[GSBN] Sheathing and straw bales proposed
Jacob Deva Racusin
buildnatural at googlemail.com
Sun Jul 20 22:27:34 CDT 2014
COStraw Nostraw - catchy! 'Strawbale Mafia' has a ring to it too...
Yes, we've been working through a few iterations of what we are calling
the StrawCell system, I've been involved with three or four builds of
this strategy and have a pretty decent handle on some of the
peculiarities. Here are a few notes, both solicited by your questions
Ian, and unsolicited but probably relevant:
I'd be very hesitant to use batts in this assembly. The reason you can
get away without the exterior plaster coat, as you correctly identify,
is that there is a thick blanket of insulation outboard of the bales,
which effectively moves the dewpoint further to the exterior away from
that junction, placing the exterior bale plane in the middle of the wall
(for all practical purposes - more on that below). Batts strike me as
having the following problems:
- very difficult to install to fully fill the cavity, and more
likely to allow interstitial convective cycling (or, air movement) which
could cool the assembly and potentially lead to condensation-related
issues, as well as reduced thermal performance
- must be installed prior to bales, greatly increasing the
likelihood of air movement between the insulation planes, potentially
leading to condensation-related problems and reduced thermal performance
(more on this later)
- cellulose does a much better job of spreading and safely storing
liquid moisture should a wetting event occur
Regarding the sequencing, yes, the benefits of being able to close the
building in and install the bales in the winter is pretty incredible for
us cold-climate folks. We just did a bale raising in January - that is
a tremendous coup for those of us who have had to perform schedule
acrobatics for years, driving the feast-or-famine seasonal lifestyle of
the northern bale builder. I've installed bales both before and after
blowing the cellulose. From a performance perspective, I really
advocate for installing the bales first and then blowing the cell - this
ensures (providing you have set up the cavities well and done your
planning right back in framing) that the cellulose will make complete
contact against the straw and fully fill the wall cavity - a critical
installation detail. This presumes appropriate dense-packing technique
(min 3.5 lbs/cu ft, etc). When you install the cell first, the bales
just don't make a really tight fit - they themselves are wonky, and the
cellulose tends to bulge a bit in the middle of the cavity (esp if using
InsulWeb or comp fabric membrane to retain the insulation), creating
little chimneys at each stud. We stuffed each of these with cellulose
at each course as we were baling, but it was a hassle, and less than
ideal. Of course, it meant we were dried in and insulated before the
bales even showed up and could blow the cell from inside the building,
so that's pretty attractive from a sequencing perspective. Otherwise,
you are blowing from the outside - better for mess, worse for access
(staging for two-stories). Also means you either need to net the
exterior prior to sheathing (an extra step, but nice to see the cavity
visibly), or blow through your sheathing into blind cavities. I've done
a lot of this and feel confident with blind blows, but it's a tricky
move and hard to verify without cutting extra holes and/or doing precise
bag counts, and the installer needs to be really experienced. You can't
conduct IR quality control, as there is a wall of straw on the other
side of the cavity! But, the end product is arguably better, if the
blow has been done right...The other concern there is blowing the bales
out of plane. 3.5 lbs/cu ft is a lot of pressure - more if air builds
up in the cavity and can't escape if blowing through sheathing - and
we've added a lot of extra prep work by not supporting bales well
enough, esp at windows/doors and tops of walls. We are now doing an
interior pinning-type strategy, where we sew 1/2" x 2" strapping through
the bales with baling twine and double-staple it to the studs before the
sheathing goes on. We can plumb the walls as we go, and ensure that
when the bales get pushed away from the studs they are held on layout by
pre-plumbed strapping - works really well. It also allows us to cut all
the strings on the face of our edge-laid bales and feather out the bales
so we have a fraction of the straw-clay stuffing, much less prep time on
the interior plastered surface!
