Tiny House Building Science: The Walls

** NOTICE **
At least one aspect of the following post differs from that which we implemented
in the end-design and/or the actual construction of our Tiny House.
For the most accurate and up-to-date information please refer to our eBook.

Perhaps the largest single issue in the design of our tiny home has centered on how to construct the building’s envelope.  “Envelope” is just a fancy word that describes the shell; the floor, walls and roof that form the parts one sees when looking at the outside of a structure.

This is the 2nd of 4 posts detailing the methods we have chosen (after much research, consultation and contemplation) to protect our home from the elements and shield it from the negative results of human habitation.  Read the first post in this series to learn more about Building Science.

Building Science & Tiny Houses:  The Learning Curve

I’d like to be able to say that the process of choosing how we were going to construct the shell of our tiny house came easily.  But saying this would be a stretch.  In reality, it took many hours of research at our public library, the generous assistance of participants of online forums and direct consultation with building science proponents in order to come to terms with the unique interrelation of the forces of gravity, heat and cold, high and low pressure, and the remarkable movement of water in both liquid and vapor forms at play in a modern tiny dwelling.  Most tiny house proponents seemingly down-play or completely fail to directly address the intricacies involved in managing these factors in a small structure.

Larger buildings, by the very nature of their increased size and mass, are better able to absorb higher amounts of negative forces or are far more forgiving of poor design.  In other words, the chances are greater that the structure won’t fail the building, or the building its occupants.  However, everything is magnified in a tiny structure.  To illustrate this point, how long do you think it would take to fog-up all the windows in a multi-story 2000 square foot home if someone were to forget to switch on the range hood over an uncovered pot of boiling water?  Now contrast this same scenario in a structure of 100 square feet?  You get the idea.

So, what are we planning to do about all of this?

Keep it Open, Keep it Dry

Our plan for our tiny house envelope calls for a moderately “open” design (“Vapor Open Assembly”) in which we acknowledge that no matter what we do, all of the layers of the wall will, at some point in time, get wet.

Now, don’t misunderstand me to mean that our design will allow rain to pour through the house every time a storm blows through.  On the contrary, the exterior portion of our wall will weather such instances quite well.  What I mean is that stuff gets wet no matter what steps we think we’re taking to prevent this from happening.  And also that stuff gets wet as a result of all kinds of powerful forces.  Rain driven rain, capillary action, solar driven moisture exchange, vapor diffusion, freeze/thaw cycles, unchecked air movement, differential pressure gradients – all of these forces (and more) cause water in one form or another to reek havoc on structures.  However, if building science has taught us nothing else, it has taught us that we need to permit whatever it is that gets wet to dry out.  In fact, its only when you exceed something’s capacity to dry out that you encounter problems related to moisture.

The Walls

To illustrate how we’ll do this, I’ve included a sketch of our envelope, starting with the four walls,  along with a detailed description, starting from the outermost exterior layer proceeding to the innermost interior layer (from outside to inside).

a

You’ll recall from an earlier post that we found a deal we just couldn’t pass up in the form of 2000 linear feet of redwood boards that were cast-offs from a milling project.  These boards, laid up as horizontal clapboards, will comprise the outermost face of the wall.  Rather than nail them directly onto the sheathing, as is most common, we are going to implement something called a “rain screen” system.  We do this for two reasons.

Number one is to hold the redwood off of the house wrap to prevent the naturally occurring tannins in the wood (the tannins are natural poisons and are what make this material so rot and insect resistant) from destroying the housewrap’s waterproof characteristic.  Other woods that exhibit this same tendency are white and red cedar (and black locust).  This is the same reason it is mandatory to use stainless steel fasteners with these species of wood; the tannins are so caustic they’ll eat right though galvanized steel or aluminium if given half a chance.

