Questions For Chris - Moisture permeability, air exchange, paint, rainscreen, vapor barrier

Hi Chris,

Welcome and Thanks a lot for taking the time to do this - I have had a great interest in strawbale construction for sometime, and since you're based in Ontario your climate experience is very relevant to me! I hope to build in New Brunswick within a couple of years and although there are a couple examples of strawbale in the area, it seems generally 'unproven' and I've developed a couple of questions I'm hoping you can help me address.

There's been lots of talk on this forum regarding.. and I hope I'm using the right terminology here.. moisture permeability (not to say, breatheability) of structures and the need for mechanical air exchange and moisture control VS completely passive systems. In one of the videos posted in your introductory thread you mention 20 year old builds that have been continuously monitored for moisture and never had a problem.

Can you comment on that further with regard to the systems in place to prevent moisture problems, or is there simply no inherent issues beyond that of conventional builds..OR does strawbale and natural plaster perform better than conventional in that sense?

I'm interested in the systems in place in these structures to deal with excess moisture. Is moisture and air exchange dealt with in a conventional way in these structures and if not, how is it dealt with?

You mention using vapor barrier on the ends of your SIP's but what about in situations where you install cladding? It is my understanding that vapor barrier/retarder is a no-no when used to create a complete envelope around a straw insulated structure as it stops or retards moisture it can create a build up of moisture inside the wall from excess moisture in the building. Is that correct? Do you use vapor barrier on any of your structures to create an envelope? If so is it only to comply with code (as it seems to be in the code in many places in Canada) or is it useful? I can imagine that if vapor barrier is used, that mechanical air exchange would be required.

The main question buried in there is if course: Are mechanical systems required when natural plaster, with natural paint is used on interior and exterior of a strawbale wall (henceforth referred to as 'all natural'..for the purposes of this post), or is the 'moisture permeability' of these materials not great enough to allow the excess moisture pass through the wall and out? I'm sure this partly depends on how much moisture is generated, what type of heating is being used, and a myriad of other factors but let's imagine the bathroom has a vent of some kind so we're only dealing with people and pet body moisture and of course cooking..or what you might think of as an average situation.

I notice that you use cladding on at least some of your structures. If mechanical air exchange is not required when going the 'all natural' route does cladding change that? I imagine that cladding a strawbale and plaster wall would add another barrier for the vapor to pass through and possibly condense on. Do you employ a rainscreen to mitigate this or is it an issue at all?

As I mentioned or implied I'm personally interested in using strawbale construction and avoiding vapor barriers and mechanical air exchange. I also hope to be able to clad the plastered straw bale (using a rainscreen gap) in order to avoid taking in moisture from rain coming in direct contact with the plaster since NB is no desert.

Many Thanks!!
j

Hi Chris,
What suggestions do you have for dealing with persistent black mould? Bane to asthmatics the world over.
I realise that this touches on all the subjects listed above.
We are encouraged to insulate well and have damp proof course installed on a traditional brick bungalow but in the damp British winter drying clothes is an in house affair and that generates moisture that takes power to pump outside.
I'm (not very successfully) growing English Ivy in the affected rooms but I wonder if you have come across a natural earth substrate that is resistant to mould spores.

Hi Jay,

Your questions are extremely relevant, and at the forefront of what most natural/sustainable builders are thinking about these days.

It has certainly been my experience that the bale buildings I've been involved with have not experienced any moisture issues. That's upwards of 50 buildings in Ontario, dating back now 20 years. So that's good.

However, having moisture "problems" like mold is not the same thing as having well balanced humidity levels inside the building. To some degree, bale buildings (and other natural wall systems) do a great job of handling interior humidity levels by acting like giant "sponges" with a huge amount of accessible storage capacity for moisture. So when a building is being loaded with moisture (boiling pasta, showering), this humidity can be absorbed by the walls (and other natural materials in the building, like wood ceilings). Most conventional homes experience moisture problems quickly because few or even no surfaces in the house are able to absorb humidity (latex paints, sealed floors, etc). The ability of natural wall systems to allow this moisture to migrate in either direction (back into the building, or out the atmosphere) means that even high levels of humidity tend to get re-dispersed quickly. All good things!