I'd suggest you upsize to 2x6s, for the following reasons:
- no additional labor cost
- the extra cost of wood will be easily recovered by the increased
thermal performance and reduced operating costs, providing you are 1)
not heating with free fuel, and 2) aren't looking for a 2 year ROI or
something silly like that
- it makes blowing cell MUCH easier, esp if blowing through sheathing
- it adds more insulation outboard of the straw, further increasing
the likelihood that the ext bale plane will be above the dew point temp
That last note speaks back to the question earlier about batts vs bales
- really, the key here is making sure that you have enough quality
insulation outboard of that straw to keep the ext temp above dew point.
That depends on a few environmental factors/operating conditions, such
as delta-T, RH, envelope air leakage, building pressure, and also on the
amount of insulation outside of that bale. If you only have leaky,
incompletely-installed R-12 batts outside of an R-30 wall, I could see
you easily hitting that dew point. If you have R-20 dense-packed cell
outboard of that R-30 bale, that looks better from a hydrothermal
perspective. This is similar to the issue of adding foam board
insulation to the exterior of an insulated stud wall in a retrofit - you
need to add enough to ensure that you are keeping that sheathing up to temp.
Speaking of hydrothermal dynamics (yes, lets!), the balance between
interior and exterior assembly surface permeability levels is a hot
topic (in some small, weird circles). If your interior lime plaster is
a perm of, say, 12 (and I'm talking US perms here - my apologies to the
metric community for my conversion illiteracy!), and that exterior
plywood sheathing is only a perm of 5, then you are allowing vapor to
diffuse into your cavity at a faster rate than it can diffuse out.
Providing an air-tight assembly, reasonable pressure balance (or ideally
slight depressurization), and RH below 50%, diffusion alone is unlikely
to cause a major problem from condensation, but that's a lot of
provisions. We use either wood board sheathing or fiberboard to ensure
a permeability that is at least equal to, and ideally significantly
greater, than the interior surface. Throw a vented drainage plane in
there before your siding goes on and you have a very robust, very
durable wall assembly. John Straube answered a question I asked at the
CO ISBC about this permeability balance, and his reply informed the
content of this paragraph. I am still on the hunt for a way to easily,
and with verification, adjust the permeability of interior plastered
surfaces down for cold climates to address this issue. Haven't quite
gotten deep enough into that yet.
And we apply lime plaster over lime-stabilized, very well-scratched clay
base coats regularly. The hyphenated words preceding 'clay base' are
the keys as to why it works, in my opinion. Of course, if you are only
plastering interior without freeze-thaw and horizontal rain pressures,
so much the better. Hey, this wall system is starting to look pretty
good for us northerners!
Hope that helps. Best of luck with the project Ian, keep us posted with
developing details and questions!
New Frameworks Natural Design/Build
Montgomery, Vermont, USA
On 7/19/14, 2:29 PM, Derek Stearns Roff wrote:
> Hi, Ian,
> The Vermont COStraw Nostraw has made a good case for blown in dense-pack cellulose in direct contact with the outside of the bales. (The COStraw Nostraw is an inner circle of the strawbale mafia, which insists on putting Cellulose Over Straw, or using no straw at all.) The dense-pack cellulose can compensate for the fire, air movement/convection/thermal performance aiding, and vermin impeding qualities that the exterior plaster normally adds to the wall. I’m skeptical that batt insulation could be an effective substitute. Gaps are universal with batt insulation, and good installation just means fewer and smaller gaps. Given the irregular shape of a strawbale wall profile, I expect there would be too many gaps between the straw and the batts, and the presence or absence of sufficient tight contact would be impossible to inspect and verify. The idea that batts will “settle tight against the bales” seems very unlikely to reach the desired levels of contact between the two materials. Maybe our resident COStraw practitioner will offer his opinion. If your batt insulation is really settling, doesn’t that imply gaps at the top of the wall?
> I’ll add the usual warning, that lime over clay needs to be tested with the specific lime and the specific clay you will use, since some combinations refuse to collaborate.
