Stick-frame Retrofit Suggestions

Mid-Vancouver Islander here (for sake of climate info)-

I'm wondering if I can pick your collective brain about cob/slipstraw building- retrofitting a stick-frame 20'x30' building with a trussed roof on a cement pad. There is plywood on the exterior and, apparently, it's structural, so I'm not allowed to remove it. We're trying to figure out if there is a safe way to insulate the walls and roof of this shack without using synthetic materials which will off-gas or get all up in my nose, eyes, skin, and throat with little prickly horrible bits.

We already have planned a little cob extension on the front of this structure (south side) and we will be loading that up with glass and windows to let in light. We'll be making a thermal-mass floor for the entire building. Because of the cob section (its look and feel) I would really like to extend the earthy, rounded, smooth feeling to the rest of the building's interior, so I thought that maybe a slipstraw infill between the studs, and then plastered, might work. But by many accounts, slipstraw doesn't have a very high R-value. I'm also a little worried about moisture moving in the plywood, and the need to put up tar paper on the exterior of the plywood in order to plaster it (cutting off the path vapour, possibly causing condensation?) I would really like to earthen plaster both the interior and exterior walls. Any suggestions (even outside-the-box ones) are greatly appreciated! I've been searching and searching for a solution to this, but I think a discussion might be what I need most. Cheers!

Kat

I've had these same exact thoughts about retro fitting light clay/straw into an existing stick-framed house. Just so you know, yes the plywood sheathing is structural, but it doesn't meant there aren't ways of getting around it, since having a sheathed wall with slipstraw behind it defeats a lot of the purpose. That being said, doing a major remodel and redoing some of the structural elements would best be left to a contractor, or at least have one help you. I'm all for owner-built homes, but there are times and places that a pro's expertise is helpful, and this would be one.
One option would be to basically build a parallel wall, either inside or outside, so that the walls would be thicker and thus take further advantage of the light clay/straw (slipstraw) . Putting a wall on the inside of course cuts down on space and you'd have to bring all the electrical to the surface, while putting it outside cuts back on your overhang, which you'll want plenty of for good wall protection. Either way you might have to revisit your foundation to ascertain if its strong enough to support a good amount of heavy wet material.
There are other optinos for insulation besides slipstraw, such as cellulose (paper) blown in or wool batts (great but a little bit spendy). I suppose you could just stuff straw in the wall cavaties, but I'm not sure if thats code approved or how well it might function.

perhaps wrap it in straw bales?

Thank you for your ideas! I think I would love to do a straw bale wrap, but the biggest problem with that is the existing roof doesn't create enough overhang. I love the idea of insulating on the outside, but the roof just makes that difficult. What seems to be the popular option here is just using denim batt insulation and then plastering the exterior and interior of the walls. I'm feeling a little set back because I wanted to do a real "green" building, but perhaps the most permaculture-minded thing to do here is work with what I've got, and what will be the most convenient in that case. Is it okay to earthen plaster over a vapour barrier? I'm thinking that I would put wire/hardware cloth over the vapour barrier and plaster that. I'm absolutely firm on not using drywall, so I've at least got to find an alternative to that.

Can you do blown in insulation in the walls?

I had a similar issue with my house when we remodeled it. It's a 1927 Montgomery Ward bungalow with plaster lathe walls. There was little to no insulation in the walls. We were going to do blown in insulation but because of how difficult it would have been to repair the plaster we ended up using foam board insulation on the outside under the siding. When we installed the foam board we took care to tape all the seams to help reduce air infiltration. While the added R value did help with heating costs the biggest difference I notice was that when the wind blows hard outside I no longer "feel" it inside.

I know styrofoam is not a green material, and if you don't want to use it you might consider using a housewrap like Tyvek or similar.

In my opinion I weighed the "greeness" of conserving energy over the life of the home as being greater than the one time production and use of styrofoam.

So for me I considered it an appropriate technology for rehabing an 85 y/o house, some might not.

Ok as I understand it priority 1 is insulation. With only a 20x30 building, you'll really feel it if you try to insulate with straw bales and don't extend the foundation outward.

