Clay as a medium for walls in the PNW

Hey All

A friend of mine, thinks it’s silly to entertain using clay in a wall structure in the PNW (I’m considering a hemp clay or straw clay wall system.) I have considered hemp lime mixture but am sensitive to the added cost and not thrilled about the caustic nature of lime.

I am just East of the Cascades but we still have damp long winters and long dry summers. I would provide for ample drying time and seal the walls before the rain begins. Would love to get feedback from the community on this. Is this the wrong climate for this medium or is there more to consider?

If you're on the east side of the Cascades I'd think you're in a good spot to do earth building. Jim Reiland is likely to have some really good advice. He's located in the southern part of Oregon and knows the climate as well as having massive experience with natural building techniques.

Hi Emily,

I'm not an expert -- more an enthusiastic enthusiast.

But the way I understand it is that clay walls will be just fine in your region if you take the necessary precautions.

If you're able to stop your walls from coming into contact with driving rain, you should be good. Which means your walls should be raised high enough off the ground to account for splashback, and your roof overhangs should be deep enough to protect the walls from rain.

What I'm concerned about is when you say the walls will be 'sealed'. Sealed with what exactly?

Clay walls are problematic when they're sealed in such a way that prevents water vapour from exiting the wall -- like with concrete or plastic paint. The moisture then becomes trapped in the wall leading to failure. Many heritage building are destroyed by using the incorrect materials for renovation.

If you're sealing your walls with something breathable -- like mineral paints or a natural plaster -- the vapour will be able to pass through this barrier and exit the wall, leaving it dry.

TLDR: Clay is fine in light clay straw in almost any environment provided you have generous roof overhangs. Clay based plaster is probably fine provided the aforementioned generous roof overhangs but I think lime plaster is better.

The long response:

With significant roof overhangs an exterior clay based plaster should be fine. We did our whole exterior light clay straw house with earthen plaster and it's held up fine for the past five years. There is one fairly exposed corner with only two feet of roof overhang that I re-plastered a few months ago. Our climate is probably drier than yours but we still get ~32 inches of rain a year and when it usually comes in the form of blowing wind storms.

That being said since building our cabin I've switched to mostly using lime plasters for a few reasons. Note, these are anecdotal observations and maybe over time I've just become a better plasterer.

1) Lime based plaster dusts off less than clay plaster
2) I get less shrinkage with lime plaster than clay plaster.
3) Hairline cracks in lime plaster will heal themselves to an extent.
4) Once lime plaster cures it is pretty hard wearing
5) If you're adding a tint to your plaster it's easier to maintain a consistent color.

Regarding cost and causticness

A lime plaster is mostly sand (~80%) so a 50 pound bag of type S hydrated lime  (~$20 in my area) will go a long way. You could probably plaster the interior and exterior of a 200 sqft house with less than six bags. I've found that contact with hydrated lime mostly just dries out my skin. I'll typically wear light gloves and safety glasses when I'm plastering.



 

Protected clay is fine as others have stated. I'd be more concerned about having adequate insulation balanced with well positioned thermal mass to moderate cold outdoor temps, and a sturdy structural frame (heavy timber?) to support significant snow loads if that's a feature in your area.

Mike Harris wrote:Hi Emily,

I'm not an expert -- more an enthusiastic enthusiast.

But the way I understand it is that clay walls will be just fine in your region if you take the necessary precautions.

If you're able to stop your walls from coming into contact with driving rain, you should be good. Which means your walls should be raised high enough off the ground to account for splashback, and your roof overhangs should be deep enough to protect the walls from rain.

What I'm concerned about is when you say the walls will be 'sealed'. Sealed with what exactly?

Clay walls are problematic when they're sealed in such a way that prevents water vapour from exiting the wall -- like with concrete or plastic paint. The moisture then becomes trapped in the wall leading to failure. Many heritage building are destroyed by using the incorrect materials for renovation.

If you're sealing your walls with something breathable -- like mineral paints or a natural plaster -- the vapour will be able to pass through this barrier and exit the wall, leaving it dry.