Number two is to allow the backs of the clapboards to dry out, which accelerates drying of the entire board.  Remember that it is not a matter of “if” but “when” water finds its way behind the clapboards.  So, when the wall gets wet, it can properly dry out, thus extending the life of the clapboards.  Think of it like a separate buffer layer, the first line of defense to keep water from easily migrating into the wall structure.

b

The next layer is made up of vertical strapping, narrow ½” thick boards we will rip out of larger material and fasten vertically to the wall to create a ½” space between the redwood and the layer of housewrap discussed next.  This is the crux of the rain screen system.  It features a technique to install metal screen along the bottom and top of the channel to prevent critters from setting up shop in this desirable cavity.

c

Next is the single layer of Tyvek housewrap.  Tyvek is a waterproof/breathable material not unlike Gore-Tex or other high performance laminates (Ultrex, Aquanot, etc.) commonly used in rainproof outerwear.  The material’s pores are so small that they prevent liquid water from passing through but are not so small that they stop water vapor.  This layer is redundant to the clapboards and serves two purposes.  It stops any liquid water that gets past the clapboards from entering the wall but also allows water vapor to pass freely back and forth.  The latter characteristic is what permits drying to occur.

Another option we could have employed at this layer of the wall would have been to hermetically fasten to the sheathing what’s known as rigid foam insulation, the two most common forms of which are XPS (Extruded Polystyrene) and polyiso (Polyisocyanurate).  These are desirable materials since they share impressive R-values and, when installed at this location in the wall structure, act as a blanket keeping the entire framing system warm.  This is an especially good thing since moist air that is warm doesn’t tend to condense into liquid water.

However, aside from its toxic nature, we opted to not use rigid foam because (a) we don’t have the room to install the recommnded 2 inches to completely eliminate the chance of condensation, and (b) water vapor cannot easily (or in some cases at all) pass through it.  In this way, it can actually prevent drying.  Assuming no other layer deeper in the wall structure is not also a “vapor barrier” (an industry term for any material that allows no water to pass) then the wall is theoretically capable of “drying to the inside”.  We opted, in the end, for a wall that will be able to dry both ways, to the inside and to the outside.

d

Next comes the plywood sheathing.  There are pros and cons of this material (like most anything else) but we went with it for the additional strength it provides to the structure.  Plywood is made up of odd numbered layers of very thin wood veneer glued together in a way that gives the end result incredible strength when compared to a natural wood board of the same thickness.  Were we to build this house on a stationary foundation, we would use locally felled and milled “native” boards installed on the diagonal in place of plywood.  Plywood is not a local product and it has an undesirable footprint relative to our local alternatives.  However, given our tight dimensional requirements (we only have room for ½” of sheathing material) and the need to provide hurricane resistant strength to the structure, plywood is really the only way to go.

One note – plywood is a significant “vapor retarder”, another industry term meaning that it dramatically slows down, but does not entirely stop, the flow of water vapor.  It is so effective a retarder, though, that we plan to drill holes in it to better allow water vapor to pass through it.  These holes have come to be known as “Vapor Diffusion Ports” and have been shown to improve OSB’s ability to dry.  The same study that showed the advantages of drilling holes in OSB found that it that it made no significant improvement to plywood’s ability to dry.  However, the folks who undertook the study only used a couple of holes each at the top and bottom of each stud cavity.  In comparison, we’ll install a uniform amount of holes to the entire stud cavity – doing so certainly cannot hurt and should in the end improve the plywood’s ability to dry.  This layer will be fastened to the layer immediately underneath it using construction adhesive and screws.

e

The next layer is another layer of strapping.  The purpose of this second layer of strapping, this one fastened horizontally across the wall, is to add a little bit more depth to the stud cavity but mostly to reduce the amount of thermal bridging caused by the framing.

One way heat is transferred from cold to warm occurs when something cold is introduced into a heated space.  Aluminum windows are a great example of this phenomenon, known as “conduction“.  If you’ve ever been in an office building or other commercial space, where these windows are commonplace, you will have felt the effect that the ice cold metal window frames have on the warm side of a window.  Of course, regardless of their construction, windows are not unlike big holes in the walls they are mounted in, since they lag way behind their neighboring walls’ insulating ability.  However, windows made out of metal (steel or aluminium, for instance) only accentuate this problem since they have little innate ability to resist the temptation to conduct heat.  Cold outdoor temperatures transfer right through the frames, chilling indoor air in the process. Likewise, conventionally installed wooden studs, rafters in a cathedral ceiling and joist over an open or unconditioned space act similarly in this regard, transferring cold or hot directly into the living space.  And although wooden studs conduct somewhat less then their steel stud counterparts, studies have shown that the transfer of heat (i.e. heatloss) via conduction through wooden framing can reduce the whole-wall R-value by upwards of 10%!

