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This article explains the insulating and heating properties of log homes, comparing solid log structures, slab-sided log homes, and conventionally framed homes.
This series of log home construction and maintenance articles provides information on the inspection and diagnosis of damage to new and older log homes and includes description of log house and log siding insulation values and alternatives, and also a description of the characteristics of slab-sided log homes as well as all other types of log home construction.
We include illustrations of log structures from several very different areas and climates in both the United States and Norway. Our page top photo shows a modern kit log home constructed in New York State. For modern kit and factory-sourced log structures we include details of common construction and building defects that cause water and air leaks and ultimately rot damage and we point to key problem areas that need to be inspected carefully when buying or maintaining a log home.
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When a solid log wall is built using logs rounded on one or both exposed sides, the nominal log diameter does not give an accurate estimate of the wall's insulating value.
That is because portions of the wall are constructed at a thickness less than the full log's diameter.
The average wall thickness should be used to calculate the "R" value of a solid log wall when rounded logs are used.
The beautiful log cabin in our photo (above left), located on Elk Lake in Michigan, was built more than 50 years ago as a summer retreat.
While thick native logs were used in its construction, this cabin is not comfortably habitable in the dead of winter, principally due to drafts, leaky windows, thin upper walls and an uninsulated roof. But even in an uninsulated building the most rapid heat loss points are typically around and through windows and doors.
Lighting the huge stone fireplace and burning wood continuously provides comfort to Lon (photo at left) who is sitting around the fire, while the same fire's draft draws freezing air into the rest of the structure.
The average wall thickness over the entire log wall area of this Michigan cabin is about 5" - R-5 in terms of resistance to heat loss. When we consider that the upper portion of this cabin is sheathed with 3/4" lumber, and the roof is un insulated, the average R-value of this beautiful, but chilly structure is about R-2 over its entire surface area.
But a tight, non-drafty solid log home will have tremendous thermal mass and can be very comfortable and easily heated, as we discuss below. It's not accurate to compare an antique (and drafty) log cabin such as the one above with a a modern log home nor with a conventional wood-framed and insulated structure. Modern milled-log homes that are properly constructed are quite comfortable in cold weather.
Thermal Mass & R-Values of Solid Log Homes Compared with Insulated Wall Wood Structures
A second error (after draftiness) that plagues comparison of the relative comfort and heating efficiency of different types of homes is the failure to consider all of the areas on the building that should be insulated. We like to inspect homes that have their heat on in freezing weather - there is a lot one can learn.
This Pennsylvania log home has tight, thick solid log walls and it is not drafty. But the builder didn't bother to insulate under the roof. The huge icicles hanging from the log home's eaves tell the story.
If there are high heating bills for this home we'd start by reviewing the ceiling and under-roof insulation of the structure.
While the "R" value of a solid log home is almost certainly less than that of a modern conventionally-framed stud wall home insulated with fiberglass or other products, the wall "R" values alone do not accurately describe the comfort level of a log home.
Provided that the log construction has been well-built without drafts or leaks, the thermal mass of solid log walls is considerable.
But as we discuss at ENERGY EFFICIENCY of LOG HOMES, solid log homes are more massive than lightweight wood-frame insulated structures, but a National Bureau of Standards study found that the energy that a log home saves in the swing seasons (spring and fall) through added mass (and comfort for occupants by heating or cooling systems needing to cycle on or off less often) does not amount to much on an annual basis.
See THERMAL MASS in BUILDINGS for details about this effect.
Average R-value or Insulating Value of a Solid Log Wall
Solid wood has an R-value of about R-1 per inch of thickness. A round log wall using 8" diameter walls is not, however 8" in uniform thickness; that wall thickness is reached only across the center of each log.
The scalloped wall surfaces will certainly be less than 8" (R-8) in thickness where log faces meet one another. Compute the average wall thickness of solid wood to arrive at a reasonable R-value estimate for a solid log wall. Typically for an 8" log the average log wall thickness is around 6 to 6.5".
Consider Thermal Mass in Addition to R-Value of a Log Home
Log home enthusiasts argue that in measuring comfort one should not only consider the "R" value of the building walls and roof but also the thermal mass of the building. High thermal mass (provided by the mass of solid logs in a log home) means that the building will be slow to change in temperature.
We agree that overcoming drafts and un-wanted air leaks is the first priority for making a building comfortable and for reducing heating or cooling costs in cold climates. INSULATION R-Values & Properties provides detailed estimates of the insulating values and properties of various insulating materials.
A large thermal mass in any building tends to make temperature changes occur more slowly than in structures lacking that feature. As a result, occupants of solid log homes often assert that they find their building very comfortable in both heating and cooling seasons.
Log Slab Sided Log Home Insulation R-Values
Slab-log sided homes such as the one shown here and discussed in detail at Slab Log Cabin Siding are generally built over conventional wood-frame walls that allow conventional wall insulation.
This cabin was renovated using 2x6 wall studs to permit extra in-wall insulation as well as the application of solid foam insulation on its exterior walls. The walls of this building were framed to about R-20. Very important as well, the builder did a great job assuring that the home would be draft free.
After a horribly incompetent installation of an in-floor radiant heat system (by the same builder who did so well on framing and insulation) we had to abandon the heating system for this Minnesota cabin. Luckily the cabin is so tight and well insulated that we discovered that we could heat it for at least three seasons using just a few portable electric baseboard heaters.
Continue reading at LEAK DIAGNOSIS & CURE in LOG HOMES
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Frequently Asked Questions (FAQs) about log home insulation and the insulating value of solid logs
Questions & answers or comments about the insulating, heating, and cooling properties and comfort of log homes and other solid wood log-constructed buildings
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