Modeling Ventilated ("rain screen") siding in WUFI

[graham]

I am currently trying to analyze a wall with ventilated siding in WUFI and have found numerous suggested approaches, with perhaps the most involved being an iterative estimation process described here (p. 18): http://www.buildingscience.com/documents/reports/rr-0907-ventilated-wall-claddings-review-performance-modeling.

Does anyone have a good sense of what is acceptable practice?

[Adam Cohen]

This is a tough one, the biggest challenge is the air changes behind the siding.  There has been some testing on this by ORNL and the measured changes vary widely.  They vary by configuration as well as by orientation and absorbtivity and emissivity of finish.  When I look at these type of things I tend to be very conservative.  One can get amazing results by  increasing air changes.  i am not sure in the real world what the answer is.

Bottom line, if the results do not pass the gut check, rerun more conservatively

On Mon, Nov 3, 2014 at 6:16 PM, graham <gra...@essentialhabitat.com> wrote:

I am currently trying to analyze a wall with ventilated siding in WUFI and have found numerous suggested approaches, with perhaps the most involved being an iterative estimation process described here (p. 18): http://www.buildingscience.com/documents/reports/rr-0907-ventilated-wall-claddings-review-performance-modeling.

Does anyone have a good sense of what is acceptable practice?

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Adam Cohen
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[Linda Whaley]

In one of the WUFI training classes Achilles talked about ventilation verses venting and recommended using a 50ach for a ventilation setup in WUFI.  This falls in line with what is mentioned on page 7 of the article you linked to and with what Adam is saying about being conservative: 

"For a 1.22 m wide by 2.4 m high brick wall with a 20 mm deep cavity with two open

head joints (no bug-screen) at top and bottom, ventilation rates were predicted and

confirmed to be between 0 to 90 ACH or 0 to 0.50 l/s/m2 of cladding."

"It's not what you look at that matters, it's what you see." Henry David Thoreau

[Hayden Robinson]

Here is an excerpt from my notes from a late-2012 WUFI workshop with Achilles and Andre:

 

        To create a ventilated cavity (air has both a way in, and out), create a material layer using "Air" from "Generic Materials" Then, on the "Assembly/Monitor Positions" window, click on the "Sources, Sinks" button. Then click on the "New Air Change Source" button. See instructions for creating a leak, below. Under "Spread Area", select "Whole Layer". Under "Source Type", select "Constant". At "Air Changes [1/h]", enter the appropriate value. (Air-change rates for ventilated cavities are between 25 and 50 for most cases, and there is not much difference between those two values; see ASHRAE TRP 1091.) For a drainage plane (such as can be created by the adhesive beads used to adhere exterior insulation), 5 - 10 ACH - assuming venting flashings are provided at top and bottom.

 

 

Hayden Robinson Zertifizierte Passivhausdesigner

hayden robinson architect

206.691.3445

[graham]

Thanks everyone! I think I have enough to make a start at this. I may try the iterative method if I summon the ambition.

It would be SUPER helpful to have a published concise set of conventions for using WUFI. It is a program that is super sensitive to the inputs.

[graham]

Along similar lines, does anyone have insight into how to model a "vented" vs. a "ventilated" arrangement? Specifically, that which is created with stucco wrap and similar products? What size gap does one use?