Vancouver Field Exposure Facility: Analysis and Comparison of HardiePlank Walls Research Report - 1306 17 May 2013 Jonathan Smegal http://www.buildingscience.com/documents/reports/rr-1306-vancouver-test-hut-hardieplank-walls

So HardiPlank with rainscreen is better than stucco without a rainscreen in a very humid rainy climate with intentional water leaks on the inside and outside. Is HardiPlank better than stucco? I could not discern from the results whether it was the HardiPlank or the rainscreen that made the difference in the moisture content of the sheathing.

It looks like an interior side polyethylene vapor barrier outperforms latex paint in their walls.

Posted By Lee Dodge on 17 May 2013 11:01 PM
So HardiPlank with rainscreen is better than stucco without a rainscreen in a very humid rainy climate with intentional water leaks on the inside and outside. Is HardiPlank better than stucco? I could not discern from the results whether it was the HardiPlank or the rainscreen that made the difference in the moisture content of the sheathing.

Its the rainscreen, stucco is porous,
even Vinyl siding needs a rainscreen

Chris,

We can speculate, but they had a test all set up where they could have made a proper comparions between stucco versus HardiPlank with the same rainscreen and/or lack of rainscreen, but they failed to make that test.

I'm not convinced vinyl siding needs a rainscreen in most climates, but putting up stucco without rainscreen is to court danger in any climate, even if it (almost) made it in the Vancouver hut testing. Vinyl is inherently back-ventilated, and not very porous- it takes a huge driving rain to push much bulk moisture in, but it lets it out pretty fast.

To draw any true comparisons between the Hardie & stucco would have required testing both with and without rainscreens for both siding type. It's not a highly spectulative conjecture to state that stucco WITH rainscreen would do a lot better than stucco without, independent of what they were using for an interior side vapor retarder.

BTW: Vancouver is a very rainy climate, but it's not a very humid climate. The outdoor dew point averages are nearly as low as Colorado in summer. Winter dew points bounce against the outdoor air temps a lot (the foggy-dew coastal mist), but only for a few hours a day. Most of the time the air is dry enough even when it's raining to provide significant drying, provided there is a rainscreen gap to work with. It's the bulk-water wetting sans rainscreen that causes most of Vancouver's weather related moisture problems in buildings.

Dana1, all it takes is a spashing off the ground or a roof/side wall to find it's way behind any cladding.Raw wood needs protected behind sidings or stucco, and stucco now in our area requires 2 barriers.

More data here. Note the effect of large overhangs - a one or two story house with good overhangs might have no moisture problems in a wall with minimal or no rainscreen.

Posted By Dana1 on 20 May 2013 06:26 PM
...snip...
BTW: Vancouver is a very rainy climate, but it's not a very humid climate. The outdoor dew point averages are nearly as low as Colorado in summer . Winter dew points bounce against the outdoor air temps a lot (the foggy-dew coastal mist), but only for a few hours a day. Most of the time the air is dry enough even when it's raining to provide significant drying, provided there is a rainscreen gap to work with. It's the bulk-water wetting sans rainscreen that causes most of Vancouver's weather related moisture problems in buildings.

From WeatherSpark for Vancouver:
The relative humidity typically ranges from 55% (mildly humid) to 96% (very humid) over the course of the year, rarely dropping below 43% (comfortable) and reaching as high as 100% (very humid).

So Weatherspark considers Vancouver mildly humid to very humid.

From WeatherSpark for Colorado Springs:
The relative humidity typically ranges from 24% (dry) to 81% (humid) over the course of the year, rarely dropping below 10% (very dry) and reaching as high as 97% (very humid).

Range of average relative humidities for Vancouver over the year:
Daily maximum: 89% to 96%
Daily minimum: 55% to 74%

Range of average relative humidities for Colorado Springs over the year:
Daily maximum: 71% to 81%
Daily minimum: 24% to 33%

Posted By cmkavala on 20 May 2013 07:00 PM
Dana1, all it takes is a spashing off the ground or a roof/side wall to find it's way behind any cladding.Raw wood needs protected behind sidings or stucco, and stucco now in our area requires 2 barriers.

