How do I fix the high air exchange in my straw-bale home?
Two years ago, I purchsed a straw bale home in Minnesota. When I closed the house was in poor shape and the exterior had to be rebuilt. Our first winter in the house was cool and drafty and we were spending a fair amount on heat. We did as much research as we could, but we have struggled to figure out how to fix and maintain our home.
Fortunately when we removed the old rainscreen the bale was in good shape. It had not been plastered and the bale had settled in some places leaving air gaps. We repacked the gaps before residing the house with Hardiboard. There is a Tyvek moisture barrier between the bale and the siding, but the bale remains unplastered.
I thought that this would make a big difference in our heating costs, but last winter we continued to spend much more than we anticpated to heat the home. I suspect that the bale in the roof has settled as well, but I am loath to remove the tin roof to fix the bale.
I was wondering about blowing some kind insulation into the gaps. I also had an energy audit done on the house and they found that the house exchanges air 11.6 times per hour. The second floor ceiling is rough beam and there are gaps between the beams. On top of the beams there is a layer of felt beneath the bale. The inspector recommended caulking between the beams to help reduce the air exchange. I am worried that this could cause moisture to build up under the bales in the roof.
I would really like to shore things up before the snow flies again in order to reduce our natural gas usage this winter. Any advice would be appreciated.
Thanks,
Eddy
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Eddy,
Your house is unusual, and it's hard to diagnose what's going on over the Internet.
I assume that the 11.6 ach refers to ach @ 50 pascals of depressurization. That's very leaky.
As you probably know, most straw bale homes are plastered on both sides. The plaster provides both structural strength and reduction of air leaks. Yours isn't plastered on the exterior. That mistake by the builder was unfortunate.
It's also probable that your ceiling is very leaky. Without a site visit, it's hard to know what type of retrofit work would make the most sense.
"Fortunately when we removed the old rainscreen the bale was in good shape."
Just because they looked good on the surface does not mean they were in good shape. Did you check the center moisture content with a bale moisture meter? Code requires no more than 20% the weight of the bales before rendering. If you have access check ten at random especially where the wind blows the most, another code requirement.
"There is a Tyvek moisture barrier between the bale and the siding, but the bale remains unplastered."
Bales need to breath not have a barrier, permeate both to the interior and exterior. Moisture is regulated with a natural earth plaster per code usually. The exterior bale surface should have a scratch coat plaster, a rain plain 1.5-2 " air gap, 2x2s, nailed to studs or post siding attached to it, the siding can have vapor retarder not less than 5 perms per code. Windows need plaster air detailing correctly to trim. That is what you should have done when you had the siding off and the only way to get it right is do it again. Follow code below. Same applies to the roof. WRB does not belong against the bales, they need plaster. Don't caulk the beams.
I suspect due to the lack of plaster and air infiltration your center bales are soaked causing high reductions in r-value, that is the first to check. That would call for removing the bales, they will be difficult to impossible to dry at the core and they probably have rotted. If that is not the case, educate yourself with code, and the book I recommend below, stop listening to people that do not know about bale construction around you obviously.
As far as filling gaps from settling use straw or mineral wool since it is chemically inert or cellulose without any corrosive chemicals like fire retardants.
A good blower door tester will have infra-red and smoke to tell you were the big air is coming from, you need to focus on CFM and location or pressure differences around holes(start with large) get the total ACH down to 3 or less...focus on plaster detailing and protecting the bales from moisture or wind driven rain.
Good luck!
Code: http://www.ecobuildnetwork.org/images/PDFfiles/strawbale_code_support/IRC_StrawbaleConstructionAppendix_Approved_10.4.13r3.pdf
Book: http://www.amazon.com/The-Natural-Building-Companion-Comprehensive/dp/1603583394/ref=sr_1_10?ie=UTF8&qid=1394399248&sr=8-10&keywords=stabilized+rammed+earth
Find your twin and sell.