Retrofitting Straw bales For Interior Insulation?

Hi everyone, this is my first post to permies, but ive been a fan of this site for quite a while now.

I've also been very interested in straw bale construction, and the different ways that people have integrated alternative building strategies into existing structures.

I know of many people who line the exterior of an existing house with straw bales for insulation/aesthetic value, but I'm thinking about retrofiting the INTERIOR walls of an existing brick structure for straw bale insulation.

Here in Detroit, the city has tax auctions every now and then to sell off forclosed houses for extremely low prices. Many/most of these houses usually aren't in the best condition, but for what I have in mind, that's not so much of an issue.

Essentially, my idea is to gut the inside of a house down to the bricks, apply an interior layer of earthen plaster over the exposed bricks, and then to bring the straw bales INSIDE of the house to use as interior insulation, which would then be finished in a final layer of interior earthen plaster.

So this would be the composition of the walls, from the outside in: brick exterior, then the middle layer of earthen plaster, then the straw bales, then the final interior finish layer of earthen plaster.

Obviously this would cut down on the amount of interior square footage, but that isn't so much of an issue for me.

It seems to me that having the straw bales on the interior of the house would eliminate any risk of moisture from the outside infiltrating the bales, assuming a proper gutter system and foundation.

What I'm more worried about is any moisture being trapped inside, resulting in mold. Earthen plaster is a "breathable" material that will not trap moisture inside of a structure, which is why most straw bale houses use it to finish the interior and exterior. But for my project, there will be an additional brick layer enclosing the exterior walls. Could this cause condensation problems? I believe that brick is porous enough to allow moisture to pass through, but by that point any moisture generated in the house would have had to travel through many layers! 2 layers of plaster, the straw bales, and now a brick wall... I am wondering if ventilation might be nessesary between the brick wall and the straw bale wall to carry out any moisture before it builds up? Or are all the materials breathable enough to allow the moisture to pass right through, to the outside?

What do you guys think?
Has anyone had any experience lining the interior of an existing structure with straw bales, or can anyone direct me to any research that has been done in this area?

Thanks

Howdy Miles, welcome to permies!

I really have no experience with this but am interested in the concept and looking forward to an answer also.

Is Detroit making a comeback?

I belive a treatment of borax(or is it boric acid?) would not only adress the mold issue but any insect infestation issues and provide fireproofing.
I would also like to suggest setting up one or more of these house as grow houses!
Not for weed, but for oyster mushrooms. No sunlight or grow lights needed.

Filling the roof with windows of some sort, and adding windows to the walls, could allow a green house to be born.
Knock out the first and second floor, fill the basement with compost, and gtow full sized trees!

I don't see why you would want to do this. You would be moving free thermal mass to the outside of the envelope and loose space. Also the structure would not necessarily be equipped to handle the loads on the interior. seems to me for the same or less work you could do the outside and have a lot more on the plus side then the minus.

Hi Miles, and welcome to Permies.

There's a process called "slip straw" that may be exactly what you're looking for. Here's a great youtube video that shows how it's done: and there are pleanty of other videos to be seen as well, just search for slip straw.

While am an absolute proponent of straw bale construction, I've got to admit that when dealing with the detroit climate, as well as retro-fitting a 100 year old home, you may want to consider skipping the straw idea and research spray-foam insulation. Especially since you're going to be gutting the interior plaster walls. There are just so many nooks and crannies that you'd be missing with the straw method. They've got a new product out now that uses soy-beans to make the spray foam, so it's quite environmentally friendly, and once it's installed, it's 100% air tight and offers fantastic R value insulation. Naturally the down side is the cost, at around $4000, but there's the added benefit of one day service as well as being able to slap on conventional sheet drywall, which would be far more appealing to prospective buyers when it comes time to sell.

Just my .02 cents

Hi everyone, thanks for the quick replies!

Miles, while the city of Detroit is still barely able to function, it is definitely on the rebound. There is a lot of potential for investors, and many people like me who aren't able/don't want to apply for a loan from the bank in order to own a house. There is also a thriving artist community in the neighborhood that I live in, many of whome create "art houses" from once abandoned properties. Google search "hinterlands detroit", "design 99", "detroit contemporary"... My next door neighbor is actually totally off the grid, using a passive solar roof/woodstove for heat, solar/wind for electric, and rainwater harvesting system. Also with all the vacant lots and empty land, there is a growing community of urban gardeners in Detroit as well.

