Thermal Mass of Log Walls

[MountainDon]

A recent topic got me to thinking more about thermal mass in walls, specifically as a substitute for wall insulation as in log cabins/homes. So I've done some googling and will try to summarize what I've learned.

The Department of Energy has done a lot of work on the science of energy efficiency. My understanding is that most places in the country should have some sort off minimum standard for new construction. It varies by state and often by areas within the same state. There is a software program available for checking compliance. When I enquired about what was required to obtain a building permit I was told to use the program and to submit the form it could generate to show the calculations with the application for the permit.

On the DOE website I found a document that explores the methodology used for developing the REScheck software. There are also documents specific to log homes. In among all the data there is an explanation of how wall mass is used in calculating energy efficiency. A mass wall could be a solid wood wall as log construction is considered to be. Mass walls also include masonry walls and that gets complicated if the cavities are filled with insulation or concrete. My curiosity was mainly around log walls so that's what I looked at. To qualify as a mass wall the wall must have a certain minimum Heat Capacity. When you work through all the terms, take the moisture content of the material into account, factor in the species of wood, and so on, you end up with a "Thermal Mass Credit" that is used in the REScheck program.

What it all boils down to is that for a wall to be considered as having a Thermal Mass Credit it must have a density of 20 lbs per square foot of wall area, or greater. I found some interesting tables.

Table One: This is from a DOE document. The chart lists species in the left column. The columns shaded in blue indicate thicknesses that meet the requirements for consideration as a mass wall.




It can be seen that there are few species that qualify in a 5 inch size. Let it be noted that the size listed is not across the round, not the diameter of the log. It is the cross sectional square, as illustrated below.




I ran some numbers in REScheck for a basic small 10 x 12 log cabin. I used a floor R-value of 25 and a ceiling of R-38 as they are the recommended minimums for my mountains. For the walls I selected 7 inch Ponderosa Pine. REScheck comes out saying it passes by 0% better than code. Pretty much the minimum requirement then. However, installing a single 36 x 48 inch window (vinyl frame low-e glass) drops the rating to 6.7% worse than code. That would have to be made up someplace/somehow.

Table Three may be of general interest:



From the table it can be seen that dry Ponderosa Pine has a density of 28 lbs / cu ft. That would be equal to a density of 2.33 lbs/per inch per square foot. So to achieve 20 lbs/ square foot of surface area that computes to 8.5 inches. That's a little greater than the 7 inches that Table One suggests. I think that may be party due to differences in the moisture content used in the software and the density chart I found elsewhere. Other factors may apply as well. However, it's probably close enough to suggest that the software writers may have done their homework.

References used:

REScheck software    http://www.energycodes.gov/rescheck/download.stm

Methodology for REScheck software   http://www.energycodes.gov/rescheck/documents/rescheck_tsd4.4.pdf

Log Homes in REScheck     http://www.energycodes.gov/training/pdfs/logwall_final_02_14.pdf

Wood densities     http://www.engineeringtoolbox.com/weigt-wood-d_821.html

[PFunk_Spock]

Good info.

For some real world applications, I cannot speak highly enough of log homes and the thermal mass they provide and their benefits.

While Im currently renting, the cabin I currently live in has between 7-8" diameter logs, and you can really feel how much heat, as well as cold they hold onto.

Summer days of 90+ rarely bring the cabin indoors to a temp greater than 80, consistently holding below 70 until late afternoon. Overnight, the temps rarely drop lower than 60 due to the heat they hold in. This is with no HVAC on at all, just letting it go.

That being said, on a hot summer day, you never get that cool feeling at night indoors, as the cabin walls hold the heat from the day. Also, sunny spring morning rarely feel as warm as they are, due to the cooling effect. A small price to pay.

[John Raabe]

Nice bit of work Don. Thanks for digging into the ResCheck data.