f

 We decided to go with dense pack cellulosein the wall stud cavities for the following reasons:

1)  the product has a low-embodied energy and is comprised of up to 85% recycled paper content plus it is recyclable,

2) celluloseis non-toxic; even its additives are safe (the Borates/boric acid used as a fungicide and fire retardant are only slightly more toxic then table salt),

3) we can install this product ourselves,

4) there is some debate on this, but dense pack cellulose is said to be a great air barrier,

5) cellulose can store water vapor – keeping it away from framing, sheathing etc. – and also promotes drying by carrying moisture to dryer areas of the wall assembly, and

6) it’s a great sound insulator.

g

For over the studs, but under the next layer, we may use nothing or we may use an interesting product called MemBrainmarketed by Certainteed.  It’s a unique product that is apparently comprised of pores that expand or contract depending upon the relative humidity (“RH”) it “senses” in the ambient air.  In other words, when the RH is high (like in the winter when its cold outside, warm inside and we take a shower or when cooking produces a lot of steam) the pores will tighten and impart a vapor barrier quality to the product.  In this case, it will keep water vapor generated indoors from passing into the wall on its way outside, which it wants to do since moisture most always want to move to where it is dryer.  According to the manufacturer, when the RH is low (like in the summer months) the pores in this material actually open up and the perm rating (a graduated scale of how vapor permeable something is) increases from around 1 (when closed-off) to up around 10.  For relative comparison, plastic sheeting’s perm rating is practically zero and Tyvek’s is upwards of 50.

h

Since the start of this project we have wanted to use local pine or hemlock tongue and groove boardsinstalled horizontally for the interior wall and cathedral ceiling finish.  We may use white cedar for the bathroom just for the antiseptic qualities it imparts, it’s wonderful smell and it’s resistance to rot.

Much of the fuss in figuring out how to build our walls and ceilings came due to the fact that natural boards do not offer much in the way of an air barrier or vapor retarder.  The common “go-to” interior finish which provides both of those qualities is sheetrock with one coat of latex paint.  Since I am not a fan of either of these products from an environmental or aesthetic point of view, I assumed others would share my feelings and that there would be lots of information out there to assist the builder in designing a wall in which this natural product would comprise the inner finished surface.  Not so!

When we did receive advice, it arrived commonly in the form of “Put up sheetrock and one coat of paint [sometimes spelled out as “vapor barrier primer”], then install the boards over that.”  This seemed crazy to me.  Much of Scandanavia utilizes interior wooden boards in lieu of sheetrock (or similar products) on walls that comprise the shell of the building and they are located in Climate Zone 6 and higher.  I assumed that some of that knowledge of best practices when it comes to this material would have found its way across the pond.  Yet, it was tough finding anyone online who had much to offer regarding a substitute method for imparting an air barrier and vapor retarder.  We did finally find an expert who referred us to the “MemBrain” product discussed earlier, but we’re still searching for someone who has actual experience using the stuff.  As far as the finish goes, we will likely use a linseed oil and pigment mixture as sold by IKEA and others to lightly “pickle” the wood.

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This is Part 2 in a series of 4 posts in which we examine building science as it pertains to our tiny house.  Our next post in this series will will focus on the construction of our roof.

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4 comments to Tiny House Building Science: The Walls

  • Hei,

    I am a Norwegian who has moved to Seattle. I have been researching building a tiny home for quite some time now, and just came across your blog today. There is a ton of great information here! I noticed yours and your dad’s name, and figure you must be Scandinavian too. I hope you are having a great bike ride wherever you are, and that I can perhaps email you for advice in the future.

    Hilsen,
    Einar

  • beezwings

    Hi! I’m trying to figure out the best way to build my walls as well. I am insulating with wool, which has a lot of moisture-absorbing properties. I’m curious what elements of the wall science you changed since this post–I’m thinking I will follow much of what you have done!

    Thanks!

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