[GSBN] windscreen

[jfstraube]

A rainscreen is very poorly defined and means different things to different people.
My definition is that a rainscreen is a cladding that allows a proportion of impinging rainwater to penetrate.  Hence brick, stucco, fiber cement, wood siding, shakes, etc are all rainscreen claddings and all require other measures to deal with the water that penetrates.  In common practise, a drainage gap (from 1/16" to 2") is provided, and a drainage plane (#15 felt, Grade D paper, Tyvek etc) is used to drain the water down to flashing, which is a waterproof component tat redirects liquid water to the exterior via drainholes (weep holes, as in, "if you forget them, you will weep").
In strawbale, stucco is applied directly over SB and hence must use a storage approach: all water that penetrates must be safely stored in the mass of the stucco and straw.  As the stucco is kind of thin (like 1-2") and the straw moisture susceptible not much can be tolerated.
The reason we like overhangs, is that they reduce the amount of water on the wall, extending the range of use of a SB wall.  Adding a screen over an airgap, will extend the range of performance even further when combined with overhangs.  If we have a highly perforated screen, the protection offered is less than if the screen is mostly solid.  So a 20 ga sheet metal screen with 1/8" holes at 1/4" spaces will work rather well, but it would be better to have no holes and lap the joints to reduce the water that penetrates.  If you were to install an insect screen (like the window stuff) it will improve performance, but perhaps not enough to work in a highly exposed and windy location.

The other possible function one can add to a cladding, rainscreen or not, is to make it ventilated.  By opening air gaps top and bottom, we can improve the ability to dry out the backup wall or allow vapor from interior to escape, and to allow drying of retained penetrant water.  A very open screen does this very well.  A closed sheet metal screen less well.  So it is not always obvious that a more rain leaky screen wont work as well or more poorly than a tight screen because ventilation drying can be a big deal.

My slam dunk solution I meant was the 5 cm air gap.

Build an SB wall like you always would, with concern for overhangs, windows, splashbacks etc.  Then attach vertical strips of wood, say 2x3's, to the stucco at 24" centers or so. Now add cladding of your choice to the furring leaving insect screen protected inlet and outlet openings that are at least 1" in size continuous slot at both the top and the bottom.  This approach will work quite well in even extreme climates.

John



On 2011-01-21, at 1:12 PM, Bob Theis wrote:

> John Swearingen's comment about bending rainscreens brings up an interesting point I'd like clarified:  If the primary function of rainscreens is to break the force of the wind, then the "screen" needn't be shingled planks as we generally see, but any manner of screen with smallish openings set away from the wall behind, yes?
> 
> If that's the case, is there any guidance regarding  how small the openings should be?
> 
> Yes, yes, it will depend on wind speed,  the thickness of the screen,  and variables I'd never imagine, but I'd be happy with an oversimplified  generality to guide evaluating screen material possibilities. If, for example, I want to make the screen a grill on which I grow vines, how dense would it have to be to work, absent the growies?
> 
> ...and my team wants to know how to do the slam dunk home run John Straube mentions.
> 
> 
> On Jan 21, 2011, at 3:51 AM, John Straube wrote:
> 
>> I like both mineral slicate paint (which usually has added water repellents) and the slam dunk home run solution of adding a ventilated and drained rainscreen cladding.
>> A less extreme solution that may work is a siloxane / silane blend which is clear, penetrates into pores and makes the waterproof. Drying can happen faster than with Xypex and thompson water seal (pore blockers, one with mineral crystals the other with organic wax and gum).
>> 
>> Sent from my BlackBerry®
>> 
>> -----Original Message-----
>> From: Darrel DeBoer <Darrel at DeBoerArchitects.com>
>> Sender: GSBN-bounces at greenbuilder.com
>> Date: Thu, 20 Jan 2011 23:59:27
>> To: <GSBN at greenbuilder.com>
>> Reply-To: "(private, with public archives) Global Straw Building Network" <GSBN at greenbuilder.com>
>> Subject: [GSBN] Exterior plaster sealer?
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John Straube
www.BuildingScience.com