[GSBN] Strawbale retrofit project

[Mark Piepkorn]

The kind of thing Joyce was asking about, but it's probably months 
off. This is a contribution to the thought pool, but comments and 
suggestions are welcome.

Subject: an 1880s building in southern Vermont (long, very cold 
winters), double- (perhaps triple-) wythe structural clay brick, two 
stories. Given the era, type of building, and that there are no 
turned bricks, the core might be grouted solid - one of many 
questions to examine. It hasn't been painted or sealed. It looks like this:
http://potkettleblack.com/TEMPORARY/P5221899.jpg

The proposal is to retrofit with straw bales from the inside, 
converting it to separate living units. The exterior needs to be 
preserved, so insulating from the outside - or even installing a 
rainscreen - isn't an option. The current interior has painted wood 
panels and plaster. It's not known if these are applied directly to 
the brick or if there's framing.

Whatever is done to insulate will change the thermal and moisture 
dynamics of the 120-year-old walls, offering real potential to do 
more harm than good over time, including spalling of the brick faces, 
and rot in the joist ends, which are probably set into pockets in the 
masonry wall.

I spoke with Peter Yost about it a bit (some of you know him - it's 
great to have a world-class building science guy as a colleague) and 
we discussed conventional retrofit techniques. When I brought up the 
idea of using bales, he (of course) referred me to Dr John.

Straube, that is.

The bales would have earth plaster on both sides. Even though studies 
of conventional insulation indicate that only one side of the 
material really needs to be adhered to, or at least have a tight fit 
against, the sheathing to perform properly, there's enough evidence 
that SB should have a direct seal on both sides that I'm reluctant to 
stray from that. (It's also recommended that the insulation material 
be monolithic, or be layered with staggered joints, neither of which 
is amenable to SB.)

The ceilings are a dozen feet high or more. I don't know the size, 
spacing, or condition of the joists, but I expect that the load will 
be acceptable, especially so close to the edges. If need be, an 
additional bearing system could be built. Something for a consulting engineer.

I include the following aborted path simply to feed the knowledge of 
the commons:

I was wondering, with water on the brain, if having a plane - vented 
or unvented - between the bricks and the plastered bales would help, 
hinder, or be irrelevant. Rather than trying to create an air space, 
I was thinking about using some kind of nonwoven mesh up to 1" thick 
(generally used for soil stabilization, and which are becoming 
available made from recycled plastic).

I wasn't worried about liquid moisture; I just had a hunch that it 
might facilitate a better drying regime. There was also a concern, 
however, that it could end up channeling moisture to those pocketed 
joist ends, even if the sealing of the air space was meticulous, 
using the bricks and mortar as the transfer medium.

Then I found a paper that singled out my two big concerns as the two 
big concerns - it's nice to be right now and then - and the authors 
could have been using the subject building as a baseline. What 
fortune. "Interior Insulation Retrofits of Load-Bearing Masonry Walls 
In Cold Climates":
http://www.buildingscience.com/documents/digests/bsd-114-interior-insulation-retrofits-of-load-bearing-masonry-walls-in-cold-climates

Even more startling, without ever mentioning straw bales with earthen 
plasters, the paper describes the qualities of those materials - 
including high safe moisture storage capacity and low permeance - as 
being ideal. (That said, their favored approach is a couple inches of 
high-density, closed cell poly foam - 1 perm, R12 - or 5" of an 
open-cell, semi-permeable foams - 13 perms, R20 - "if the interior is 
kept at a low humidity during winter and the outdoor temperature is 
not too cold." So that needs to be thought about.)

However, the air plane notion I had is nixed in no uncertain terms as 
likely to cause serious condensation issues. They absolutely warn 
against any opportunity for convective loops in the plane between 
brick and retrofitted insulation. This makes me feel pretty good 
about a wet-applied clay-based render directly against the brick, 
maybe 1" thick, with slipped bales smooshed into it. Check the 
embedded moisture sensors until it's dried, then work on finishing 
the inside face.

The report also describes how to detail around the wooden members 
pocketed into the masonry.

There are more things to consider, of course, but the bare basics of 
it seems like a go-ahead.


Mark Piepkorn
www.potkettleblack.com

You may drive out Nature with a pitchfork,
yet she will always hurry back.
   - Horace