I haven't yet seen a straw bale wall that was less than 14" thick, meaning you're taking away over two feet from both length and width of your interior. If you want to extend your foundation to add on so little, you'll probably not get your money's worth.

What I consider a better option would be to choose a better insulation to put in the wall.

Off the top of my head there are only 3 other insulations that aren't completely synthetic and have adequate performance. Cellulose, Recycled Cotton (generally blue jeans), and Wool. Unfortunately generally all three of these insulations are treated with an antifungal, i believe a borax based one is used so it's not too bad. Always check with the possible supplier though.

As for the earthen plaster you can always put it on a metal lathe if you're desperate for the finish.

Can you add a non-insulated covered deck around the house to extend the roofing coverage while planning to use the edge at the house to do the straw bale wrap?

We have a shed that we used 6" cross cut sections of a tree. Screwed on from the inside over an OSB sheathed wall. These are about 24 inches in diameter. The plan is to mud between the logs. Sort of a thin cord wood mounted on 2X4 walls. They do split and crack that's why we are waiting to mud them. Ya I'm not getting a building permit.

Kat Cearns wrote:Mid-Vancouver Islander here (for sake of climate info)-

I'm wondering if I can pick your collective brain about cob/slipstraw building- retrofitting a stick-frame 20'x30' building with a trussed roof on a cement pad. There is plywood on the exterior and, apparently, it's structural, so I'm not allowed to remove it. We're trying to figure out if there is a safe way to insulate the walls and roof of this shack without using synthetic materials which will off-gas or get all up in my nose, eyes, skin, and throat with little prickly horrible bits.

We already have planned a little cob extension on the front of this structure (south side) and we will be loading that up with glass and windows to let in light. We'll be making a thermal-mass floor for the entire building. Because of the cob section (its look and feel) I would really like to extend the earthy, rounded, smooth feeling to the rest of the building's interior, so I thought that maybe a slipstraw infill between the studs, and then plastered, might work. But by many accounts, slipstraw doesn't have a very high R-value. I'm also a little worried about moisture moving in the plywood, and the need to put up tar paper on the exterior of the plywood in order to plaster it (cutting off the path vapour, possibly causing condensation?) I would really like to earthen plaster both the interior and exterior walls. Any suggestions (even outside-the-box ones) are greatly appreciated! I've been searching and searching for a solution to this, but I think a discussion might be what I need most. Cheers!

Kat

For starters, if you want an insulation that has nearly zero (everything emmits something) off-gasses, check out Mineral Wool/Rockwool insulation. Roxul makes one that I am familiar with and have used it in my own house. It has a high R-value plus does not absorb water or mold, and is resistant to fire naturally.

Depending upon how your how as built, you could work in 4-foot high by 8-foot long runs w/o reducing your structural capacitity much, however, like said above that is something I would leave to a pro. I've done it on a few buildings and it is a pain in the ass, but that is what I get paid for.

If you do not want moisture to get into the plywood, you will need to keep between a 1/2-inch to 1-inch air gap between the cob & plywood. However, with that said there are house-wraps (Tyvek) and the old fashion roofer's felt for keeping exterior water from moving to the interior.

Hope this helps.

As a long-time builder and general contractor, my opinion of tyvek could not be lower as regards pretty much all of its qualities. Roofer's felt (tar paper) is superior in every way. Traditionally, quality stucco work was backed by two layers - one would bond to the cement, the second layer protected the home. A rainscreen or vent wall is a great technique, but the relative weight of earthen renders makes this system questionable as an add-on type of siding.

There are a lot of options for shear-bracing the walls without the plywood. Let-in steel braces were once standard on many homes, and diagonal 2x4 works great as well. In many code departments, the exterior shear qualities may be substituted for the interior sheathing structure. This mostly applies to very low quality housing - builders are allowed to use the interior drywall as the bracing and put cardboard/buffalo board on the outside, or, in many cases, they simply use interlock siding right over tyvek, no sheathing at all.

A crazy option is to simply move the walls themselves inwards a certain distance, re-anchor the plates with epoxy and threaded rod, thus making space for clay-slip on the exterior. Making a couple hundred feet of sawzall cuts plus a temp dummy wall inside seems extreme but is possible. Also, extending the eaves may not be easy but is also an option if you get creative with things.