Builders Information

The R-value of Logs

The R-value of dry softwood is R-1.41 per inch.  NO MORE.

If you are reading this, you are probably considering a log home. You are trying to understand and get to the truth about energy efficiency and logs.

What we hope to accomplish with this information is to peel back some myths and misinformation often presented about logs.

• Myth #1: Wood is a natural insulator.
• Truth: Wood is cellulose fiber, nothing more. It has an R-value based on its density, moisture content, and stability. A log full of checks and splits does not provide the same R-value as one that does not have checks and splits.
• A log with high moisture content does not have the same r-value as a log that is lower in moisture content.
• Myth #2: Logs hold heat better.
• Truth: Logs like any other material will equalize to the ambient temperature conditions they are in. This has nothing to do with their R-value. Logs do not make heat.
• Myth #3: Logs have thermal mass so they are better insulators.
• Truth: Thermal mass has nothing to do with R-value. Please click here to better understand thermal mass and how it affects your home.

What is the R-value of a log?

• What is the size of the log? Width and height. The length does not matter.
• What is the shape of the log? Round, D-shape, or Square.
• What is the wood species of the log? Hardwoods offer approximately 1/2 the R-value of softwoods.

With this information, we can calculate a true average r-value for a log.

The US DOE provides a recognized standard for determining the R-value of wood. Click here for the link.

Typical for Round Timber Logs

8   inch log with a 5" top,  8" center, 5" bottom cross section

= R-9.2

10 inch log with a 6" top, 10" center, 6" bottom cross section

= R-11.3

12 inch log with a 7" top, 12" center, 7" bottom cross section

= R-13.4

Typical for D-Timber Logs

8 inch log with a 7" top, 8" center, 7" bottom cross section

= R-10.6

10 inch log with a 9" top, 10" center, 9" bottom cross section

= R-13.4

12 inch log with a 11" top, 12" center, 11" bottom cross section

= R-16.2

Typical for Square Timber Logs

8 inch log with a 8" top, 8" center, 8" bottom cross section

= R-11.3

10 inch log with a 10" top, 10" center, 10" bottom cross section

= R-14.1

12 inch log with a 12" top, 12" center, 12" bottom cross section

= R-16.9

Average R-value for softwood   1.41 per inch
Average R-value of log = (average cross section width) x (R-value of softwood)

For most of these log sizes & profiles, an insulated log option is available.  Most will increase the R-value of the log by more than 100%.

Click this link to see by state & county what the new code is for your building site.

http://energycode.pnl.gov/EnergyCodeReqs/

If you plan to build and want to achieve the new minimum federal energy code for wall R-value, you will need a 14" to 16" log in zones 5 & higher. These would need to be square logs, with no checks & splits.  Round logs have significantly lower R-value than square logs or timbers.