March 10 , 2002
is a Warm Cavity
Now that March is here it's time to get back into the cordwood construction mode. For the last month I've been quite busy adding articles, photographs and new "Meet the Masons".
With spring comes thoughts of cordwood and plans for this year. If all things go well, all of the exterior walls should be completed by late summer. So what comes after the exterior walls? Insulation.
I have pondered various insulation options before (see January 14, 2001 journal) and the verdict is still out. Recently though, I received an email asking me about spray foam products for a double-wall cordwood house and it got me thinking again about a product called "Icynene". (See their website: www.icynene.com.)
I first learned of Icynene from a neighbor of mine that retrofitted a late 1800's brick schoolhouse. The building had no insulation to speak of and Icynene was sprayed onto the walls to give the house R-value and also prevent air infiltration. Sprayed on you say? Yes, Icynene is a spray foam product. At first, I thought I would never even consider such a product due to health reasons. BUT, this product is water based-not petroleum based and after the product dries (within the first week), there is no outgassing at all.
Okay, so it's a non-toxic product, but what about applying it to the back of a cordwood wall. A cordwood wall should breath-right? Yes, trapping moisture in any wood matarial (along with air) will cause rot. I had pretty much discounted any spray foam product for this reason until a chance conversation with a fellow who attended our open house last autumn. We got to talking about Icynene and he mentioned that he thought it was an open-celled foam. After investigating this, I have learned that Icynene is open-celled and will not trap moisture. The product is quite often sprayed onto wood supports and sheathing without any ill effects. Based on this, I would consider it a viable insulation and vapor barrier for double wall cordwood.
If there is a downside to a cordwood wall, it is air infiltration. This is not a problem that can't be overcome, but it will require a couple of over-winter seasons before all the log shrinkage voids can be caulked and log-checks sealed.
This is where Icynene really shines. Using a double cordwood wall, the problem can be prevented in one fell swoop. Applying Icynene to the wall-cavity side of the exterior wall will stop any air leaks and insulate the wall at the same time.
Secondly, applying the foam to the exterior wall prevents all kinds of mold/moisture problems from occurring in between the walls. The condensation point or dew point is never reached inside the double-wall.
Dew point is a fancy term used to describe the point (measured in degrees F or C) in which saturation occurs. For example, a nice cold glass of iced tea on a warm summer's day will quickly sweat. These droplets on the exterior of the glass form because the air in contact with the glass can no longer hold any more moisture. The dew point temperature is the temperature in which saturation occurs.
Although these droplets look inviting on a glass of iced tea, you never want to see them inside of your walls. Moisture problems typically do not occur inside a single cordwood wall, or if they do the moisture is not trapped and quickly dries out.
Building a house using the double-wall technique creates a scenario similar to a standard stick frame house. Depending how you apply insulation and a vapor barrier (if at all), could cause moisture entrapment. The key here is to prevent the dew point from ever occuring in between the walls. I have considered packing the cavity between both walls with loose fill sawdust or cellulose with no vapor barrier whatsoever. This would allow for minimal air movement without trapping any moisture.
A better option though might be to apply Icynene to the inside of the exterior wall. The advantage of this would be to prevent air infiltration, while still allowing moisture movement and to top it all off, insulate the exterior wall. This would keep the wall cavity warm.
of a Warm Cavity
This brings up another advantage... At last year's MREA fair (Midwest Renewable Energy Association) I listened to a number of professional architects and builders who won various energy efficiency awards. Their secrets to success were the prevention of air infiltration and the ability to build a house within a house. In other words, the exterior wall was not in direct contact with the interior wall- no "energy bleeding".
A double cordwood wall can be built in a similar fashion. The interior wall and the exterior walls can be built independently from each other as long as there's a method to stabilize the walls. By applying Icynene to the interior of the exterior wall, the cavity in between the walls would be kept quite warm. This would leave the interior cordwood wall as one big thermal battery, with minimal heat loss through the entire wall.
Beyond the Walls
I had never heard of this before this weekend, but there is such a thing as a "hot roof theory". The theory states: "Using the Hot Roof Theory, the attic is completely sealed. The soffit areas are packed with insulation, and the attic walls and ceiling (the roof deck) are coated with insulation. The best type of insulation for this type of a project is sprayed foam. "
Using this theory it would be possible to totally seal the house envelope using Icynene, making the house virtually air tight. Now making a house virtually air tight, can lead to other problems -- like not getting any fresh air in the house. So, an air exchanger would have to be installed. (I find this whole fresh-air health thing similar to health diets, some claim a high meat protein diet is good for you while others say it will lead to an early grave. I prefer the vegetarian diet myself. Flip a coin!)
it Worth It?
Beats me! Icynene is not cheap and can only be installed by a qualified contractor. I believe it does hold merit and if I go back to my original goal, which was to build an energy efficient house using today's dollars vs. paying for it in future energy costs, it might be worth the added expense. Being able to build an airtight house does have its merits, but there are other methods that may not be completely air tight, but cost less to build. Dense packing lime treated sawdust is an option. Although not completely air tight, it would greatly reduce air infiltration.
got more thinking to do.