Solar houses often add thermal mass to the inside of an insulated shell. This has a similar effect by setting in motion a "thermal flywheel" that wants to heat up or cool down more slowly than would a lower mass structure. The greater the mass (wood, masonry, water) the greater the flywheel effect.

The classic solar buildings were the pueblo structures of the high SW desert where you have predicable winter sunlight and the thickness of the masonry or adobe walls can be adjusted to hold heat that re-radiates at night to keep the occupants comfortable through sub-freezing nights. Later developments such as the Trombe wall improved on this by glazing the outside of the masonry to restrict re-radiation of the warmth to the outside at night.

[TheWire]

Thermal mass works well for a place thats heated all the time in the winter.  However, if you keep your cabin unheated between winter visits, it can take a lot longer to bring the place up to temperature because that mass is cold.  Same thing for places with radiant heated slabs.

[Jeff922]

I read recently that an inch of pine log has the R value of about 1.5.  Is this accurate?  I always thought log homes were terrible in cold climates because their solid walls create a huge thermal bridge that the heat system constantly has to fight.  There must be something I'm missing. Hummm....

[fishing_guy]

Thermal mass works well for a place thats heated all the time in the winter.  However, if you keep your cabin unheated between winter visits, it can take a lot longer to bring the place up to temperature because that mass is cold.  Same thing for places with radiant heated slabs.

A guy I worked with had a log cabin in northern Minnesota.  He was telling me that one winter weekend he went up.  The week before the temp had hit -40F.  That weekend it was 32F out.  The inside of his cabin was still -10F.  By the end of the weekend they had warmed it to a balmy 40F.

So there is something to the thermal mass.

[Don_P]

If the average temperature is fairly temperate then having a thermal flywheel is a good thing. When the average temperature becomes more severe it becomes an R value equation...having the logs inside a foam envelope would then be great. I believe the DOE log study houses were near DC. And yup the coldest weekend you're going to spend is in a seasonal log cabin.

[Jeff922]

So is it a fair generalization to assume that log homes, even when heated continuously through the winter months, are inefficient in cold climates?  I still can't get my head around the thermal-mass vs. insulation thing.

[MountainDon]

I read recently that an inch of pine log has the R value of about 1.5.  Is this accurate?  

What I've read is that softwoods can have a R value of up to 1.4 per inch and that hardwoods can have an R-value up to .7

Variances within species as well. Cedar was one of the better as far as R-value goes, but being lighter likely needs to be larger to count for thermal mass.

[MountainDon]

So is it a fair generalization to assume that log homes, even when heated continuously through the winter months, are inefficient in cold climates?  I still can't get my head around the thermal-mass vs. insulation thing.

Depends on the size of the logs probably. Larger logs will have more mass and more r-value.    Make the logs big enough it's gotta work, as long as there's no air leakage.

[MountainDon]

AH-HA!  I found the report from the National Bureau of Standards from 1982.

Link to the full report HERE

In summary:

A study was conducted by the National Bureau of Standards (NBS) for
the Department of Housing and Urban Development (HUD) and the
Department of Energy (DOE) to determine the effects of thermal mass
(the bulk of solid wood log walls, or brick and block walls) on a
building's energy consumption. For the test, six 20'x20' test buildings
were built on the grounds of the National Bureau of Standards, 20
miles north of Washington, DC, in the fall of 1980. Each structure was
identical except for construction of its exterior walls. The buildings
were maintained at the same temperature levels throughout the 28-
week test period between 1981 and 1982. NBS technicians precisely
recorded energy consumption of each structure during this entire
period.

Test Results
• During the three-week spring heating period, the log building
used 46% less heating energy than the insulated wood frame building.
• During the eleven-week summer cooling period, the log building
used 24% less cooling energy than the insulated wood frame building.
• During the fourteen-week winter heating period, the log building
and the insulated wood frame building used virtually the same
amounts of heating energy.

