Vapor Barries in Straw Bale Houses –

Written by Andrew Morrison

house wrapHouse wrap often creates more damage than it prevents in straw bale houses. Why then are they required in straw bale building codes? The answer is not complicated; however, the impact of vapor barriers in straw bale houses is.

For straw bale homes, the push has always been to provide the bale walls with a vapor permeable finish to allow any water vapor to escape the building. In most cases, this translates into an earth based plaster like clay or lime. These plasters have the ability to release vapor and thus allow the bales to dry out when the weather permits. Other plasters that are synthetic or cement based have limited ability to transfer the moisture away from the bales. In some cases, again depending on the weather or climate, the bales end up soaking up moisture from the environment which can cause decay in the walls.

So if it is true that the bales need to breathe and that the plaster that is applied over the surface can influence the vapor transfer away from the bales, why would anyone use a house wrap at all? So here is the simple answer: because that is what people do on conventional homes. Because much of the code language for straw bale buildings is based on codes for conventional construction, some things just never get tested before they become requirements.

I have built many straw bale homes and many conventional homes. In both cases, I find items within the code that really do not make sense for the individual situation; however, the code is not designed to address individual situations but rather large, blanket situations. As a result, I have had to fight for what I believe and then provide some type of proof or performance guarantee from an engineer. Although expensive, an engineer’s stamp is cheaper than a failed wall system.

straw bales with vapor barrierI do not use house wrap on my bale walls unless I really have to. In some cases, where the bales may be exposed to rain splash or snow drifts, I may utilize the material. In dry climates, I do not use the material at all because I believe the ability for the building to release moisture is more important than the attempt to keep it out. The picture above shows an example of a house wrap in the form of roofing felt applied to the bottom courses of bales.

In this case, the home inspector required it on the exposed portion of a wall where wind driven rain rain was a concern. I was sure to NOT wrap the felt under the bales. This allows any moisture to drain free of the bales into the gravel at the base of the wall. Do not confuse the need for a house wrap material with the need for roofing felt over wood exposed to plaster. They are very different and the plaster protection is definitely needed to avoid cracks in the finish. The stripes on the wall of roofing felt are wood members covered before plastering.

I have learned over the years that moisture WILL get in to your house one way or another. Believing otherwise is like believing I can stop it from raining when I want a sunny day. In light of that, it makes more sense to build so that moisture can escape once it gets in. This is a simple answer to a complicated question. Indeed, there are many people and companies out there that spend countless hours and currency researching the impact of house wrap on construction projects and the results of those studies point to the inclusion of vapor barriers in conventional construction practices. Because there is limited information about the impact of those barriers on bale homes, we, as builders, are left to use our common sense and what information we can find.

One thing we know for sure is that moisture can cause irreparable damage to a straw bale house. Knowing this, it is imperative that you do whatever you can to protect your walls from water AND moisture vapor build up. Exactly how you do that will depend largely on your climate, your construction materials, your mechanical systems, and your design.

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115 Responses

  1. There are ways to build with bales that have a finish other than plaster. You need to skim coat the bales with plaster no matter what to make sure they don’t pose a fire risk, but that plaster can be cheap and sloppy (relatively so). You need a vented gap on the exterior between the bales/plaster and the back side of the siding. Keep in mind that finishing walls with plaster is actually a little bit easier than using rigid building materials in many cases. Good luck.

  2. Andrew – I live in climate where summer lasts only for 3-4 months and most of the year is much colder and winter is snowy. I was thinking that it might be good idea to try to stop hot moist air to enter bales from the inside of the house as this will happen 3/4 of the year. On the other side exterior plaster would be able to release vapor from the bales to the outside. Does it make any sense?

  3. Hi Marcin. I am not sure how you would repel the vapor from the interior. the method is key. It is indeed a good idea, but wrapping the house in house wrap or a vapor barrier is not a great approach in my mind. I’d prefer to see you use a plaster sealing material over the finish plaster to seal things up. Be sure to leave the exterior plaster free from the seal so it can release whatever it gathers. One option there is to use silicate paints as they allow vapor to move through but stop water from entering the plaster.

  4. Hello, I am interested in buying a “homestead”.
    ( In Southeast Kansas)

    One cob house (completed)

    One hay bale house
    (structure completed except inside walls are not sealed).