> On the moisture question, Joe Lstiburek of Building Science Corporation lists both OSB and plywood as “vapor semi-permeable”, and "lightweight asphalt impregnated building papers (#15 building paper)” as being vapor permeable. My memory of John Straube’s presentations at your Colorado ISBC is that plywood has variable permeability, which increases as moisture levels increase. That advantage, plus its greater resistance to mold and moisture, and its fewer objectionable chemicals, leads to my preferring plywood over OSB. It’s worth doing the calculations, but I think that in Boulder’s climate, you will be OK on the permeability question.
> I look forward to seeing your project during a future visit to Boulder.
> By the way, there is a very useful section on moisture for strawbale buildings in "Design of Straw Bale Buildings: The State of the Art", by Bruce King, et al. Parts of this wonderful book are now available as a free PDF download at the BuildWell Library, on (you guessed it) Bruce King’s website, while some chapters can be downloaded individually at low costs. An excerpt of the moisture chapter (written by John Straube) is available free at the second link below, and the whole chapter can be downloaded from the third link for $6. The entire book can be downloaded from the same link for $20.
> On Jul 18, 2014, at 10:48 PM, Ian Smith <ian at lopezsmolensengineers.com> wrote:
>> Hello GSBN,
>> This is my first time posting here... hope I'm doing this right. Quick intro of me: I am a structural engineer that started working on straw bale projects in 2003, with Jeff Ruppert. Jeff is over in Paonia now, and I'm still in Boulder. Pretty sure I'm also still the Treasurer of COSBA (Colorado Straw Bale Association), although we're on some sort of extended vacation after hosting the ISBC in 2012.
>> Anyway, my wife and I have recently bought a small house in town and we're hoping to do a strawbale addition on it. The addition will be rectangular with a gable roof. We're thinking of building a wall system that would be relatively experimental for us here, although I understand that others are doing similar things in Vermont, and maybe Europe...?
>> I just wanted describe our proposed wall assembly and see if anyone has any comments, etc.
>> The assembly would be, from interior to exterior:
>> "Conventional" 3-coat interior clay plaster with lime finish coat(s) applied directly to straw bales, stacked on-edge.
>> 2x4 (4x9 cm) wood studs at standard spacing with blown-in cellulose or batt insulation.
>> 1/2" (1.25 cm) Structural wood sheathing where needed (structurally), and fiberboard elsewhere.
>> Exterior finish A) On walls under eaves (south and north) - double building paper and 2-coat lime plaster
>> Exterior finish B) On walls exposed more to weather - weather resistant barrier, vertical wood nailers, and horizontal wood (or similar) rainscreen.
>> This wall assembly potentially allows for the complete construction of the exterior walls - with finish and windows installed - and roof, before the straw bales and stud insulation are installed. Therefore, installation of the straw bales can (potentially) occur at any time of year or in any weather, since the exterior of the building is 100% complete. No need for temporary bracing. It also provides some savings in labor since the need for "normal" exterior plaster is greatly reduced (although additional, expensive finish materials are needed).
>> Yes, the exterior side of the bales will have no plaster. Although, it could be possible to do a slip (thin) coat on each bale while stacking. The batts or blown cellulose between studs would (theoretically) settle tight against the bales, mitigating the potential for air movement there. I assume the insulation would also contain a fire retardant. Differential settlement of the bales would be mitigated by tying the bales to the studs, and providing proper support for the interior plaster at the foundation level.
>> Most of my concerns with this proposed assembly are focused on building science and moisture issues. I think I need to use the fiberboard sheathing wherever possible to achieve the highest (overall) permeability possible, but I'm not fully sure... Also, are there any permeability issues with the double building paper and lime plaster when applied directly to the sheathing? Should I maybe install a drainage plane behind the plaster?
>> Let me know if you think of anything else. Thanks very much, (and Dirk, congratulations on the big SB building!)
>> Ian Smith, P.E.
>> Boulder, CO, USA
>> GSBN mailing list
>> GSBN at sustainablesources.com
> Derek Roff
> derek at unm.edu
> GSBN mailing list
> GSBN at sustainablesources.com
Jacob Deva Racusin
New Frameworks Natural Design/Build
Author, The Natural Building Companion
Chelsea Green Press, 2012
jacob at newframeworks.com
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