Why not use strawbales for slab insulation, and just say no to foam?
2tePuaao2B
| Posted in General Questions on
The Archibio Sandwich Slab has demonstrated observed performance that can make them very attractive for very cold climate situations. With almost no option other than petro based materials, bales represent an ideal low-cost solution for quality comfortable floors.
The strawbale insulated slab uses approximately 20% more concrete than a 6″ thick conventional slab for an equivalent floor area. The cost is between 50 cents to 1 dollar per square foot of R-50 insulation, no other insulated slab can be so affordable.
The Archibio Slab is like a big sandwich made of 2 concrete slices with a layer of bales in between.
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Straw bales have certainly been used as insulation under a slab, and they do provide excellent R-value. However it should be noted that everything I have read about this method mentions that it uses more concrete than a standard slab. Anywhere from 20 - 40 percent more concrete. Considering the high embodied energy cost of concrete, the associated CO2 pollution, and presence of heavy metals increasing its use could very well be counter-productive. After energy production concrete production is one the largest sources of CO2.
Roy,
Several years ago I visited this straw bale home and spoke with the owner/builder - https://www.greenbuildingadvisor.com/homes/straw-bale-home-minnesota-woods
He noted that he wouldn't recommend using straw bale under the slab because he found signs of rot in his installation.
In my opinion it would be impossible to keep the straw bale dry in a subslab application and the probability of it subsequently rotting is high.
If they are encased in concrete (below, sides, and above) I don't see why rot should be an issue.
Water works its way through concrete via hydrostatic pressure and capillary action.
We are all familiar with the capillary break required between footings and foundations walls.
Straw bale is extremely prone to moisture damage/rot. Straw bale is more problematic in cold climates also because of vapor diffusion into the assembly during the winter.
I've worked on cob/straw bale construction with a natural builder who goes through great lengths to avoid the use of any 'unnatural' materials. But even he compromises where the wall meets the foundation with the use of rigid foam board and wet applied capillary breaks to ensure water is not wicked up from the ground into the straw bale.
I just read the book quickly online.... rot is a problem, lots of concrete, complex.
My thoughts as of today. TOO MUCH WORK FOR SOMEONE MY AGE TO DO AND TOO MUCH NEW TO LEARN!!!!!
Was I yelling? Sorry, had to. May even need to grab an IPA. Changed my mind, am grabbing one pronto.
I want to know more about Roxul costs? Who knows the cost of the under slab rated Roxul?
Oh and I did note the formaldehyde someone says it has but I think under slab no problemo is my guess.
Thanks J Chesnut, for providing info concerning rot issues, most informative.
AJ, I am trying to contact a Roxul rep about the cost. I will post once I find out.
Anyone familiar with Faswall or Durisol?
I think they make poor wall assemblies but seems like they could make a rugged subslab insulating "SIP" panel. The product is basically a sandwich of the mineral fiber insulation and cementitious wood fiber, which I believe is inert to rotting.
So we need prices and assembly info for Durisol and Faswall too.
Don't remember the cost precisely but Roxul is easily several times the cost of eps or xps. I'll look thru my records and see what I can find. And not to be crass, but I have severall pallets of Drainboard on a jobsite in southern Maine right now that I don't need if anyone is interested.
J- what leads to the opinion on Faswall and Durisol making poor wall assemblies? I am familiar with a couple of successful projects but I have not personally worked with them. Thanks for the thoughts.
With proper installation that includes a layer of compacted sand and stone hydrostatic pressures , capillary action has proven not to be a problem. Casual misunderstandings of the proper technique had resulted in some very unfortunate experiments back in the 90's. I believe these are what are still being refered to here. The original procedure has been vastly improved through the years. I'm working on getting the current detailing to share.
- the R-value is not good enough for cold climates. It is only a little better than a 2x6 frame wall.