As we all start to make our buildings more air-tight (a necessity for cold/extreme climates if you care about energy efficiency), we eliminate leaks in the building that are "fast" routes for excessive moisture to move through. This is a good thing, but it means that the walls become the only medium for storage/exchange. From all experience, they seem to do this quite well even in fairly air tight buildings. However, higher than average humidity in the building can mean seeing condensation on windows in cold weather and a sense of "dampness" if levels rise too high (above 70-75% RH). Wall permeability may not be able to keep indoor RH at or below these levels, even though the walls themselves are not experiencing any issues that would cause failure.

So the question becomes, how much and what type of air/humidity exchange is needed. And the answer is: Nobody knows! Some research has gone into creating ventilation standards for conventional buildings, and that's what's been enshrined in codes. However, these standards assume non-permeable walls, no moisture-open surfaces in the building, and a fairly high degree of material off-gassing. This means that code requirements for ventilation are many times more than is necessary for natural homes. But how much more? And how to deal with codes when looking to install "alternative" systems for ventilation.

I do believe that mechanical ventilation is a good idea, in any kind of house. Fresh air is good. Balanced humidity is good. So a system to help provide those conditions is a good thing. For some reason, people in the natural building world tend to be dead set against this, claiming that "having machines help the house to breathe" is anathema. However, many of these same people will have a refrigerator, a computer, a mechanical heating system, a pumped solar hot water system... in other words, use machines to make their homes comfortable. A Heat Recovery Ventilator (HRV) is actually a very simple machine, with only a fan and no other moving parts, and can be extremely effective in keeping indoor air humidity balanced, and providing tempered fresh air. They can be controlled automatically, or manually for those who are off-grid or don't like the machines to make decisions. A small unit isn't very expensive.

There are simpler systems that are more "natural". Solar air heaters can draw in fresh outside air and bring it into the house pre-heated. Earth tube ventilation is also a possibility (though it can have moisture issues of its own). Fresh air intakes near wood-burning devices can draw outside air and deposit it near the heat source... In other words, there are multiple strategies to get fresh, tempered air into the building in a cold climate. Strategies abound! Having these strategies accepted by code officials can be more difficult. If you're choosing to use an "alternative" system, you will likely need an HVAC engineer to design the system (or run calculations on the system you have designed). Be sure the permeability/porosity of interior surfaces is being considered, as this storage capacity will greatly affect the amount of air that needs to be moved.

Code officials tend to dislike systems that require manual input to function, or systems that can be altered in the normal course of building occupancy. For example, you might have a house with all-porous materials inside. Great. But if the next homeowner slaps a coat of latex all over that, the ventilation assumptions go out the window and the system may not work. However, simple solar hot air collectors can be an automated system, as can other small, fan-based systems.

At this point in my building career, I would not ever opt for no ventilation at all. It just makes a building much more comfortable. However, in my current straw bale home, we run the HRV relatively infrequently, especially now that construction moisture is finally starting to subside (after almost 2 years!). But it is great to have it. Given how little we use it, we could definitely have gotten away with a smaller, alternative system. However, prices for HRVs are getting so competitive, it would be just as expensive to build my own collectors, buy fans, a PV panel to run the fans, and the controllers. Both would work.

When cladding over a natural wall system, we only use "rain screen" cladding systems, in which a strapping material is used to create open vertical channels between the siding and wall, and these are open (though screened against critters) at the top and bottom of the wall, allowing for air to circulate and move moisture. It may (don't have any studies to show) slow down moisture moving to the outside, but it definitely drastically reduces the amount of moisture that gets on/into the wall from precipitation.

For us, choosing a cladding is a very design-dependent decision. Plaster finishes are definitely vulnerable to bad weather. In some designs (one story, good overhangs, etc), these factors can be mitigated and I feel fine going with plaster. But many designs/locations don't suit finished plaster surfaces on the exterior, and many owners don't want to maintain exterior plaster. Many owners and builders will choose a high embodied energy cement-based plaster to help withstand these conditions. The impacts of this decision on the environment can be worse than adding a layer of sustainably harvested wood cladding.

The book I wrote is an attempt to help people figure out the balance they'd like to achieve when considering questions like cement-based plaster vs. wood siding.

Long answer!

What suggestions do you have for dealing with persistent black mould? Bane to asthmatics the world over.
I realise that this touches on all the subjects listed above.
We are encouraged to insulate well and have damp proof course installed on a traditional brick bungalow but in the damp British winter drying clothes is an in house affair and that generates moisture that takes power to pump outside.
I'm (not very successfully) growing English Ivy in the affected rooms but I wonder if you have come across a natural earth substrate that is resistant to mould spores.