Hi Mike yes was referring to plaster, specifically lime plaster re sealing and thank you for your comment and thoughts.

Aaron Yarbrough wrote:: Clay is fine in light clay straw in almost any environment provided you have generous roof overhangs. Clay based plaster is probably fine provided the aforementioned generous roof overhangs but I think lime plaster is better.

The long response:

With significant roof overhangs an exterior clay based plaster should be fine. We did our whole exterior light clay straw house with earthen plaster and it's held up fine for the past five years. There is one fairly exposed corner with only two feet of roof overhang that I re-plastered a few months ago. Our climate is probably drier than yours but we still get ~32 inches of rain a year and when it usually comes in the form of blowing wind storms.

That being said since building our cabin I've switched to mostly using lime plasters for a few reasons. Note, these are anecdotal observations and maybe over time I've just become a better plasterer.

1) Lime based plaster dusts off less than clay plaster
2) I get less shrinkage with lime plaster than clay plaster.
3) Hairline cracks in lime plaster will heal themselves to an extent.
4) Once lime plaster cures it is pretty hard wearing
5) If you're adding a tint to your plaster it's easier to maintain a consistent color.

Regarding cost and causticness

A lime plaster is mostly sand (~80%) so a 50 pound bag of type S hydrated lime  (~$20 in my area) will go a long way. You could probably plaster the interior and exterior of a 200 sqft house with less than six bags. I've found that contact with hydrated lime mostly just dries out my skin. I'll typically wear light gloves and safety glasses when I'm plastering.



 

Thanks Aaron!

I’m not planning to use clay plaster I’m considering using a clay straw (light straw clay) or clay hemp wall material (would add a splash of borax to the mix in both instances), which I would then lime plaster over once fully dried. Using hemp Crete would involve quite a lot of lime which becomes rather pricey when compared to clay.

Ben Brownell wrote:Protected clay is fine as others have stated. I'd be more concerned about having adequate insulation balanced with well positioned thermal mass to moderate cold outdoor temps, and a sturdy structural frame (heavy timber?) to support significant snow loads if that's a feature in your area.

Thanks Ben,

Yes the clay straw and hemp clay or hempcrete are both non structural, I would timber frame the structure and then use the clay and or lime mixtures to create the wall structure, these are in fact insulative (Hempcrete has r value of .8-1.2 per inch, straw clay has r value of 1 - 1.5 per inch.)

Hi Emily,

Thanks for the clarifications.  Either wall system should be protected from wind-driven rain by either or both adequate roof overhangs and a suitable plaster.  

Note that both straw-clay and hempcrete could be plastered, but they could also be sided if you were concerned about wind-driven rain. (Addendum. Straw bale walls can also be sided, but it's best if the exterior bale surface is plastered first. At the very least a scratch coat provides an air barrier, resists fire, insects, and rodents. Adding a brown coat is better as it supplies a better air barrier.  Adding siding to a straw bale wall requires some planning. One method is to let 2x ledgers or posts into the bales on whatever centers the siding requires, usually 2', and make a note of where they are.  Once the plaster covers the wall secure furring strips through the plaster to the ledgers or posts, then secure the siding to the furring strips. This results in a "proper" rain screen with an air gap (the thickness of the furring strips).  When stacking the bales against plywood sheathing--a wall system that works great in some climates plastering the exterior isn't possible, and may not be critical as the plywood takes on some of the function of the plaster.  Add siding to the exterior surface of the plywood according to your area's best practice. Rain screens are a great practice, but may not be absolutely necessary in some climates.  Be sure to install insect screen at the top and bottom of the wall so it's not an open space for critters to occupy.)

Clay plasters are lower cost and user friendly but can erode from the wall if hit by too much rain.  Sealing them with some silicate based product or linseed oil helps, but also makes it difficult to fix as a repair plaster won't stick as easily to the treated plaster.  