- the wall system is relatively resource intensive because it requires concrete and rebar for structure
- the modules are not precise in dimension and because they are dry stacked problems arise trying to keep the wall plumb and level (you can see shims in the attached picture)
- takes longer to build than frame walls
- has most the drawbacks of detailing a CMU wall
- a CMU sized module with some insulation stuffed in the cavity is just not a well conceived system in my opinion
I'll be interested to hear more about this. I would expect that any recommendations for such systems will depend highly on local conditions including soil types. I would be skeptical of any "universal" method that would claim that an organic material buried under a basement slab will not accumulate moisture at some point and then rot.
I am working with a slab on grade, not a basement slab, but I'm sure that the technique will work in either situation. Was there a point or valid contribution to this thread with post # 18 ? ...
J. your right, high clay soils require more sand and stone, but the system is working. Oil hasn't funded the research so the old proven test of time, ( with moisture monitors) is at work here.
I couldn't of said it better J
Roy, how can you not understand my point? I am posting that I feel exactly as J does and sharing that with whomever would care to know that now, two people have chimed in with this view.
Why are you asking? You didn't understand or you were offended, what?
Yes, there is a point to my post. And there will be others that take note even if you don't.
We all need to work our butts off to be less in each others face Roy. Don't you think so? Yes?
I do. Enough said.
I agree. I would be too nervous to try to put strawbales below grade...
Are there any examples of strawbales being used above grade as a "sub-slab" or sub-floor insulation?
Here's a less than half-baked idea:
Maybe some type of platform structure lifted a short distance above final grade by piers and strawbale infill underneath the platform.
Loose fill cellulose on top of the strawbales between the joists of the platform structure.
The base under the strawbales could be well drained gravel with an impermeable membrane between the ground and the straw.
A parged exterior but maybe with a rainscreen cladding as well for suspenders.
I imagine the strawbales and cellulose would have to be able to dry to the inside of the platform structure above.
Wide overhangs, and all-round kick-ass drainage would be critical.
I looked it up - we paid about $50/4x6 sheet of 2-3/8" Roxul Drainboard. So over $2/sf, as opposed to under $1/sf for 3" EPS.
Dan,
The 2 3/8" Roxul DrainBoard is R-10, while the 3-in. XPS is R-15.
So the Roxul is more than 3 times as expensive as the XPS in terms of $/R.
I'll be getting the detail package for strawbale insulated slab a little after noon. After I review it I'll be posting for those that are interested.
Great day to all!
Lucas,
I like the way you are thinking with post 20. I'll give it a more than half baked rating.
The sandwich slab consists of a web of concrete that has a percentage of chopped straw incorporated into the mix. The mix is filled solid around all bales creating a web of support between and around the the slab.( Kind of like a tension box in woodworking terms) Large overhangs with proper "kick ass" drainage are always a part of the strawbale and other natural building home designs that add to the prevention of water entry.
Regarding J Chesnut's observations on Durisol. Having used Durisol for my house I very much agree with his observations. I was enamoured with the concept of thermal mass, among other things, and FHB article helped drive me toward using it. Typical framing is a heck of a lot easier, knowing what I know now I would have much preferred a PERSIT/REMOTE approach, what Thorsten Sclubb has written about.
It sounds a bit like Thermomass w/straw subbed for the EPS?
What would you do at the edges?
I'm thinking not just of moisture but of pests -- I hail from Termite-Land.
Joe
Here is a concept sketch to go with #20.
This idea is less than half-baked - there are many unresolved issues that I haven't thought about like air sealing, attachment of rainscreen cladding over parging, drying potential of under-floor insulation, etc, etc.
Also there is no scale and proportions are not necessarily correct.
@ Lucas post 27. Why bother with straw bales on the ground? Why not suspend insulation from the floor joists? Put in a suspended system and fill with cellulose to top of the joists, vapor barrier on surface of ground like with a conditioned crawl space, air barrier at subfloor. The structure would have to be designed to support the added weight of cellulose on the joists, but you don't have any contact with the ground at all except at the foundation and internal posts.
David,
There was some previous discussion about using strawbales for sub-slab insulation. This was just an idea I had for using strawbales to the same end but without having to put them below grade.
