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Question DetailsAsked on 6/13/2017

should asphlat felt be used on exterior walls?

I have plywood sheathing and have been advised to put felt on the wall prior to wood shakes. I don't want to cause moisture problems in the wall.

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Assuming you do not have another vapor retarder within the wall (except for likely a vapor barrier directly under the interior drywall or plaster), then yes you should have a vapor retarder 9not a vapor barrier) under the exterior siding. This assumes you do not have a vapor barrier under the exterior siding, which is used instead of an interior surface vapor barrier in some very constantly wet or hot humid environments - but you should never have two true waterproof or vapor barriers in a wall because they will trap moisture in side the wall.

Normal construction for your situation, assuming interior vapor barrier (you should be able to see by removing an electrical outlet cover and see if there is vapor barrier edges showing there and by opening up a small area of the exterior - remove a piece of sheathing to check what the wall layering is made of), would be a Type I vapor retarder (not a totally waterproof layer) under the shakes. Old-school they would be nailed directly into the sheathing through the vapor retarder which would by TYpe I felt (15#) - modern method would more commonly not have the plywood sheathing and the shakes would be installed with a synthetic vapor retarder sheet like the appropriate Tyvek housewrap under them, placed over or under horizontal firring strips at the correct exposure spacing which would be over and nailed/screwed to the studs.

With the plywood there, it is better practice to have firring strips anyway even if not needed for the support, (meaning the sheathing is intact and at least 1/2 inch thick for proper nail grip), because an air gap between the sheathing and the shakes provides airspace to allow for dissipation of the moisture which WILL come through the shakes. Putting the water barrier OVER the firring strips protects the house wall from moisture better but means the backs of the shakes stay wetter so shortens their life, putting it under the firring strips (which would mean they should be cedar or treated wood for rot resistance) provides the better airspace under the shakes - but the firring strips should then have a small space behind them so any water on them can drain down the face of the water barrier without getting trapped on the horizontal strips. Some of the best siding contractors either cut drainage saw-kerf slots in the back of the firring strips for drainage or put spacer headed nails or staple a 1/8" or so spacer piece like a roofing water barrier plastic nail washer (which the firring strip nails/screws go through when the strip is put on) on each stud at the firring strips to space them out a bit from the water barrier. Others put a piece of 1/2x2 lath vertically over the water barrier at each stud then fasten the firring strips through that to the studs - providing a 1/2" gap all along between the water barrier and the firring strips. Others ignore that issue entirely and count on any water buildup finding its way between the firring strips and the water barrier on its own - not ideal but usually adequate and generally allowed per code.

Felt versus synthetic - the felt is easier to do alone, but the synthetic is supposed to be longer lived (has not been around long enough to prove that) but does take 2 people to handle the much larger pieces - though also takes a lot less time to put up. I would not obsess about one versus the other - I have seen 40-60 year old buildings with sidign leakage which had zero to minimal water penetration past the roofing felt - though granted that was true 15# felt, not the much thinner and lower asphalt content so-called #15 felt made today.

Answered 3 years ago by LCD

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