William, I think that your greenhouse suggestion could be a really great idea if done properly! I like the idea of growing oyster mushrooms, but it seems like it could have the potential to become some kind of giant pathogen incubator if not monitored closely!
But I am a big fan of greenhouses, and would like to integrate some kind of passive solar design into the roof for this project for additional heat.

You make a good point, Sean. A house may, or may not need additional structural support to be able to handle the weight of the straw bales. I guess it would really depend on the structural integrity of the individual house? Its definitely something to look into, and I'm glad you brought it up. But another reason I have been considering bringing the straw bales inside is "camouflage"... I mean, we're talking about using straw bales, a pretty unconventional building material, in the middle of a city, without code approval... Bound to raise some eyebrows. Plus, I also want to incorporate cob rocket stoves for my main heating, a composting toilet (I don't even want a flush toilet) and a lot of other "unconventional" methods that break pretty much every rule in the book. Its one thing to utilize these methods in a rural environment, and another thing entirely if you're in the middle of the city.
I'm not opposed to building codes or anything, they're there to protect people from shotty construction after all... But they can also impede new (or, rather old) experimental earthen building technologies, and hinder green progress.
My thought process is, if it just looks like another house, I will be less likely to run into any trouble with building authorities. Whereas If it looks like a (beautiful) straw bale house, it might attract unwanted attention from city officials.

Darryl, slip straw looks very interesting and straight forward, but I'm wondering what the R value of a couple inches of packed straw would be? And whether or not it would be suitable for Michigan climate?

My idea was to gut the old plaster off of the brick wall (which typically isn't in the best of shape in these old foreclosure houses) and then apply my own layer of clay plaster directly on the interior bricks BEFORE installing the straw bales. Then a final finishing interior layer of plaster over the bales. So the straw bales are kind of sandwitched between 2 layers of plaster. That way, the first plaster layer over the bricks would fill in all of the nooks and crannies of the brick wall, creating a (relatively) smooth surface for behind the bales.

I do want to use spray foam insulation for the attic, but to insulate the entire house with the stuff simply out of my budget... That's one of the things that turned me toward straw bale construction and earthen architecture in the first place. They have the potential to be within my means to finance without relying on a bank loan.

I've taken a tour of the "strawbale studio" Workshop in michigan a couple of months ago. To my knowledge, they are one of the main strawbale workshops in the US, and their bueatiful round structures are really what turned me on to straw bale construction. They told me that they had no trouble heating in winter using only a small woodstove, and the research that I've done shows that straw bales can have an R value of between R-20 and R-50! With insulation like that, plus efficient double pane windows and a rocket stove, I'm sure straw bales houses could handle any sub zero temperatures detroit could throw at it!

Thanks for your input you guys, you've really given me a lot to think about, and I'm glad I found this forum!

Miles,
After reading your reply, I now understand your shooting for maximum savings. So, here's an idea you might consider...
Used 2X4's aren't that hard to come by on craigslist. You can even salvage them [in 4 foot sections] from old pallets. You could attach them to the existing studs in the walls with long screws, thereby making the "insulation area" between the brick outside and the plaster inside anywhere from 8 to 10 inches. That space could then be stuffed with quite a bit of insulating straw.

I worked construction for a couple years and I've got hands on experience with both sheet rock and trowel plastering... I'd go for the sheet rock any day of the week! The raw materials for store bought drywall sheets will run you about $500. A good 2 man crew can drywall the interior of a house in a weekend. By going earth plaster, your raw materials cost goes down, but you're looking at several weeks of back breaking labor... At the end of the day, it's just better economical sense to work your 9 to 5 job for the $500 than it is to break your back for up to a month to get the same results.

As to the insulation values, if a 12" bale with 1" of earth plaster on both sides is good for R-50, then an 8-10 inch packed slip straw cavity with 4" brick on one side and 1/2" drywall on the other should be good for somewhere in the neighborhood of R-40... Having said that however, the term "R-value" has many facets that work differently in different climates. Convection, conduction, and radiation are the three ways energy is transferred, and different methods come into play when your trying to keep a house warm as opposed to keeping it cool. Here's a wikipedia link that explains it, and it's worth educating yourself. http://en.wikipedia.org/wiki/R-value_(insulation)

