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Old 08-11-2013, 05:32 PM   #1
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How to Vent and Insulate a low use cabin in a hot climate


I can't believe there are so many different opinions regarding insulating and venting a cathedral/gambrel roof.

Iím having a Derksen Lofted Barn cabin built and there are some options which I canít change, specifically the roof rafters which will be 24Ē on center 2x6 boards. I really need some help deciding what to do with the roof venting and insulation. It seems like most opinions are in the context of colder climates like northern US. I donít get the same feedback from any of the builders or insulation companies down here. Iíve even had a spray foam installer (who is very reputable) tell me that he wouldnít recommend using spray foam (closed or open) since I wonít be using the cabin that much, he was concerned about creating a sealed building

Here is the situation:
-VERY IMPORTANT: I live in central texas where we get 105 degree summer days and some winter nights in the mid teens. Most of the year itís warm here and I would be running the AC (window unit) in the cabin. It does not rain here often.

-VERY IMPORTANT: This is a weekend retreat cabin. I will only occupy it for 0-6 days per month in both the summer and winter. I donít plan on running any air conditioners or heaters while no one is in the unit.

-The roof is the ceiling for the building and vice versa.

-The cabin is 16x40í with a gambrel roof which comes with a ridge vent installed but sealed off with black plastic (bridging the two sides of the roof on top of the OSB decking; I can simply cut the plastic open if I want to use the vent.

-There are no soffit vents installed anywhere and there is very little room to do so; the best I could probably do is to drill several 3Ē holes in between each rafter if I decide to use a soffit-ridge vent option

-I will be using R13 fiberglass in the walls. (Iím not sure yet whether they will be faced or unfaced; the cabin is wrapped in Tyvek.

-There is no insulation or vapor barrier below the foundation. The foundation will be on pier and beam 24Ē off the ground. I do plan on adding skirting around the building.

-The building does not require an inspection so I donít need to follow code but would like to get as close as possible.

-I donít want to say that money is not a concern, because it is, but I donít want to do something because itís cheap now and pay for it later.

Here are some scenarios which Iíve compiled from the 15 different opinions I received. My goal was primarily to make sure the cabin had no condensation or other moisture problems and secondly to provide adequate insulation considering the low number of days Iíll be using the cabin. Each of the scenarios of from the interior of the building to the exterior.

Scenario 1: with soffet and ridge vent
Tongue and Groove Paneling
Unfaced R13 Fiberglass insulation
1Ē Foam board
1Ē air gap between foam board and roof decking (I would install 1Ē pieces of wood to provide stand off between the roof decking and the foam board)
Roof decking
30# felt paper
Corrugated metal roof
-I would add no additional ceiling vents.


Scenario 2: with no soffet and or ridge vent
Tongue and Groove Paneling
Unfaced R13 Fiberglass insulation
2Ē closed cell spray foam
Roof decking
30# felt paper
Corrugated metal roof
-Add 4 ceiling vents which would allow moisture to escape through the roof. They would always be left open when the cabin is vacant and likely open most of the time when the cabin is occupied

Any thoughts. Letís see if we can get more than one person to agree on a common method.

Drawings are attached.

Thanks.
Attached Files
File Type: pdf Cabin 16x40 - PDF.pdf (63.8 KB, 29 views)


Last edited by NotoriousAPP; 08-11-2013 at 05:37 PM. Reason: add attachment
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Old 08-14-2013, 08:57 PM   #2
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How to Vent and Insulate a low use cabin in a hot climate


If you are doing the roof, have you considered just doing and over deck type venting for the roof?

Going will metal will certainly keep the heat out of the assembly more effectively than the shingles.

You are in a dry climate so moisture venting becomes less of a concern given the arid conditions and the lack of use.

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Old 08-16-2013, 01:45 PM   #3
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How to Vent and Insulate a low use cabin in a hot climate


Quote:
Originally Posted by Windows on Wash View Post
If you are doing the roof, have you considered just doing and over deck type venting for the roof?

Going will metal will certainly keep the heat out of the assembly more effectively than the shingles.
I didn't consider it but I am bought into this idea. Very common sense and I bet it would help a lot.

I already have a metal roof.
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Old 08-16-2013, 08:51 PM   #4
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How to Vent and Insulate a low use cabin in a hot climate


There are a ton of good articles about it.

That would be my approach and leave the framing depth for full insulation value.
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Old 08-17-2013, 09:28 PM   #5
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How to Vent and Insulate a low use cabin in a hot climate


http://www.buildingscience.com/docum...n-roof-venting

http://www.buildingscience.com/docum...d-roof-systems

http://www.buildingscience.com/docum...ifornia-attics

No facing on insulation; http://www.buildingscience.com/docum...commendations/

Gary
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Last edited by Gary in WA; 08-17-2013 at 09:43 PM.
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