December 4, 2000
The last few years of El Nino and La Nina weather are a thing of the past it appears and it looks like we are in for a full blown winter. The past month has found me buttoning things up for the winter. But, just because winter has a grip on things, does not stop the planning process for next year's projects. During the next few months, I'll be discussing some of the options that I am considering for our house. Presently I am considering different options for the cordwood walls that are next on the list of projects. So, lets dig into the subject of double wall construction.
If you recall, our intent has been to build our house using Cliff Shockey's double wall technique. The double wall technique was designed for northern climates (Cliff is from Saskatchewan) and at this point I am not aware of any double wall cordwood homes here in the United States. (If anyone knows of any, please send me an email.) The advantage of the double wall technique is its added R value and its reduction of air infiltration through the walls. If you are looking for optimum energy efficiency, the double wall technique is superb. In our area this technique is probably overkill but, our intent is to future proof our house from rising energy bills.
Cliff builds his walls by building the outside wall first, followed by sheathing, insulation, vapor retarder and then finally the interior wall. While reviewing this method, I started to wonder about stability issues with higher double walls. Cliff's walls were primarily one story structures in which wall stability is not a major concern. Generally, the higher the wall, the more stability plays a role. Most single wall cordwood homes have a wall thickness in the range of 12" to 24" which are usually adequate to prevent width stability issues. Double walls typically have walls that are in the range of 6" to 8". A two story double wall can cause a wall stability issue unless the walls are somehow tied to each other or some other structural object such as a post and beam frame.
I recently asked Cliff about higher walls that would be commonly found in two story structures. Cliff said that he did use blocks of wood that ran through both walls to help tie walls together that were higher than 8'. He held the blocks in place with wood boards until such time that he could come back and build the interior wall.
For our purpose, this would be rather tedious to say the least. All sixteen sides to our house are two stories tall. This would require that these long blocks would have to be prehung along with thirteen windows frames to help tie the walls together. The interior walls, while being constructed would be mortared around the existing blocks and window frames.
There are a few other options that I am considering.
The first option exclusively uses post and beam frames to stabilize the walls. Two post and beam frames hold the exterior and interior walls in place. This method creates two entirely independent walls which allow the builder to build the exterior wall first and then add insulation and a vapor retarder before ever adding the interior wall. If you were in a hurry to get your exterior walls up before winter sets in, this would be the fastest approach.
The second option is to build both the interior and exterior walls simultaneously, adding blocks of wood periodically that extend through both walls. The purpose of the blocks is to tie both walls together and stabilize them. The overall amount of time to build this wall is less than two independent walls and is less expensive material wise. This method unfortunately makes it more difficult to install a vapor retarder and it limits the number of insulation options available.
Here's a table showing the pro's and con's of both methods:
|Double post & beam||
Fast construction of exterior wall
Ability to add a vapor retarder and seal the wall before the interior wall is constructed
Many insulation options available
Electrical inspection can be done before completing wall
Additional material expense
More framing required, more labor necessary to complete walls
Temporary framing required to support windows, etc.
Difficult to add bottles or other artifacts to the wall
|Build both walls simultaneously||
Overall less time to complete walls
Easy to add bottles or other artifacts into the wall
Loose fill insulation can be added to wall cavity while building the wall.
Easier to set window frames in place
Very difficult to air seal the wall during construction although the wall could be sealed with caulk or permachink later on
Would not be able to use fiberglass insulation bats or apply a foam based insulation that seals as well as insulates
Loose fill insulation has a tendency to settle leaving insulation voids in the wall.
The verdict is still out regarding which method to use. If I can find a suitable insulation that insulates and seals air out, I will probably go with building both walls simultaneously. There are foam insulation products on the market that do this, but they have to be professionally installed and I can't imagine having a crew standing by while I mortar in logs. (It would be the longest, slowest, most expensive installation ever!) I need to find a product that I can install myself. Loose fill cellulose or fiberglass insulation is such a product, but it does not address the air infiltration issue.
I'll be writing about different insulation alternatives that are earth friendly soon, so keep close to your favorite cordwood construction web site.