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Rainscreen Question

Jun 20 '13 7 Last Comment
washingtonian
Jun 20, 13 1:48 pm

I'm designing the wall system of a shed and want to use a rainscreen concept. It'll be horizontal 2 x 6s with .5" gaps as cladding. My question is, since this is an unconditioned shed do I still need a proper breathable membrane behind it? I saw an example of another shed which simply used roofing felt as the membrane and was trying to get some feedback on whether this might perform well enough. Of course cost is an issue too so if I can avoid the high-tech membrane and still maintain durability I'd like to. Thanks!

 

wurdan freo
Jun 20, 13 2:13 pm

In my opinion, the shed typology, will not have insulated exterior walls, conditioned interior space or habitable interior space. It will be used primarily for storage. That being said, roofing felt is perfectly acceptable as a water barrier.  I would reconsider the 1/2" joints and go for 1/8". Why 2x6s and not boards? Also if you're using pine, I wouldn't expect your siding to last very long.  You could try burning it. If you buy it your self and burn it yourself, could save you money. Still not sure how carbonized pine would hold up. 

gruen
Jun 20, 13 2:40 pm

Felt will be fine. 1/2" joint is fine. 1x material is cheaper. Paint it and pine is fine, but if raw use cedar

Rusty!
Jun 20, 13 2:45 pm

What's the point of a rainscreen on an unconditioned structure?

gwharton
Jun 20, 13 2:58 pm

A pressure-equalized rainscreen system needs an air barrier behind the rainsreen assembly and cavity. Air barrier membranes are different from water vapor membranes. You can't use them interchangeably.

It also needs to be compartmentalized so that air-pressure differentials do not occur across different faces of a building. Thus, any corners need to be fully blocked to prevent air communication between faces on either side of the corner.

The 1/2" joint gap you mention will likely work for allowing the air pressure outside and behind the rainscreen to equalize quickly. Make sure the system is also open at the top and bottom.

Rusty: Using a rainscreen cladding system is for keeping water from penetrating the building envelope. It doesn't matter whether the building is conditioned or not.

Rusty!
Jun 20, 13 3:15 pm

gwharton: it's an overkill. air pressure differential will be close to 0. Ideally indoor is directly vented to the outside. unconditioned space and all.

It will look ugly too. 1/2" gaps showing roofing felt in the back.

Miles JaffeMiles Jaffe
Jun 20, 13 7:56 pm

As Rusty noted, the conditions necessitating a rainscreen do not exist in your application.

If you insist on this idiocy, 1/2" spacing is probably the minimum as a 6" board will measure between 5-1/8" and 5-1/2" depending on when and where you measure. Hell, you'll have that much variation in the pile before it gets nailed up.

Keep in mind that an exterior cavity makes a nice home for all sorts of critters like wasps, hornets, spiders, mice, etc. It also collects all kinds of pretty debris like dirt, leaves and twigs. Damp places where the sun doesn't shine have a tendency to grow mold and fungus.

Board and batten, clapboard, shiplap and shingles are all time tested and proven water barriers that don't need a membrane or sheathing. Red, white or yellow cedar and cypress are all good choices (although the quality and weatherabilty of red cedar has diminished considerably with hybridization, and yellow cedar is being clear-cut and not replanted). Using 2x material for siding is a complete waste. Even 1x is wasteful compared to clapboard. There are plenty of old barns where you can see daylight through the cracks - in the roof no less - that are bone dry in the rain.

Pine is an invitation to disaster whether painted or not. And why anyone would want the costly maintenance of exterior paint - especially on a shed - is a mystery. Painting the outside after installation will leave the inside unprotected and exacerbate cupping and movement.

All in all, a really bad, wasteful idea fraught with problems.

washingtonian
Jun 20, 13 8:59 pm

Thanks for the responses.

Miles- I understand 2x cladding is pretty wasteful- the main reason I considered it is that I have some in the existing structure (to be demolished) on the site that I could probably use for about 1/3 to 1/2 of the material. I'll definitely check out those other options you mentioned.

All of the other points regarding the durability of the material are good and I'll think about them. There is a source for cedar near me (a mill I think) that I'm going to investigate, I'm not sure if it's red or yellow cedar though. 

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