Want to use sawdust for insulation, is plywood breathable?

I want to make a breathable house with sawdust as the insulation. Im deciding on the interior and exterior sheathing to use. Im currently leaning towards plywood internally and externally, then tar paper over the external plywood sheathing, then roughsawn board over top of that (board and batten probably).

Is plywood breathable? Its wood sheets glued together, not sure what glue is used, maybe some sort of epoxy? Would this make the house not very breathable? I wont have any house wrap / moisture barriers (besides the tar paper). I want to make sure the house can breathe and the wall cavities can expel moisture to prevent mold and the sawdust to remain dry.

There is no such thing as a breathable house. Some materials can take up water and give it back if that is what you mean with breathable. Or do you mean drafty?? Sawdust is not a good insulator because it settles down over time and then the lower half is compacted (and insulates way less) and the upper half is not insulated at all. I personally would go for the good old rockwool because it stays put.

Angelika Maier wrote:There is no such thing as a breathable house.

Many of the traditional buildings, in many cultures, are described as being "breathable". Many modern, green buildings are also "breathable" by design. The materials are deliberately not air- or water-tight and will regulate their humidity, which changes throughout the year. I assume that's what Cameron means?

Cob is a good example of a breathable material. It will absorb water from the air during periods of increased humidity and release it when the humidity drops. This is one of the desired characteristics of cob - much like its ability to do this with heat, it acts as a buffer or a battery and ease transitions between extreme weathers (hot, cold, too wet and too dry). The change in humidity will create subtle air currents within the building and aid air exchange.

Many types of stone are also porous and will allow air and moisture to pass through them, albeit slowly. This assumed that the house is not rendered on the outside, or is rendered using a porous plaster such as limewash. Some types of stone (such as slate and granite) are not porous, however.

Modern houses usually include "trickle vents" above windows to allow continuous air exchange, preventing mold growth and the build up of formaldehydes and other unpleasant pollutants. In fact, building codes usually have a figure for a number of complete air exchanges that should take place each hour (meaning how many times ALL of the air inside a house should be replaced with fresh air from outside).

Cameron Miller wrote:
Is plywood breathable? Its wood sheets glued together, not sure what glue is used, maybe some sort of epoxy? Would this make the house not very breathable? I wont have any house wrap / moisture barriers (besides the tar paper). I want to make sure the house can breathe and the wall cavities can expel moisture to prevent mold and the sawdust to remain dry.

Plywood comes in various grades - most significantly, "marine" and regular. Marine ply uses a better quality of glue and typically has fewer voids between the sheets and, in general, is more durable and waterproof.

In general, I would say that plywood is not particulary breathable. There may be some air and moisture exchange through a sheet of plywood but I would imagine that most of that happens around the edges.

Personally, I wouldn't want to use plywood externally but that may be due to the damp climate in which I live. I have tended to build using a vapor barrier (e.g. tyvek) between my insulation and my breathable cladding material (e.g. heavily-overlapped, durable timber planks). I feel this allows any moisture that makes its way into the insulation to evaporate out (via the vapor barrier and through the gaps between the cladding boards). I do intend to use plywood to internally clad the shed I am currently building as I don't expect much moisture to end up inside the building - and water on boots, for example, that does make its way inside will be able to drain via the holes between the floorboards.

For breathability I want moisture to be able to escape through the walls, I dont want moisture barriers like plastic barriers choking breathability. I have inspiration from older houses before the invention of tyvek / vapor / moisture barriers. Older houses were generally thick wall studs (Balloon frame), diagonal boards (for strength) on the exterior for sheathing, tar paper was made in the 1800's so tar paper may have been used on the exterior (or nothing at all), then lath and plaster for interior walls (or more wooded boards). Wall cavities were generally filled with what ever was available, sawdust was common.

Some sort of 1" x 10" x 12' or similar boards would be acceptable for exterior sheathing. Ironically enough 1" thick boards are more expensive then 2" boards, there doesnt seem to be a lot of options for 1", most appearing to be "premium" for appearance. In the above example a  1 in. x 10 in. x 10 ft. (premium) is: $17.21. While a 2 in. x 10 in. x 10 ft. is: $12.72. ($1.5 / sq ft) .... now if you look at plywood its 15/32 in. x 4 ft. x 8 ft @ $21.45 ($0.67 / sq ft) ... so you're looking at roughly a little over double in cost to sheath the house in dimensional lumber vs plywood. What do you use the sheath your house? You mention you use "heavily-overlapped, durable timber planks". Sheathing with boards would be preferable but keeping costs low is important to me so Ill side with plywood / tarpaper. I could eventually further sheath the exterior with greenwood board + batten after construction if lumber is available on the property or I will have to find something different.