    Is there any way to check the moisture inside the bales?
    What should I check for/pay attention to while inspecting the homestead?

    Thank you,

  5. Hi David. First thing is to make sure the walls were built with straw and not hay. Hay is a food source, not a building material. You can check the moisture content of the bales with a moisture meter. I use a Delmhorst F-2000. In terms of what else to look for, that’s a loaded question as there are so many things to consider. Too much to shove into a comment. At the very least, tight walls, proper anchoring, moisture content, adequate mesh work, no rebar pinning, and many other details. Hope this helps you get started.

  6. Hi,

    I want to be able to build my own house in the future and am currently looking at all my options. I came across straw bale houses while researching insulation and really like its insulation value, the fact that it would be cheap for me, and the awesome window seats. However, I live in an area with driving rain and snow plus I don’t particularly like plaster sided houses. I would really like to have a stone house with straw as my infill insulation but I can’t really find any information about such method. Any thoughts?

    Thank you,

  7. Hi Ashley. Glad you found straw bale construction in your research and it peaked your interest. You can definitely build a bale house without having a plaster exterior. There are lots of options available to you for siding finishes. Keeping the costs low will be the harder part as stone facade is an expensive finish regardless of the house being a bale structure or a conventional home. Cheers!

  8. Hi Andrew,
    I was wondering if your thoughts about this point have changed with the development of “smart” membranes and the use of straw-cell wall systems?

  9. Hi Robert. Not really. I am looking for a positive and strong connection between the plaster and the bales, so I would not want anything in between those two that could interfere. Also, the building code states specifically that:

    AS105.6.1 Water-Resistant Barriers and Vapor Permeance Ratings

    Plastered bale walls shall be constructed without any membrane barrier between straw and plaster to facilitate transpiration of moisture from the bales, and to secure a structural bond between straw and plaster, except as permitted or required elsewhere in this appendix. Where a water-resistant barrier is placed behind an exterior finish, it shall have a vapor permeance rating of not less than 5 perms, except as permitted or required elsewhere in this appendix.

    AS105.6.2 Vapor Retarders

    Wall finishes shall have an equivalent vapor permeance rating of a Class III vapor retarder on the interior side of exterior strawbale walls in Climate Zones 5, 6, 7, 8 and Marine 4, as defined in Chapter 11. Bales in walls enclosing showers or steam rooms shall be protected on the interior side by a Class I or Class II vapor retarder.

  10. would it be a reasonable idea to use PVC pipe with holes drilled in it to vent the strawbale walls up to the top of the wall? Could such vents be sunk into the center of the bales? would that have any significant effect on fire susceptibility of the wall?

    What about using a stone veneer on the outside of the strawbales? would that be detrimental if the straw was vented? It seems like I have seen some buildings that were strawbale but had a stone veneer.

  11. Hi Katherine. Venting the bale walls in the fashion you suggest would be very difficult and not very productive. If you want to provide adequate venting, it would be in the form of a rainscreen on the exterior surface of the wall. This is also a good option when applying a brick veneer on the exterior face of the wall, so it seems like a solid fit for your needs.

  12. Hi Andrew,
    Thank you so much for this.
    I am using straw bale as insulation material in my shipping container home. Because steel sweats, I have first applied a vapour layer and am also deciding if I need to fill the spaces in between the vapour barrier layer and the straw with a second foam insulation, or if I should crush straw into that or even make a light straw clay mix to fill those ridge spaces? I will lime plaster my straw to create the walls.
    Thanks for any advice!

  13. Hi Chandra. I would suggest either providing a ventilation space between the bales and the container so that any moisture that moves through the walls towards the exterior can escape the structure. Exactly how you do this will depend on the overall design of the container/house. You could add an insulation layer between the vapor barrier and bales, but that still won’t allow the moisture to escape if needed. Cheers.

  14. Hi Andrew,

    Thanks for the site and articles, I find it very interesting !
    I am wondering if one could insulate a brick wall with straw bales, and how one could prevent condensation on the bricks ?

  15. Hi Matt. Yes, it is possible and your question is exactly the one to ask! You would need to provide a ventilation space between the two materials to make sure there is no ability for trapped moisture to find its way into the bales. Be sure to dip the bales in some type of slip before stacking on the side that will face the brick/ventilation space to reduce any fire risk that might otherwise exist in that space.

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