Lime plasters are more durable but they aren't water repellent so much as water reservoirs. In natural wall assemblies like straw bale, straw-clay, or hempcrete walls exterior plasters  function to absorb liquid moisture (wind-driven rain), then release it as water vapor. This mechanism can fail if there's too much rain and not enough dry time between rainfall events. When that happens the water soaks into whatever substrate the plaster is on--straw bales, straw clay, hempcrete--where it stays until exterior conditions pull the moisture back out in the form of water vapor.

How long that takes depends on exterior conditions--warm and dry summer weather is best for evaporating water from the walls, though very cold winter conditions help to prevent microbes from becoming active. I have seen straw bale and straw-clay walls that were wet on the exterior surface survive for months in S. Oregon winters without apparent damage because it was too cold for the microbes to start eating. Some of the repair work I did on straw bale structures in S. Oregon involved only the lower few feet of the wall where the plaster absorbed liquid water from rain splash and roofs without gutters.  

It costs more, but larger overhangs protect walls better.

RE your comment about "adequate dry time." The handed-down wisdom in the straw-clay building world of "one week of dry time per 1" of wall thickness" really depends on optimal drying conditions.  I have tested straw-clay walls many months after they were placed and found readings well over 20% moisture content.  It's possible that the measuring tools we use (moisture reading equipment designed for hay) isn't appropriate for a material like straw-clay, but lacking other reliable methods to evaluate the interior of a straw-clay wall we waited for the wall moisture content to come down before plastering either side. We began exterior plasters nine months after the walls were placed, and completed the interior plasters almost a year after the straw-clay was placed.  We also didn't rely entirely on warm, dry weather to coax moisture from the walls. Over the winter the owner used a wood burning stove to raise the interior temperatures, and also ran dehumidifiers.

Jim
Many Hands Builders

Jim Reiland wrote:Hi Emily,

Thanks for the clarifications.  Either wall system should be protected from wind-driven rain by either or both adequate roof overhangs and a suitable plaster.  

Note that both straw-clay and hempcrete could be plastered, but they could also be sided if you were concerned about wind-driven rain. (Addendum. Straw bale walls can also be sided, but it's best if the exterior bale surface is plastered first. At the very least a scratch coat provides an air barrier, resists fire, insects, and rodents. Adding a brown coat is better as it supplies a better air barrier.  Adding siding to a straw bale wall requires some planning. One method is to let 2x ledgers or posts into the bales on whatever centers the siding requires, usually 2', and make a note of where they are.  Once the plaster covers the wall secure furring strips through the plaster to the ledgers or posts, then secure the siding to the furring strips. This results in a "proper" rain screen with an air gap (the thickness of the furring strips).  When stacking the bales against plywood sheathing--a wall system that works great in some climates plastering the exterior isn't possible, and may not be critical as the plywood takes on some of the function of the plaster.  Add siding to the exterior surface of the plywood according to your area's best practice. Rain screens are a great practice, but may not be absolutely necessary in some climates.  Be sure to install insect screen at the top and bottom of the wall so it's not an open space for critters to occupy.)

Clay plasters are lower cost and user friendly but can erode from the wall if hit by too much rain.  Sealing them with some silicate based product or linseed oil helps, but also makes it difficult to fix as a repair plaster won't stick as easily to the treated plaster.  

Lime plasters are more durable but they aren't water repellent so much as water reservoirs. In natural wall assemblies like straw bale, straw-clay, or hempcrete walls exterior plasters  function to absorb liquid moisture (wind-driven rain), then release it as water vapor. This mechanism can fail if there's too much rain and not enough dry time between rainfall events. When that happens the water soaks into whatever substrate the plaster is on--straw bales, straw clay, hempcrete--where it stays until exterior conditions pull the moisture back out in the form of water vapor.

How long that takes depends on exterior conditions--warm and dry summer weather is best for evaporating water from the walls, though very cold winter conditions help to prevent microbes from becoming active. I have seen straw bale and straw-clay walls that were wet on the exterior surface survive for months in S. Oregon winters without apparent damage because it was too cold for the microbes to start eating. Some of the repair work I did on straw bale structures in S. Oregon involved only the lower few feet of the wall where the plaster absorbed liquid water from rain splash and roofs without gutters.  