The National Bureau of Standards technicians conducting the test
calculated the R-value of the log building, which was constructed with
a 7" solid square log, at a nominal R-10. It rates the insulated wood
frame building, with its 2'x4' wall and 3-1/2" of fiberglass insulation, at
a nominal R-12, thus giving the wood frame structure a 17% higher Rvalue.
Yet during the entire 28 week, three season test cycle, both
buildings used virtually identical amounts of energy. This led the
National Bureau of Standards to conclude that the thermal mass of log
walls is an energy-conserving feature in residential construction.

The full report has the specs on construction details of the 6 buildings tested, plus charts with energy usage.

[Jeff922]

Good find!  That was very interesting.  I probably had my numbers mixed up - R value of 1.5 per inch.  There is a cedar shingle mill right down the road from my house.  Their web page claims white cedar shingles with butt ends of 3/8" has an R-value of about 7 while 5/8" butts has about R-value 11.  I found that a little hard to believe when compaired to a white pine log, but who am I.  Too bad that study is so old.  I would LOVE to see the same test performed in northern New England with 2x6 R19 walls as a compairison.  And what the heck, throw in a superinsulated structure and a SIP structure.  Then I could really get down to the nitty gritty.

[Chuck Adze]

I know some of the old homes in certain Maine coastal towns had 3 or more layers of sheathing boards on the walls for added mass and insulation.
They were cheap.
They also built them leaning into the prevailing winds a bit.

In my new place, I am planning on building a small log section (16 x 24') out of white cedar, flat sided, and chink style.
I am presently chainsawing out the logs, about 6" thick and use a V notch or modified V notch.
This will be attached to the timber structure.
However the inside will be foam insulated, and drywalled (maybe two layers).

The place I am in now I built (about 2,200 sq. ft.) with 6" -2 x 6 fiberglass walls and also used 2" of polyiso.
It is a very comfortable home.
I go through about 4 tons of pellets, and a liitle cord wood for real cold nights (old tarm wood boiler).

[Chuck Adze]

AH-HA!  I found the report from the National Bureau of Standards from 1982.

Link to the full report HERE

In summary:

A study was conducted by the National Bureau of Standards (NBS) for
the Department of Housing and Urban Development (HUD) and the
Department of Energy (DOE) to determine the effects of thermal mass
(the bulk of solid wood log walls, or brick and block walls) on a
building's energy consumption. For the test, six 20'x20' test buildings
were built on the grounds of the National Bureau of Standards, 20
miles north of Washington, DC, in the fall of 1980. Each structure was
identical except for construction of its exterior walls. The buildings
were maintained at the same temperature levels throughout the 28-
week test period between 1981 and 1982. NBS technicians precisely
recorded energy consumption of each structure during this entire
period.

Test Results
• During the three-week spring heating period, the log building
used 46% less heating energy than the insulated wood frame building.
• During the eleven-week summer cooling period, the log building
used 24% less cooling energy than the insulated wood frame building.
• During the fourteen-week winter heating period, the log building
and the insulated wood frame building used virtually the same
amounts of heating energy.

The National Bureau of Standards technicians conducting the test
calculated the R-value of the log building, which was constructed with
a 7" solid square log, at a nominal R-10. It rates the insulated wood
frame building, with its 2'x4' wall and 3-1/2" of fiberglass insulation, at
a nominal R-12, thus giving the wood frame structure a 17% higher Rvalue.
Yet during the entire 28 week, three season test cycle, both
buildings used virtually identical amounts of energy. This led the
National Bureau of Standards to conclude that the thermal mass of log
walls is an energy-conserving feature in residential construction.

The full report has the specs on construction details of the 6 buildings tested, plus charts with energy usage.

That would be consistent with the local log home builder / mill who says his 6" white cedar logs are about the same as a 2x4 insulated wall.

[Chuck Adze]

Here are a couple photos of milling old gray white cedar poles;

[Chuck Adze]

Don,
Here is the article I was speaking about;

http://www.buildingscience.com/documents/houseplans/hp-very-cold-recommendations/view

[MountainDon]

Building Science is an excellent source