Hi Chris,

Persistent black mould is a problem that's difficult to remediate with just a wall substrate. You may find it worthwhile to open a window in the drying room and place a fan in the window blowing outwards. It will use some energy, but it will help the situation quite a bit.

I would suggest a traditional lime plaster as a wall coating, rather than an earthen version. The lime is naturally antiseptic, and much less likely to support mould. However, in damp buildings the mould tends to be originating from deeper in the wall. If there is mould on the framing, the lath, or any part of the wall behind the plaster, it will likely re-emerge over time regardless of the type of wall substrate you use.

The thing with mould is that the spores are everywhere (concentrations will vary), but they only grow under the right conditions. They need fairly high moisture and temperatures and a lack of direct sunlight. If you can change any one of those factors, you'll prevent the mould from growing. Since you probably want to live in a home that is within the temperature range that mould likes, you'll need to reduce the moisture.

Hi again Jay,

I was racking my brain to come up with the name of a simple, through-wall HRV system that straddles the line between full-on mechanical system and passive home-made system. Just found the link for the Lunos HRV system: http://www.foursevenfive.com/index.php?main_page=index&cPath=70_71_98_99&zenid=3e565cf586f42bb4ee7997e49a394813

So terazyme or permazyme added to earthen plaster? Adobe bricks?

Do you use c.e.b.? Why do you think we don't see commercial ceb contractors in middle america?

I'm not sure. My good friend Henry Weirsma is trying to get a CEB (compressed earth block) business off the ground in this area, but it hasn't been an easy road.

I think that one issue is scale of production. CEB really starts to get economically attractive when production is very high and mechanized. Smaller producers can't hit the production quantities that make the price work. I've had to decide against CEB several times, even though I'm in a position where I often work for clients who are willing to pay more for environmentally sound materials.

Once made and delivered to a site, the blocks are fairly quick and easy to work with, but their weight makes transportation and loading/unloading a big job too, unless machinery is involved at both ends of the delivery.

And here in a cold climate, the blocks require an insulation strategy, so it ends up being a double wall system, or there's an entire secondary system to provide insulation. This adds to cost and time.

For me, one outstanding issue is the fact that most CEB producers are including a healthy percentage of cement in the blocks, from 5-10%. This makes them not quite natural, and not so far different from concrete blocks in fact.

I do, however, hold out great hope that CEB will become more common outside its current geographical range, and that dedicated CEBers will streamline these problems away...

How about the enzyme brew for clay plasters or floors? I see a company called Fernco sells terazyme along side CEB presses. Have you heard of anybody using these products for any thing other than road construction?

Certainly there is some precedent that shows these enzymes work for roads. And I wouldn't say that they don't work for earth floors or plasters, I just don't know anybody who has tried it. I know my friend had his CEBs tested at Queen's University with and without the enzymes and found no measurable difference in strength or water characteristics. But new ideas can take a while to show up and be proven, so I wouldn't write them off...

Tim Evers wrote:Do you use c.e.b.? Why do you think we don't see commercial ceb contractors in middle america?

There are more than zero, but maybe not much more.

Here's the facebook page for Midwest Earth Builders; they're in Wisconsin. https://www.facebook.com/profile.php?id=164823956878149

Hi Chris Magwood,

I am usually am the one (not the only!) vocal here at pushing the "natural-traditional systems of architecture." It is most refreshing to read another industry leader, builder and educator making similar to exactly the same comments to questions.

From "cold roof-rain screen systems" to all natural-traditional building materials our mantras are pretty much the same. I have even had my fair share of time with CEB systems and the machines as they are little different that adobe block compression systems or civa rams.

We do differ in the HRV and mechanical ventilation concepts for architecture, yet the way you address it would be acceptable if a client or architect wanted it for a home. I do tend to be very conservative, yet I am more a traditional (historical) builder than just natural. The two for me are one and the same, as we have countless millenia of architectural knowledge that we are ignoring, and then turning around and trying to reinvent the wheel...not very round wheels I might add.