It costs more, but larger overhangs protect walls better.

RE your comment about "adequate dry time." The handed-down wisdom in the straw-clay building world of "one week of dry time per 1" of wall thickness" really depends on optimal drying conditions.  I have tested straw-clay walls many months after they were placed and found readings well over 20% moisture content.  It's possible that the measuring tools we use (moisture reading equipment designed for hay) isn't appropriate for a material like straw-clay, but lacking other reliable methods to evaluate the interior of a straw-clay wall we waited for the wall moisture content to come down before plastering either side. We began exterior plasters nine months after the walls were placed, and completed the interior plasters almost a year after the straw-clay was placed.  We also didn't rely entirely on warm, dry weather to coax moisture from the walls. Over the winter the owner used a wood burning stove to raise the interior temperatures, and also ran dehumidifiers.

Jim
Many Hands Builders

Jim! Thanks for this info so helpful. I’ve heard mixed opinions on impervious siding for natural walls. Argument tends to be that it makes it such that the wall can’t breathe and release moisture to the outside. What has been your experience with this?

The short answer is yes, we should avoid placing impermeable materials against wall assemblies made with materials like straw bale, straw-clay, etc., unless we provide other means for moisture to escape. The only difference of opinion that I’m aware of is whether some materials or combinations of materials are vapor permeable enough for a particular climate. My earlier mention of plywood sheathing may be the source of your question about impermeable materials?  

In natural building “best practice” is to design wall assemblies that allow moisture to escape. However, that depends on how much and in what form moisture can be expected to enter the walls, as-well-as the materials in the wall assembly itself. That’s why “best practice” can take on a regional quality—what works in arid Nevada may work not as well in humid and wet Florida; what works in mild coastal California may not work in frigid Vermont.

My experience is almost exclusively with straw-based insulation materials covered on both sides with vapor permeable plasters—clay and/or lime. A fairly recent development in straw bale construction has been to use clay plasters on the interior and ½” or 3/8” plywood sheathing on the exterior with a vapor permeable plaster or some other siding over the plywood sheathing.

Much of western-central Oregon south through central California presents a climate suitable for this system, at least according to the materials modeling done by members of the California Straw Building Association. Buildings in Eugene, Ashland, and Jacksonville, OR and Vallejo and Lake Tahoe, CA have employed this wall assembly. This analysis, called a WUFI (an acronym for the German words ”wärme und feuchte instationär”) shows how heat and moisture move through different materials of a wall assembly given varying climate conditions. Regions with mild winters and warm dry summers are well suited to this kind of wall assembly as this thickness of plywood is vapor permeable enough to not trap moisture. The wall assembly is more permeable if whatever siding system that covers the plywood—metal, cement board, wood, or plaster—has an air-gap.

When I replastered my straw bale house last year I used this method.  After removing the existing plaster I let 2x4 studs into the bales (which had been laid flat many years ago so no strings were cut) on 2’ centers. I gang-drilled ½” holes on 12” centers through the stacks of the 3/8” plywood panels that sheath the straw bale walls. On my project the plywood isn’t structural—it’s there to support the conventional plaster regime, i.e. a 2-ply building paper/drain mat stapled to the plywood, lath stapled to the 2x studs the plywood attaches to, and a three-coat lime plaster built out to 1” thickness.  

I’m guessing that the ½” holes (that total 4.2 square inches per 4’ x 8’ plywood panel) increases vapor permeance; I haven’t seen tests to confirm that. If the plywood were part of the building’s shear it’s also unknown whether holes of that size and location in the plywood would compromise any structural function. Still, it’s a promising system as most builders and building code officials are familiar with plywood sheathed stud walls, and any kind of siding can cover the sheathing, both lowering costs and increasing aesthetic options.

A longer explanation of vapor permeability might begin with the question “how does moisture get into walls?”

For another time?

Jim
Many Hands Builders