Yeah, you could do that too.
I've always liked pier foundations for some reason...
I'd like to learn more about strawbale building as well.
Strawbales won't hold nails or screws obviously so how could rainscreen furring be attached?
Nice job on the detail Lucas.
Here is a BSC publication on straw bales. Note the tested R-values.
http://www.buildingscience.com/search?SearchableText=straw+bale
Don't get me wrong, I would love to do a straw bale home. I wouldn't consider straw bale for subslab/ subfloor insulation however.
Note straw bale is somewhat similar to fiberglass insulation in that it allows air movement through it. Straw bale needs to be encased in stucco/ clay slip so air conductive currents don't undermine its R-value. Air infiltration carrying humidity is a big concern in cold climates.
Straw bale gets wet in the construction process. Whether you are pouring concrete on it or applying stucco to it. The trick is to allow the bales to draw out. If half the face of the bales are sitting on the ground you aren't you already loosing half your drying potential?
Lucas,
"Strawbales won't hold nails or screws obviously so how could rainscreen furring be attached?"
The only time I have seen traditional siding types installed over strawbales is when the framing or post & beam structure is place at the exterior. The bales are set either flush or behind the framing. When the framing is set to the interior side of the bales I have only seen the use of stucco or plasters.
Using a post & beam structure with the strawbale infill set flush to the framing, I believe you would want to encase the bales with a plaster (as J- recommended). Then attach your rainscreen furring strips to the P & B structure. I have not seen a detail of this in the strawbale books I have read, but you have peaked my curiosity. I will post my ideas on the details tomorrow.
ED'S NOTE: DYLAN, IT WAS THE AMPERSAND IN THE FILE NAME. I CHANGED THE FILE TO JPG WHICH MADE THE LINK WORK, BUT THE IMAGE STILL DIDN'T SHOW UP. DELETING THE AMPERSAND FROM THE FILE NAME BEFORE UPLOAD MAKES IT SHOW UP.
J Chestnut,
I believe that you are refering to a structural straw bale building process.
To build using the straw bales as infill is entirely different. What about the fire ratings?
I find this very interesting. Thank You
I think those bales would work out fine in that area. Cheep to
Dylan, the link that you posted is broken.
I would like to see what is on the other end if you have time to fix it.
Bumping this since I would like to see the file. Can you try again or email it in for GBA to post?
Thanks
I don't understand why my file is not viewable. Do I need to do something in addition to attaching the PDF file?
I don't know where the Roxul prices earlier in the post came from, but in Virginia I just got a quote for R-10 2 3/8" Drainboard 4x6 sheets at $19.03 a piece. 1" for $7.97 (R-4.3). That puts it right at the same cost as foam alternatives.
I'm also using Faswall on N/E/W sides of house and 2x6 framing south wall with all the windows. A close friend loves his Faswall house which is a big part of why I went that direction. I just received the block this week so I'll let everyone know how it goes. It's a good bit of concrete which isn't what i thought I would be building with 5 years ago. I thought I wold be building with straw, but then I tore out another friends three story gable-end strawbale wall assembly that had molded and decided against strawbale at that point. If I had found this website earlier I would probably be doing Riversong trusses or other 2x6 framing. I don't have much glazing in the three Faswall assemblies, and virtually no plumbing or electric, so it should go fairly smoothly.
I had a PH consultant model the house design and he said I was decently close to PH certification for heat loads, 6.94 kBTu/sq ft. annual. So for my temperate climate where the concrete mass helps especially with the summer cooling load according to his calculations, Faswall seems to perform well on paper.
Don't know if anyone is currently checking this post but here are my thoughts concerning a rain screen with a strawbale structure. Sorry took me so long to post, was caught up in real life, not web life.
I chose to split the slab insulation between rock wool and foam. The split is to balance cost & R-value with reduced foam & termite protection.
Thanks to Dan Morrison for the technical assist, he posted this as a jpeg above. Now that I know about ampersand problems I wanted to post the original pdf file, primarily because of better resolution.