Stucco is very porous and all the stucco homes out here in Phx during the monsoon rainstorms get saturated with moisture and the water goes behind the stucco. The Styrofoam backing for the stucco has too many gaps in it and water finds it's way behind that. The tar paper is the only thing that stops the water but if the tar paper is ripped, improperly lapped over, you will get water leaks like I had and thousands of homes out here get every year.

Water was literally pouring in through the drywall on the west side oh the home during a wind driven rain. It was a solid wall, no windows. When they ripped open the side of the home they found the tar paper was ripped during installation and water found its way in. Out here they lay the tar paper across 16" between two 2x4s, no OSB backing. All it takes is someone to lean on it or lay a ladder on it later on and it will rip. Not to mention when they put up the 1" of EPS, they run staples & nails everywhere to tack on the chicken wire. This causes hundreds of rips and tears in the tar paper.

I am not a fan of conventional stucco. I prefer the hybrid types which some of them are waterproof. I like StuccoMax but it is not widely used.

Posted By jonr on 20 May 2013 07:09 PM
More data here. Note the effect of large overhangs - a one or two story house with good overhangs might have no moisture problems in a wall with minimal or no rainscreen.

It seems like overhangs of the right width provide protection against solar radiation on the south side in the summer when you don't want solar heating, while allowing the winter sun to provide warmth, and at the same time can be very effective against water damage from a range of sins.  The overhangs also reduce weathering of the paint and/or siding.  It looks like in a rainy climate, both overhangs and rainscreens would be desirable on all sides of a house. 

I am not a fan of conventional stucco

Seems to me if you have to add a backerboard to a stucco wall to get a rain screen, you might as well just use Hardiplank and skip the stucco. Maybe in the right climate and with overhangs, two layers of StuccoWrap would work well.

Exterior insulating finishing systems (EIFS) if you back drain these you would loss the "Insulating" part would you not?

Many of the Vancouver stucco issues turned out to be poor flashing detailing. Even in Calgary there has been problems with EIFS buildings but again some people don't understand that the top edge of the flashing has to be under the housewrap/tar paper.

Posted By Lee Dodge on 20 May 2013 07:22 PM

Posted By Dana1 on 20 May 2013 06:26 PM
...snip...
BTW: Vancouver is a very rainy climate, but it's not a very humid climate. The outdoor dew point averages are nearly as low as Colorado in summer . Winter dew points bounce against the outdoor air temps a lot (the foggy-dew coastal mist), but only for a few hours a day. Most of the time the air is dry enough even when it's raining to provide significant drying, provided there is a rainscreen gap to work with. It's the bulk-water wetting sans rainscreen that causes most of Vancouver's weather related moisture problems in buildings.

From WeatherSpark for Vancouver:
The relative humidity typically ranges from 55% (mildly humid) to 96% (very humid) over the course of the year, rarely dropping below 43% (comfortable) and reaching as high as 100% (very humid).

So Weatherspark considers Vancouver mildly humid to very humid.

From WeatherSpark for Colorado Springs:
The relative humidity typically ranges from 24% (dry) to 81% (humid) over the course of the year, rarely dropping below 10% (very dry) and reaching as high as 97% (very humid).

Range of average relative humidities for Vancouver over the year:
Daily maximum: 89% to 96%
Daily minimum: 55% to 74%

Range of average relative humidities for Colorado Springs over the year:
Daily maximum: 71% to 81%
Daily minimum: 24% to 33%

Relative humidity is RELATIVE (to the temperature.) The %RH of the outdoor air doesn't by itself define a humid/dry climate- DEW POINT does.

Dew point is a measure of the absolute humidity, and is a better determinant of how well a wall can purge interior moisture drives.  Interior wintertime dew points are higher in Vancouver than in Denver, but the outside dew points are still far lower than interior dew points.

