Replies

1. 
   Guest_ | Apr 22, 1999 07:29am | #1

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   Scott, well, er, no. Transpiration of moisture, condensation, etc... space is a good thing.

1. 
   Guest_ | Apr 22, 1999 08:23am | #2

   *ScottThere's an awful lot of old buildings around with claps and shakes/shingles nailed right onto the sheathing with nothing but tarpaper in between.

2. 
   Scott_Lewis | Apr 22, 1999 10:22am | #3

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   Patrick,

   Are you saying that those old buildings that you mention do or don't have moisture problems as a result of simply tacking siding to tar paper over sheathing? Those old buildings almost certainly have very leaky building envelopes, so moisture from the interior can probably escape quite easily. With the much tighter building envelope systems of modern, energy-efficient construction, however, is the that air gap adviseable or not?

3. 
   Guest_ | Apr 22, 1999 03:25pm | #4

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   Check out Building Science Corp's "Builder's Guide" for Cold Climates, Appendix 1 Good details and discussion. It'll give you more insight.

   The short of it: go w/ the screen (air space).

   http://www.buildingscience.com

1. 
   Guest_ | Apr 22, 1999 09:55pm | #5

   *Never put a gap on one.Never seen anyone put a gap on one.Never gave it any thought.But the reasoning for the gap, usually on roof shingles is that the shake becomes wet, won't dry out, and will rot and decay faster, without such a gap.Siding Shingles ought not to have that problem. They don't get direct water, and assuming a nice overhang (a detail missing in modern homes), at least 6-8' of the upper wall will stay nice and dry. Gutters will take any roof water away from the siding and on to the ground.So the answer is no, I wouldn't do it.

4. 
   Jim_T. | Apr 22, 1999 10:36pm | #6

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   I have a 100 year old house in NE Ohio with pine claps(painted) for siding. My walls have no insulation and no sheathing. Had to rip off the the bottom two courses of clapboards for the porch I'm installing. The two courses, after 110 years were in nearly perfect condition-the integrity of the unprimed underside was great. In essence, due to the lack of sheathing, it could be said I always had an air gap ( and a pretty big one at that) If I were in your shoes, my instincts would tell me to install an air gap. Good Luck!

1. 
   jcallahan | Apr 23, 1999 02:23am | #7

   *Joe L. and Building Science Corp. has forgotten more than I'll ever know about moisture and buildings but in this case I respectfully disagree. I've seen and worked on buildings both old and new with wood siding applied without an air gap and they have performed well in a cold climate (maine). The devil is in the details. As Scooter says, broad overhangs really help and other things like backpriming do, too. Buildings that I've worked on that had space behind the siding (vinyl, skip sheathed cement siding) were roach, mice, and snake motels- that screen won't last forever.

1. 
   don_hummer | Apr 23, 1999 05:20am | #8

   *Scott' I'd put in the gap particularly if you use Tyveckwith cedar or fir. It's being found that something in the wood is breaking down the ability for the housewrap to breathe. People are having to replace siding and sheathing due to rot and mildew.

5. 
   Guest_ | Apr 23, 1999 06:09am | #9

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   An answer drawn from personal experience(25 yrs.)
   I haven't seen a back-primed siding job fail yet. Air gap or no air gap. In roofing I've learned that there is no such thing as too much ventilation. I'm sure this applies to siding as well. But is it worth the extra cost in labor and material? That's a call only the customer can make. I did it on one job. I used lath strips and Cobra ridge vent cut down to 3"x14 1/2" strips between the lath at the bottom course. But I still back-primed!-John

6. 
   gerard_yankowski | Apr 23, 1999 06:47am | #10

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   Scott

   You will here alot about homes that are 100 years old that the claps are nearly perfect no air gap installed over plank sheating that is true. The problem is the quality of lumber 100 years ago or for that matter even 30 years ago was substantially better. I know of homes that have no air gap built in the 60's and 70's and the siding is still in perfect shape, that siding was also "Clear Heart Wood" or "Clear and Better" that when installed cost was not a factor as there was plenty of "Clear Cedar" of "Large White Pine" left. Keep that in mind the higher in quality you go in the wood you use the less of a factor an air gap becomes. I have seen 100 year old homes on the shores of Lake Michigan in far nothern Michigan in a harsh winter enviroment, (Blowing water, deep snow, drifts..) that the siding was in perfect shape, again virgin "White Pine Logs" that were "Quartersawn".