If you're going forward with the straw walls, then I'd advise against using spray-foam in the attic. The straw has to be able to breath, and by sealing the attic like that, it would be like putting a tupperware lid on it. Instead I'd go for blown cellulose. Specifically the type made from recycled newspaper. It's available at the local home depot, and if you buy enough of it at once, they'll usually loan you the machine to blow it in for free. As an added benefit, blown cellulose is super cheap. It shouldn't cost more than $200 to put a 16" layer of the stuff up there, and 16" of cellulose will have an R-value of close to R-60 [@ R-3.8/in]. Again, another wikipedia link to help you. http://en.wikipedia.org/wiki/Cellulose_insulation

If you're going for a super insulated structure, you cant ignore the floor. If you're sitting on a concrete slab, there isn't much you can do, but basements and crawl spaces can be upgraded. The most cost effective, as well as the most straight forward solution would be conventional faced fiberglass bats stapled to the floor joists. If your house has a basement, and your service systems are located there [i.e., water heater, etc] you'll also have to give some consideration to protecting them from the cold as well.

And finally, stopping air movement is going to be the key to all of this. That means good windows & doors, and patching any cracks or openings on the outside brickwork. You'll have to research your home to see if it has portland cement or lime mortar before you tackle a re-pointing project as the two DONT mix very well. Most likely, a home built before 1930 will have lime, which is actually a superior product to cement.

Before I close, there's one last idea I'd like to throw out there for you to mull over. I know that a lot of homes in the north-east use a boiler and radiator heating system. You've already said you're wanting to look into heating with a rocket stove... If your home does have an existing boiler, it might be possible to do one of two things. The first is to construct your rocket stove out of stainless steel [think old beer keg] and circulate water thru the jacket, then connect it to the existing radiator[s] in the house via a small [or even existing] pump. The second is, if you get a house that has a city owned vacant lot next door, consider doing a Jean Pain style compost mound on the vacant lot and connect that to the radiators. Unfortunately I have zero hands on knowledge with rocket stoves, boilers, or compost mounds, but I'm intelligent enough to believe this could be an option and would be worth exploring. Personally, I'd go for a compost mound because it could provide heat, hot water, as well as heating a pool or hot tub in the summer months. Again, craigslist is your friend. Around here [Kentucky] people give old hot tubs away for free if the heater or motor burn out, and the beauty of a compost mound system would be that you wouldn't need the heater or the pump

I hope you hang around and share your progress.
Your idea is magnificent and brave, I hope you can get it to work.

OK I'm going to inject some building science into this discussion.

1. Putting structural brick on the outside of a permeable and even an "impermeable" insulation material is not exactly wise. During cold times which do happen in Detroit you'll have a higher level of humidity inside the house than outside. As this humidity moves through the straw bale/slip it will come into contact with the much less permeable brick which also happens to offer a cold and comparatively impermeable surface. That surface is the perfect place for humidity to condense and turn to moisture/water. Water in a wall system means mold, mildew, rot and basic failure of the structure.

2. The brick is free thermal mass, putting it outside your thermal barrier means that its now a temperature conductor. In the summer is will gain heat and reduce the insulation performance of the straw, in the winter it will conduct heat away from the thermal barrier and again reduce performance. Put the thermal mass inside the structure and it does the exact opposite it helps regulate interior temperature and thus increases overall performance of the building. So inside VS outside, you want the insulation outside.

3. Straw slip has a lower R value per inch vs bales, it is really meant more as a semi-structural material for cob based architecture. I don't see the value in this application. If I lived in a very temperate dry climate then I would say straw slip has good value.

4. Glass bats are my personal LEAST favorite insulation material, they hold moisture, are hard to install properly etc.... I much rather recommend spray foam for the basement, foundation or crawl space.

5. Minor point of interest, there is some debate on the R-value of a straw bale and generally the numbers are between R-30 to R-50. R-50 is on the high end so don't assume that to be a hard consistent number. That said R30 is probably more then enough for this type of application in that climate.

Hope that's some useful information for you.

Hi there and thank you for starting this thread.

I also have dreams of using straw bales for interior insulation/retrofitting.

In the meantime, I have been looking at Rock Wool/ Roxul insulation on the web to insulate inside rooms in the part of a home I'm sharing for now. The review for it are all high marks. It repels water, does not allow for vermin, etc. It is not fiberglass and folks say it is much easier to use.

I've been looking at the R-30 Roxul.

