.

The rigid foam as a vent also provides more insulation value at that level than does the baffle and is less likely to get manipulated and blocked.

if you seal up the top plates where the drywall meets the top of the wall, you will largely seal up any moisture movement via air loss.

If code requires a vapor retarder (Class I), you have to use it.

Thanks!

I am assuming you mean that something like the Foamular 250 which has a permeability of 1.1 wouldn't b considered a class I vapor retarder? I'd still need to hang 6mm poly.

To summarize:

Cathedral/Crawl space: Baffles, R-38C unfaced, 6mm poly, 1" rigid foam.
Walls: R-21, 6mm poly, and of budget exists, 1" rigid.

Seal/tape/fireblock.


Getting there?
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Old 09-26-2012, 06:59 AM   #11
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You are correct. If you need a class I, a perm rating of 1.1 is not low enough. At a 1.1, you are actually Class III.
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Old 09-26-2012, 11:37 AM   #12
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Thanks for all the info.

What do you recommend for baffles? I have read a few negative things about the duro-vents.

Any specific order on the materials?

Baffles, R38C, rigid foam, 6mm poly for the last piece before the pine T&G.

Just want to make sure I have it right. I know that the pine T&G is going to leak air - so the poly is a must.

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Old 09-26-2012, 01:40 PM   #13
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Okay, so now I am getting confused. I just read on another forum, that using 6mil poly will CREATE problems for me in my walls in the summer time. Winter it's fine, but in summer it will lead to moisture problems. Not sure what to think at this point! I suppose since it's code in Zone 6, I'll install it.

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Old 09-26-2012, 03:36 PM   #14
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Code is code so if they are inspecting, you are going to need to show it.

The T&G will certainly leak air but if the rigid foam is there, it should serve as the finite and complete air barrier.

Moisture drive via summertime conditions is less of an issue when it comes to rot and other stuff as compared to wintertime diffusion and air leaks.

I personally don't care for dedicated vapor barriers and I think air is the bigger culprit but code is code.

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Old 09-26-2012, 11:23 PM   #15
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Quote:
Originally Posted by Windows on Wash View Post
If you are going to use foam board on the interior, why not foam to the exterior and just treat it as unvented assembly?

2" Iso to the exterior and 12" batt will get you the R-49.

Be sure to have a completely air tight drywall layer and you shouldn't have any moisture or condensation issues either.
--- you need at least R-25 foam, not R-14 for your location with a closed roof, since you are not going that direction- mute point anyway. Just don't want other readers in Zone 6 to get the wrong info.

With the foam on the inside- you are keeping the f.g. cavity insulation warmer (bringing the first condensing surface to the back of the inside f.b.) during the summer and run the risk of wet f.g. from night-time radiation at other times. I would use 1-1/2" wide 1" f.b. strips on the rafters and the 2" f.b. in the cavities (against 1-1/2" f.b. spacers) instead of the plastic baffles. Any moisture in the cavity can still dry through the f.b. (which would eliminate the radiation/condensation on the f.g.).

http://www.buildingscience.com/docum...bout-diffusion

4th para. below fig.3: http://www.buildingscience.com/docum...on?full_view=1

To use an interior v.b., you might get away with less foamboard, you need to figure the calc's., otherwise you would have a double vapor barrier (vapor sandwich) with a vapor barrier on EACH SIDE of the cavity insulation. Depends on the vapor drive/temperature/pressure differences.
2" would barely let moisture through; http://docs.google.com/viewer?a=v&q=...4EyM707m0gPdXQ

Gary

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