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insulating a truss and purlin roof


william_04_x's Avatar
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Join Date: Nov 2006
Posts: 39
NV

01-20-07, 09:21 AM   #1  
insulating a truss and purlin roof

hey guys,

i'm going to be building a log home.. i am a little concerned with the method of roof/ceiling construction and how to insulate it to a min R-38..please look at this drawing..

http://www.putfile.com/pic.php?img=4558361

as you can see there is maybe 4" to work with and a rigid foam must be used..the home is sold as a kit and the home mfgr recommends using this product...

http://www.p2insulation.com/index.asp?id=2

in a 2" application they claim an R-54 value..it is 2" XPS foam board with foil on one side and a vapor barrier material on the other..it's hard for me to believe that this product could be this effective, especially when their pitch-man is a styrofoam cup..

does this type of roof/insulation system require ventilation? any advice on how to properly insulate this type of ceiling would be greatly appreciated...
TIA,
william

 
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resercon's Avatar
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Join Date: Nov 2001
Posts: 1,873
NJ

01-20-07, 09:37 PM   #2  
Cold Roof

The water vapor molecule is so small that it will permeate all materials. This is one of the reasons why all building materials have "Perm Ratings". The lower the Perm Rating, the longer it takes for moisture vapor to permeate the material. For example a material to qualify as a vapor barrier it must have a Perm Rating of 1 or less. In other words, vapor barriers do not stop moisture flow, it slows it down. This is because no material can stop moisture flow through it. Most roofing materials by design have low Perm Ratings. Even though vapor barriers slow down the moisture flow through insulation, as it reaches the cold side of the insulation, the Relative Humidity (RH%) increases. This is because as the temperature drops, RH% increases. This brings us to the primary purpose of attic ventilation, that is to bypass the low vapor permeability of roofing materials.

Once you installed the insulation as in your diagram, you should install a cold roof. This is done by installing 1x2 slats from your eaves up to the ridge 16 inches on center. You have to use screws long enough that will go through the insulation to the framing below it. Install at least 1/2 inch plywood over the slats. At the bottom edge of the roof install a perforated drip edge.

http://www.popularmechanics.com/how_to_central/home_clinic/1275281.html

Like the one on this site. At the top of the roof, after you installed the roofing, install a Ridge Vent.

Basically what will occur is air will enter through the drip edge vent and travel through the gap between the plywood and insulation created by the 1x2 slats. Then out the ridge vent, taking moisture that manage to get through the insulation with it. For your information, the primary processes for this moisture transfer are known as Convection and Equilibrium Relative Humidity (ErH%).

 
william_04_x's Avatar
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Join Date: Nov 2006
Posts: 39
NV

01-21-07, 08:07 AM   #3  
that drip edge vent is a perfect solution to my problem...thank you for the link resercon..

william

 
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