Just wanted to share this information since all of us on the Earth Transition Team are all learning as we go!

miles rose wrote:

Essentially, my idea is to gut the inside of a house down to the bricks, apply an interior layer of earthen plaster over the exposed bricks, and then to bring the straw bales INSIDE of the house to use as interior insulation, which would then be finished in a final layer of interior earthen plaster.

What do you guys think?

Thanks

Hi Miles, welcome to Permies!

I hope your project succeeds! As much as I'd like to see Detroit reborn as an artist community, I'd much rather see it reborn as a hub of cold-climate alternative building.

I spend some time looking at houses in Detroit (as well as the rest of lower Michigan), so here are a couple of important factors for your plan:

First, it sounds like you might be picturing these houses as having load-bearing brick walls. Almost none of them do. That practice was pretty much over before Detroit's building boom, so they'll almost all be brick veneer over stud framing. Seriously, I'd bet that 96% of the brick houses in the city are brick veneer and 99.5% of the houses available for auction. That's going to change your approach- now you've got to cut a 1.5"x3.5" notch in your bales every sixteen inches for the studs OR leave a 3.5" gap between your bales and your bricks. Neither of these is desirable.

The other thing is that the $500 houses are mostly built before the era of open floorplans. Your contemporary tract houses, not such a big deal if your turn an 18'x 26' living room into a 16'x24' living room. Still fine. But you turn a 10'x12' room into a 8'x10' room, and you'll regret it.

If it were me, I'd be heading in the direction of doubling the studs and deciding how to insulate a twice-as-large-as-usual cavity, whether with slipstraw or rock wool or something else.

Good luck!

Mike Cantrell wrote:

miles rose wrote:

Essentially, my idea is to gut the inside of a house down to the bricks, apply an interior layer of earthen plaster over the exposed bricks, and then to bring the straw bales INSIDE of the house to use as interior insulation, which would then be finished in a final layer of interior earthen plaster.

What do you guys think?

Thanks

Hi Miles, welcome to Permies!

I hope your project succeeds! As much as I'd like to see Detroit reborn as an artist community, I'd much rather see it reborn as a hub of cold-climate alternative building.

I spend some time looking at houses in Detroit (as well as the rest of lower Michigan), so here are a couple of important factors for your plan:

First, it sounds like you might be picturing these houses as having load-bearing brick walls. Almost none of them do. That practice was pretty much over before Detroit's building boom, so they'll almost all be brick veneer over stud framing. Seriously, I'd bet that 96% of the brick houses in the city are brick veneer and 99.5% of the houses available for auction. That's going to change your approach- now you've got to cut a 1.5"x3.5" notch in your bales every sixteen inches for the studs OR leave a 3.5" gap between your bales and your bricks. Neither of these is desirable.

The other thing is that the $500 houses are mostly built before the era of open floorplans. Your contemporary tract houses, not such a big deal if your turn an 18'x 26' living room into a 16'x24' living room. Still fine. But you turn a 10'x12' room into a 8'x10' room, and you'll regret it.

If it were me, I'd be heading in the direction of doubling the studs and deciding how to insulate a twice-as-large-as-usual cavity, whether with slipstraw or rock wool or something else.

Good luck!

If this is the case then it will be hard to retrofit the structure to be more inline with a high performance envelope. Structural brick is a lot easier to work around than veneer.

You're probably not going to like this but the first decision you need to make about your project is: How will you quantify performance?

-energy efficiency?
-living space?
-useful space?

Buildings are very much machines, how they serve you is entirely up to you.

MIles, I salute your urban homesteading instincts. But be aware, a brick exterior, whether veneer or load bearing, is NOT waterproof. You have to design for water on the interior face of an exterior brick wall, because it does end up there.  The folks at buildingscience.com have extensive experience with energy retrofits of masonry buildings, and they know their building science, so they will have solid,  tested recommendations for safe ways to add insulation to a brick building.  Best of luck.

Darryl and Sean have raised some excellent points, and I’m glad Bob Theis chimed in what I was going to add.

Moisture management is everything in straw bale structures.  You might be wise, in the long run, to hire a consultant with expertise in thermal envelope performance, moisture management (incl. drainage planes, condensation issues, capillary breaks, etc...) PLUS the needs of straw or light straw clay.

Maybe this forum could help you find a skilled person, or a green building group near you that could help.  Otherwise, you could end up with brick walls surrounding a pile of compost 😨