7. 
   Guest_ | Apr 23, 1999 04:34pm | #11

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   Scott

   I think the jury's still out on whether to gap or not. My advice would be whatever you decide to do try to think of the building envelope as a system, not just as individual components. Should you decide to gap it understand the gap works as part of a rainscreen system. That is, if you accept that the outer skin is a weather permeable layer then the pressure-equalized (vented to the exterior) gap behind it allows any wind-driven water to drain away instead of being sucked into the building by capillary action. This system presumes that the true weather-barrier is behind the exterior finish, which is in fact what we assume when we use sheathing materials like Typar or felt paper. If you intend to sheath the building with styrofoam, as is advocated by the Building Science Corporation in their Cold Climate System Design then I would definitely use an air gap by strapping the surface of the styrofoam. I have seen one instance of catastrophic failure of wood siding (on the south face only) where the builder ingnored the instructions of the siding manufacturer and applied a pre-finished expensive wood siding product directly on the surface of the styrofoam. The siding curled and buckled so badly the first year it was amazing. The rest of the house was fine. My guess is that in direct contact with the insulation, the siding's temperature fluctuations caused the problem. Perhaps someone out ther has another theory.

   Finally I would always recommend back-priming, with or without an air gap.

1. 
   Guest_ | Apr 23, 1999 05:15pm | #12

   *Back prime over felt...and if foam is involved and in contact with wood sheathing and or siding too...well all bets are off as to whether weather will wipe wood out with rot!Near the stream, where sunny sides and damp sides are difficult to detail at best,J

1. 
   Guest_ | Apr 23, 1999 05:36pm | #13

   *I was thinking about this myself the other day. I Haven't made up my mind yet, but if I was to space it, I would think the ultimate installation would be a layer of cedar breather between the sheathing and the siding to eliminate the infestation possibilities. Could get real pricey though.I've seen 100+ year old claps in ground contact that were still healthy. They just don't make wood like they used to.Steve

8. 
   Scott_Lewis | Apr 24, 1999 04:23am | #14

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   Steve,

   What do you mean by a "layer of cedar breather"? Would this be a form of strapping? I'm trying to picture how infestation possibilities could be eliminated.

   Scott Lewis

9. 
   Guest_ | Apr 24, 1999 06:45am | #15

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   Scott

   When Steve said it might get expensive he wasn't kidding. . . he plans to buy a whole truckload of those little boxes of aromatic cedar clost liner and cover the whole exterior to keep the buggers away, LOL

   Actually the theory of aromatic cedar as a bug repellant has been pretty much discredited. . . I found out the hard way having built my wife a dovetail blanket box with the cedar liner to store all our precious hand knit winter woolies in. . . one year moths selectively made a mess out of some stuff that hadn't been carefully washed before storage. . . the moths go after the body oils cedar or no cedar!!!

1. 
   Guest_ | Apr 24, 1999 07:23am | #17

   *Scott-I believe the cedar breather that Steve is talking about is a plastic mesh laid over roof decking and under cedar shingles. It creates an air space so that the cedar shingles can dry out from underneath. It is about 3/4" thick and mosly air. If you have ever seen Cobra brand conceiled ridge vent, its basically the same material-just in bigger rolls. Don't know the price-just heard that its very expensive.

10. 
    Scott_Lewis | Apr 24, 1999 07:23am | #16

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    Should wood siding necessarily have an air gap between it and the wall sheathing behind it in a cold climate? The argument for it (that it allows inadvertent moisture from interior vapor, rain, etc. a way to escape) sounds logical, but couldn't the gap become an open invitation for termites and exterior vapor as well? Assuming that interior vapor infiltration is controlled by a well-designed interior vapor barrier and that rain infiltration is controlled by carefully-installed and carefully-painted siding under sizable roof overhangs, couldn't conventional back-primed wood siding installed directly against housewrapped (e.g., Tyvek) sheathing be the better, safer design?