We are trying to build a 'no-foam' home, so we need to use the most environmentally friendly insulation we can find. For us that is cellulose, a product made from simply shredding old newspapers (pretty darn eco-friendly we think!).
The issue is how do you you get enough cellulose into your wall assembly to get the high R-Values we need? Since cellulose is so light and fluffy, you need to add something structural to the envelope to hold the it in, and to attach all your siding too as well.
We chose a tried and true system of attaching vertical wooden I-Joists to the structure of the home on the outside of the plywood sheathing.
This works GREAT!
This is also a giant pain in the butt!
First, you have to make sure you have built the house so that the 2nd and 1st story studs line up with each other, so the I-Joists can span from the very bottom of the house to the very top plate.
Second, you have to install them everywhere! Basically we are re-framing our project. A huge time consuming effort as well as a hit to the pocketbook, and a big increase in the embodied energy of the home. We dodged the energy part by using local wood, and we were hoping to use FSC certified I-Joists. Sadly we couldn't get the FSC product in time, so had to use 'normal' ones. Still, adding wood product, grown in at least marginally managed, local forests is far better than framing the project even once in steel or concrete, so we feel OK about that.
Finally, it took about 3.5 weeks for us to install all the roof & wall I-joists. Since we had never done this before, we estimated it would take about half that time. This is likely the reason that some other companies have opted to try their hand at attaching insulation like Rockwool to the exterior instead. Talking with them about this, that technique seems to have it's own difficulties as well!
In the end we have a really strong, bullet-proof system, but in the future we may also be looking at other ways to bump our R-value.
If this was a 'from-the-ground-up' project we would have gone with a double stud wall cavity. But, even this has the issue of where to put the air barrier, so I guess there is no magic bullet yet, but we are working on it!
I have to say though...we got LOTS of people stopping by when we had the I-joists exposed on this project. Builders from all over the country and the world have somehow just happened on our site and stopped to ask us what is going on! One guy was biking by on a trip from Santa Cruz to Virginia! Another was here visiting his daughter from DC. Everyone wants to check out the details and all had really lovely comments on the quality of work we were doing.
Funny how so many people LOVE to see a well-built house, but how few of them are being built these days. We are really happy to have the opportunity to do this level of work, and proud to be involved in the Passive House revolution!