Another place we may differ is I do not promote, design or build "air tight" structures. Instead I focus on "draft proof," structure. As an Outdoor educator, I learned decades ago that Goretex is a great concept (like tyvek) in the laboratory where it was developed and tested, yet in real world application, you often end up more wet from the inside moisture accumulation than from what the weather outside is doing. Here again, I have learned that a sweater at base camp in the mountains, a down and felt jacket while hunting, or similar easily vapor permeable is very often more practical than any of the "high tech" gadgetry that some many think works...but doesn't. Even in a driving rain many of these systems working in concert with a loose waterproof poncho and big brimmed hat is best. Same for architecture.

We agree completely that code must rapidly start meeting these more sustainable building practices.

Thanks for all you do, for sharing here, and for now being part of Permies...

Warm Regards,

j

Hi Chris Kay,

If you PM me I maybe able to put you in touch with some fine Chaps that deal with moisture in buildings in the U.K.. I write and confront PCA members all the time with there nonsense about "rising damp," and all the other nonsensical solutions they so often give to remedy this. As a traditional builder, and Historical Restorationist, our solutions tend to be pragmatic and practical.

Hi Jay C.,

You're right that there is a tendency to ignore historical systems of architecture in the natural building world (though people do like to ooh and aah over photos of old cliff dwellings). There's a tendency to want to throw out everything that has to do with "conventional" architecture without realizing that many of those conventions are a result of centuries of trial and error that have brought us to a point where we know how to make really good buildings. It's really the materials and design decisions that have gone off track with conventional building, but the traditional know-how that underlies much of conventional building is based on that hard-won concept of "common sense", which often takes generations to arrive at!

In terms of air tightness, we're probably not as far apart as it may seem. I'm not a Goretex fan, I'm more of a wool sweater person. That's the beauty of natural building systems... they work more like the sweater in that they are capable of absorbing/storing a lot of moisture and giving it up when the conditions are right. But if we're not building as air tight as possible (and this can be done with all natural materials... natural plasters are great air barriers when properly applied), we're not building as energy efficient a home as we can. Worse, those air leaks we leave in the building enclosure end up "ventilating" the home with less than ideal air quality. Air that's drawn through an insulated cavity is picking up dust, allergens and after a few decades probably mouse feces and other things that naturally accumulate in walls and attics. Air that leaves the building through those leaks introduces a huge amount of moisture to the building enclosure (in Ontario, studies show that a 2cmx2cm hole allows 30 litres of moisture into a wall/ceiling each heating season, whereas a wall surface with no leaks allows 1/3 litre).

All these reasons compel me to make buildings air tight. The comfort of living in an air tight home seals the deal. The straw bale house I'm currently in is extremely air tight, and we can set the thermostat at 19 C and be extremely comfortable. Most people are shocked when the see the thermostat setting, because they think the house is being kept really warm. That level of comfort and energy savings (we use over 80% less heat than a new code-built home) makes it worthwhile for me, and definitely worth the tradeoff of running an HRV for a few minutes a day. Our HRV "strategy" is that we turn it on each time we use the bathroom, and it's set to run for 5 minutes each time, so it typically runs 30-45 minutes a day). This level of ventilation is entirely adequate in a home that has lots of porous, absorbent materials, and could easily be achieved in a more passive way.

If codes would adapt to understand that natural materials offer huge advantages while being able to be air tight and require much less ventilation, we'd be on our way to much better buildings!

Hi Chris,

Wonderful response, I do agree that we build and think more alike than otherwise. It could be a case of academic semantics as well, as the average GC and their concept of "air tight," is much different that what I see some in the natural building world doing

I also should stress that a "draft free" design is just that. It just is not sealed in ways that inhibit the permanence of the building materials and only natural "sealing" methods are typically employed (often like you would find on a traditional sailing ship.) I am not surprised at all by your thermostat level, and lower, as with proper heating systems (radiant) and an adjustment in the "normative culture," a home with a ambient "air temperature" of 13 C to 16 C is more than comfortable. Inclusive of a radiating thermal cline in different locations.

The one place we may differ, or at least I still have more questions/concerns is in the reliance of HRV systems. I want my architecture to work at complete efficiency without the need for augmented technology (or the worry of its failure, maintenance expense, and necessary use) to make the structure perform well. I am not adverse to the employment as you have described if a client truly wishes to use them, yet I will not go out of my way to rely on them for the architecture to work.

I hope that SB continues to grow in popularity in all locations that it is sustainable and can be part of the architectural fabric, as long as those bales are coming from the immediate area, and not being ship half way across a province, or country...then I would look to alternatives if that was necessary.

Regards,

j