# How to Vent and Insulate a low use cabin in a hot climate



## NotoriousAPP (Aug 11, 2013)

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

I’m having a Derksen Lofted Barn cabin built and there are some options which I can’t change, specifically the roof rafters which will be 24” 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 don’t get the same feedback from any of the builders or insulation companies down here. I’ve even had a spray foam installer (who is very reputable) tell me that he wouldn’t recommend using spray foam (closed or open) since I won’t 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 it’s 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 don’t 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 16x40’ 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 3” holes in between each rafter if I decide to use a soffit-ridge vent option

-I will be using R13 fiberglass in the walls. (I’m 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 24” off the ground. I do plan on adding skirting around the building.

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

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

Here are some scenarios which I’ve 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 I’ll 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
1” Foam board 
1” air gap between foam board and roof decking (I would install 1” 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
2” 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. Let’s see if we can get more than one person to agree on a common method.

Drawings are attached.

Thanks.


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## Windows on Wash (Aug 30, 2011)

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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## NotoriousAPP (Aug 11, 2013)

Windows on Wash said:


> 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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## Windows on Wash (Aug 30, 2011)

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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## Gary in WA (Mar 11, 2009)

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

http://www.buildingscience.com/documents/reports/rr-1110-hygrothermal-analysis-california-attics

No facing on insulation; http://www.buildingscience.com/docu...heet-310-vapor-control-layer-recommendations/

Gary


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