I'd like to see a study on residential internal building pressure and wall moisture. Pushing/pulling warm, moisture laden air into a wall and then cooling it isn't good. But I suspect that it happens in almost all homes. For example, think what happens when you turn on the bathroom exhaust fan (or clothes dryer) on a hot humid day. You get rid of room moisture, but you pull exterior moisture into the walls. A pair of push/pull fans with intake and exhaust wouldn't do that.

Actually relative humidity is defined as the actual absolute humidity divided by the absolute humidity at saturated conditions at the same temperature, and expressed as a percentage. The difference in absolute humidities is the driver for evaporation; that is, it is included as a linear term in the equation to compute evaporation rates for water. When the relative humidity approaches 100%, which implies this driving term approaches zero, evaporation rates for water approach zero.

WeatherSpark says for Vancouver, "The relative humidity typically ranges from 55% (mildly humid) to 96% (very humid) over the course of the year, rarely dropping below 43% (comfortable) and reaching as high as 100% (very humid)."
If you say this is not a humid climate, then your disagreement is with WeatherSpark, and not with me.

Please do not confuse the argument by talking about indoor dew points or humidity. This discussion involves wetting of exterior surfaces that are approximately at the outside temperature with water through the siding, and then computing the evaporation rate of the wet sheathing. I will come back later and present results for evaporation rates of drops at different air temperatures and humiidities.

Having lived & built in that region, despite the foggy dew aspects and near-daily wintertime rainfalls, I s'pose I'll just have to disagree with Weaterspark's characterization (at least from a building-science perspective.)

Cover a house with wet stucco/cement board/wood and air pulled through/next-to it will always be high humidity. Even with a rain screen.

But the outdoor air won't be higher humidity than the air in the rainscreen cavity, especially under "bright clouds" conditions where stucco & cavity air are at both higher temp & higher humidity than that nearly saturated 85-90% RH outdoor air.

Vancouver's humidity is moderate at times but nothing like the true high-humidity temperate rainforest regions on the SW coast of Vancouver island a several 10s of miles away, or the west slope of the nearby Olympic peninsula in WA. The humidity averages in Vancouver are quite manageable, even with reservoir claddings like brick, despite the comparatively high wintertime interior RH in those houses.

Posted By Dana1 on 20 May 2013 06:26 PM
TW: Vancouver is a very rainy climate, but it's not a very humid climate. The outdoor dew point averages are nearly as low as Colorado in summer. Winter dew points bounce against the outdoor air temps a lot (the foggy-dew coastal mist), but only for a few hours a day. Most of the time the air is dry enough even when it's raining to provide significant drying, provided there is a rainscreen gap to work with. It's the bulk-water wetting sans rainscreen that causes most of Vancouver's weather related moisture problems in buildings.

I have quantified water evaporation rates for Vancouver compared them to Colorado Springs using WeatherSpark values for minimum and maximum daily relative humidities and temperatures for summer and winter. I have assumed the minimum relative humidity occured at the maximum temperature.

Comparisons were made by computing evaporation times for water drops 100 micrometers (0.1 mm) in diameter.

Vancouver summer
Max. rel. humidity: 55F = 12.8C, 89% RH, 104 seconds to evaporate
Min. rel. humidity: 68F = 20.0C, 55% RH, 21.3 seconds

Colorado Springs summer
Max. rel. humidity: 58F = 14.4C, 76% RH, 49.1 seconds
Min. rel. humidity: 83F = 28.3C, 27% RH, 6.2 seconds

So water evaporation times are 2 to 3.5 times longer in Vancouver than in Colorado Springs. Differences in winter are about a factor of 3 longer in Vancouver than in Colorado Springs. The evaporation times for water soaked sheathing are much longer than these times, but the driving mechanism is the same.

Summer dewpoint maximums in Vancouver (at max. rel. humidities) are 48F to 56F while in Colorado Springs the corresponding values are 44F to 55F, so quite similar. So the dewpoints alone are not a good indicator of outdoor water evaporation rates.