Really my concern was coming down to breathability of plywood, but if im using tar paper on the outside (which isnt very breathable) so I suppose the plywood vs planks doesnt make a difference. I plan to use sawdust for all wall and floor cavities for insulation.

Considering through I may sheath the roof with wood planks (2 x 10 x 12) or similar dimensional lumber diagonally with tar paper on top then a metal roof. This gives strength strength, more breathability (even if the tar paper kinda defeats this) and greater resistance to moisture / rot and the nails for the tar paper wont literally go through the dimensional lumber like it would for plywood.

Hi Cameron,

I think if you refine your search to "vapor permeable materials" or "vapor permeance of plywood" you'll find more useful information.  The characteristic of a material or wall assembly of being vapor permeable is often incorrectly, but understandably termed "breathability."  

What you are looking for are materials that, when assembled in a wall, allow for moisture in the form of water vapor that somehow gets into the wall to get out again.  

Water vapor moves towards an area of lower pressure, so moist air (high pressure) will tend to disperse towards dry air (low pressure).  Do a good job of air sealing the interior by making sure electrical outlets and plumbing perforations don't leak air that carries water vapor.  If you air seal both sides of the wall to prevent air currents from carrying water vapor in or out, you've solved most of the problem.  If the exterior sheathing and interior finish materials are vapor permeable but air sealed, then water vapor inherent in the building materials (wet lumber, wet plasters) can eventually escape the assembly through those materials, though it can take a while.  

Moisture can get into walls in a number of ways, but assuming there is no bulk water intrusion (roof leaks or wind-driven rain penetrating the wall, or interior floods), the most common problem is water vapor in the form of high exterior humidity trying to get into a cool, dry space (e.g., an air conditioned space in a humid climate) or warm, moist air from inside trying to get out (e.g. a warm, humid interior in a cold, dry climate).

Saw dust was used as an insulator in ice houses (before refrigeration), so it can't be all bad, but it probably does settle.  

I can't answer the specifics about plywood's vapor permeance except to say that in relatively dry, temperate climates like what we have here in N. California and S. Oregon vapor permeance modeling  (research WUFI) indicates that 1/2" CDX plywood is at the vary edge of being acceptable in a straw bale wall assembly meant to be vapor permeable to both the interior and exterior.  I have used 3/8" CDX plywood as the exterior sheathing on several straw bale building projects, including my own house.  I wouldn't use plywood thicker than 1/2" and expect much water vapor to move through it unless it were drilled with holes to facilitate that.  How many holes, what size, etc., isn't studied that I know of, and the number and size of the holes could compromise the material's function if also meant to be a shear wall.

When we talk about how vapor permeable a material is, we're talking about a given thickness.  For example, 1" of lime plaster is considered to have a permeance rating of 10 US perms (give or take).  Clay plasters are somewhat higher.   If you apply 2" or 3" of these same plasters, the permeance drops a lot.  The same thing is true of materials like latex paint.  At one or two coats latex paint is considered vapor permeable.  But a few paint jobs (six or more coats) and the built-up layers of paint can be water proof.  I once lived in an apartment that was at least fifty years old--I'll bet there were as many coats of paint on those walls!

In my remodel work I have removed a fair amount of plywood and OSB sheathing from straw bale walls and found no damage caused by vapor condensing against the sheathing and decomposing the bales, but I understand that OSB typically has much more glue--and you're right, that's the part that limits vapor movement.  If I worked in a more humid climate, I'm not sure I would have found straw bales in good condition.

This is not a well-studied subject, and most of what we know about straw bale walls and plywood sheathing is anecdotal, but WUFI modeling suggests that this wall assembly--straw bales plastered on the interior with a plywood exterior sheathing (and and either plaster, metal, cement board, or wood siding) it's OK in places like S. Oregon, much of California, and probably throughout the arid Western United States.

Good Luck!

Jim
Many Hands Builders

Sawdust insulation is common in the houses from the 1930s and 40s where I live. My parents' house is one, as is the house of my colleague. Most of these houses are fairly small by today's standards, so they've often been lower cost housing for people. Since the people living in them don't have a tonne of money, a lot of them have never had the insulation upgraded. A few years after my parents bought theirs, they replaced the sawdust with modern insulation. The heating bill went way down and the house stayed warmer.  Those sawdust walls are always cold. My colleague found the same thing.

I don't know what your winters are like and maybe you don't mind cold and draughty anyway cause you're sitting on your rmh bench. If you're going with sawdust, might be a good idea to figure out how to do it really well, though.