Insulating under wood or engineered wood siding


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Old 08-09-07, 09:55 PM
J
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Insulating under wood or engineered wood siding

Hello, I would like to replace my existing 40 year old lap wood siding. Am looking at either cedar tongue and groove or Temple Inland engineered wood (either siding panels or siding). Current siding is over about 1 inch black celotex (impregnated fiberboard). I want to tear off the old siding, insulate with polyiso foam and put on new siding. I need to keep the celotex for structural purposes.
Questions:
1. Do I need strapping (the little strips) underneathe engineered wood?
2. Do I need to leave an air gap between the (engineered) wood siding and the foam sitting between the straps?
3. Does the (additional) insulation layer act as an outside moisture barrier trapping the condensation inside the walls?

Any comments on this would be great.
Thanks!
Joern
 
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Old 08-10-07, 10:46 AM
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1). If you are referring to vertical furring, such as to create a rain screen / air sapce behind the siding, the answer would be no... you do not NEED it, but rain screens are an excellent idea that promotes drying which theoretically helps paint jobs to last longer.

2). If I understand you correctly, and you want a rain screen air space behind the siding, you would not want to put the foam between the furring. You would first install the foam (taping all seams to make it a weather resistive barrier), and then you would install the vertical furring over the foam to create the rain screen / air space. If you do not want the rain screen / air space behind the siding, you could certainly put foam between the vertical furring. But all those seams and edges will reduce the effectiveness of the foam somewhat. It would also not be a weather resistive barrier unless you planned to use a case of flashing tape to tape all those seams.

3). It could. But it does not mean that it WILL create a problem. There are houses all over the country that have interior vapor barriers and they have foam sheathing on the exterior... effectively creating a "double vapor barrier". But there have been no large scale problems related to this, or it would have been written into the IBC years ago. Sheathing that is effective at stopping air (like foam) that has all joints sealed with building tape actually helps prevent air infiltration into the wall system. It is actually air loss / air intrusion that carries moisture into the wall cavity. So in theory, if you can prevent the air exchange you are practically home free.

Most of the nightmare-ish stories about double vapor barriers occur in the deep south where dewpoints are always very high in the summer and mold is always a potential problem when you combine high humidity and dewpoints with air conditioning and air intrusion. When such walls are covered with EIFS and are not flashed correctly, this adds to the problem.

The only other thing I might add is that if you intend to build out your sheathing, have you considered what this will do where the built out siding meets windows and doors? This is a potential problem area since water infiltration can occur in these areas. Especially if the siding will be built out past the window sills. When taping the seams of the foam (creating that weather resistive barrier) you also have to seal the perimeter of every door and window or else you are creating a leak potential.
 
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Old 08-14-07, 03:07 PM
J
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Thanks and follow up

XSleeper , thank you so much for the detailed answers!

Some more comments on the whole project:
I thought about house wrapping/tyvac-ing the (existing) impregnated fiber, taping around the window trim boards.
Then apply 0.75inch vertical strapping and 0.5inch polyiso between strapping (over the tyvac). This way there would be a 0.25in air gap in between the foam and the wood. The gaps in the foam sheathing would (sort of) enable the wall to breath.

I only thought about the strapping because the current wood siding does not hold well close to the windows, probably due to a lack of studs unerneath the blackboard or lack of nails going into the studs - I don't know until I rip siding off and test it.

If I use 0.5 insulation then I would have to nail extra trim on top of the window trim. The window sills are sticking out 0.5inch past the trim, so maybe I get by with both beeing flush.

Alternativly if I could just nail directly through the foam and blackboard into the studs then I would tape the foam off as a complete vapor barrier.


Hope this clarifies the whole idea some more. I generally like wood siding and hope only having to paint every 5 years.

Joern
 
 

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