Thermal Mass & Logs

There is a lot of misinformation and misconceptions on the topic of thermal mass and logs for log homes. What we hope to accomplish with this information page is to clear up some of the fallacies and myths often encountered when discussing thermal mass and logs.

Let’s begin by clearing up some thermal mass misconceptions. 

Thermal mass is not insulation. Thermal mass is an estimate of a material's ability to store and release heat.  Logs do have a thermal mass, but so does concrete, stone, masonry, glass.  In fact, all materials have a thermal mass. 

Thermal mass is not used to calculate a materials R-value.  R-value is actually a measure of thermal resistance.

One myth that is often heard is that you do not need to be concerned about the r-value of logs and their thermal mass characteristic’s replaces the need for insulation.

This is just not true.

Here are some comments you may hear in a typical response to your question:

“What is the r-value of your log?”

"Wood is a natural insulator"

While this is a true statement, it does not tell you anything regarding R-values.  You are being told this in an effort to avoid having to answer your question.

"Logs hold heat naturally"  

 It is a piece of wood. No more than that. Logs do not make heat!

"Log homes have been lived in for years" 

True, but they were not energy efficient then and are not energy efficient now.  New technologies can make them significantly more energy efficient.

To put it in simple terms, thermal mass as a means to store and release heat is great, but only if you are not buying the energy to produce the heat that will be stored in the log. If your log wall is absorbing heat from your furnace and slowly releasing it back into the living space of your home, it is also releasing that stored heat at the same rate to the outside. Logs are not smart.  They do not know the inside from the outside of the wall.  That is your energy dollars being radiated to the outside of your home.

Another important factor to understand is that insulation does not store heat. Insulation deflects heat and slows its migration. Insulation works in the daytime and at night. In the northern latitudes, there are many more hours of projected heat loss for a home during the heating season than there are hours where you could expect a heat gain from solar energy. The more heat generated with your energy dollars that can be retained in the interior of your home the better, and the less energy dollars wasted.

The U.S. Department of Energy has published that a log’s thermal mass can affect an apparent gain of 0.1 r-value per inch in appropriate conditions. That’s 1/10 of an r-value. I can speak from personal experience, having been born and raised in Michigan, there are very few days during the heating season that there is sufficient sunlight to accumulate any appreciable heat gain. All of the heat needed is coming from sources created with your energy dollars, and all of the heat loss also comes from those same sources. You need less heat if you don’t lose it to the outside. Insulation slows the migration of heat. If you need less heat, you pay lower energy bills. Insulation allows more of your heat to be kept in the home.Let me show you how insulated logs are free.  The energy savings are real, can be proven, and are calculated.  There is no guesswork. 

Financing Energy-Efficient Homes

Here is a link you may find useful.  Financing an energy efficient home may be easier.  Our engineered insulated logs more than double the R-value of a typical timber log option.  

http://energy.gov/energysaver/articles/financing-energy-efficient-homes

Insulated logs: How it works.

We use a high density EPS core in our insulated logs.  The EPS provides a very high R-value as compared to wood. 

EPS insulation is R 4.7 per inch of thickness

Softwood is R 1.4 per inch of thickness

Link to reference:

http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10170

Our engineered insulated log is designed and built as a Box Beam.  The log has a high density EPS core.  This makes our insulated log a much higher R-value than timber logs.

An example we like to use is:

If you buy a cup of coffee at a drive through.  The coffee is typically 140 degree F.  You can hold that coffee cup in your hand and not be burnt.  You can set the cup down for 20 minutes and still have a hot cup of coffee.  The cup is 1/8" thick and made from EPS.  Similar to the material we use in our insulated logs.  In our standard 8" logs, the EPS we use is 38 times as thick as that coffee cup.  You can see now how the EPS insulation slows the transfer of heat in your log walls. 

You only need to invest in insulation one time.  If you choose not to insulate, you will pay the increased energy costs each and every year.  Energy costs are continuing to increase and I do not know anyone that believes they will ever come back down.

A typical full timber 8 inch D-log has a cross section of 7 1/2 inch at its center and will be 6 1/2 inch at its narow points top and bottom.  An average thickness of 7 inch.

This would give you a calculated R-value of:

7 inches {average log thickness} x 1.41 {R-value of dry softwood} = R-9.9

The R-value calculation for wood assumes no checks or splits and that the timber is dry.  How many whole timber logs have you seen without checks and splits.  Not many.

Our insulated 8 inch D-log nets a calculated R-value of R-21.  More than twice the R-value provided by the best natural wood log. This example nets a 112% higher R-value.

Log Raising in Northern Minnesota near Lutsen

Log raising in Northern Minnesota, on the shore of Lake Superior. The home will be built using our 8"x12" inch, insulated D-log profile. {R-22} 

Day one:

Nice view.  North Shore of Lake Superior.

Click on photo to enlarge.

 Wall logs are up.  Ready to set loft floor system.

Big Sky, Montana

These are construction photo's of a mountain home being built near Big Sky, MT.  The log used in this home is our insulated 10" x 12" D-log, R-27.  The log is made from Western Red Cedar.

Click on photo to enlarge.