1 August 2002 Number 38. NEWS. Thermal Properties of Log Homes By Dalibor Houdek, , Forintek Canada Corp. t Reprin In the last few years, as I became more involved with walls, where the thicker wall provides better Thermal Edition ILBA, I noticed an increased number of requests for performance. At least the R-value ( Thermal resistance). reliable technical information on the Thermal perform- of the wall will be higher. By the way, even in this case The International Log ance of log Homes . the larger the number, the better the wall should per- Builders Association is As I studied this topic I came across various reports form.
2 I am using should because it is only one of the pleased to provide you and articles that said everything from log Homes are features needed to achieve energy efficiency. very energy efficient to log Homes are energy ineffi- Thermal resistivity of different softwood species with this free reprint cient. So, what is the truth? Are log Homes energy varies, but not significantly. Generally speaking, the from our newsletter, efficient or not? higher the wood density, the lower the Thermal resis- Log Building News #38. While there is no definitive answer to such a general tivity. In the following table, please compare the R-val- The ILBA is a not-for- question (considering the number of various log ues of common wood species used in log construc- profit, educational building systems), I will bring up some points to help tion.
3 Values in this table are approximate and should association dedicated to you understand the basics and the complexity of the be used with caution; actual conductivities may vary furthering the craft of problem. by as much as 20 Note that the resistivity log home construction. The energy efficiency of a home is determined by increases with decreasing moisture content. The spe- It publishes books, so-called R-values (resistance to Thermal flow) of build- cific gravities also may not represent species averages. articles, Log Building ing segments such as walls, floors, roof, windows, Comparing Thermal resistivity of 1" of wood (rang- News, and the Log etc.)
4 , and by the air tightness of the building envelope. ing approximately between ) with for Building Standards (the In log Homes (and other heavy structures) there is also aluminum, for steel, for concrete, or building code for log the heat mass that influences the heat-holding ability for glass indicates that wood is not doing all of the structure, thus influencing heating bills. that badly. Even when comparing wood to mineral Homes ), and conducts Let's start with the basics. Log Homes are made of wool insulation with a resistivity of approximately 4, annual conferences and wood (well, parts of the walls are anyway).
5 The cross the wood is still quite competitive. workshops for log home section of the log under the microscope appears like a It is apparent that the logs used for log Homes are customers and log bundle of straws. Those straws are filled with air only as large as is practical for a log builder. In my home builders. Contact when the moisture content of the wood is lower than experience the majority of companies use logs averag- information for the ILBA so-called fiber saturation point (somewhere around 28 ing between 12-16 inches in diameter at midrange. is at the end of this percent). That, combined with the average width of the lateral article.
6 It is common knowledge that air is a very good groove between ranging between 2-4 inches, almost Thermal insulator, and the more layers of air in the sys- predetermines the maximum R-value one can expect tem, the better its insulating Properties . Therefore, from such a log wall. wood seems like an ideal mate- The National Research Council of Canada (NRC). rial to separate the warm and conducted a study 1 on R-values of log walls for the cozy interior of your house National Energy Code in 1996. To facilitate the calcu- from the freezing winter condi- lation of the R-value for log walls using a simple hand tions outside, right?
7 Bigger is calculation, the NRC introduced an adjustment factor not always better except in log called profile factor to accommodate the profile of scribed log wall (thickness of the wall at the joints is less than the diameter of the logs). The profile factor Dalibor Houdek for scribed fit log walls was found to be dependent on Number 38 REPRINT EDITION LogBuildingNews 1. the log diameter and the wall joint detail Table 1 Thermal Resistivity of Various Softwood Species Used in Log Construction and to range between and The air close to the wall (Interior and exterior) Thermal Resistivity Wood Specific (h ft2 F/Btu in).
8 Contributes to the total R-value of the wall Species Gravity by approximately Oven dry At 12% MC. The R-value of a round scribed wall can be then calculated by using following Western Red Cedar equation: R-value = [(mean log diameter x White Spruce wood resistivity) + resistivity of air films] x Eastern White Pine profile factor. Western White Pine It is important to understand that this Lodgepole Pine very simple calculation is based on numer- Eastern Red Cedar ous simplifications and assumptions ( Red Pine disregards the contribution of the mineral Douglas Fir 1. insulation in the lateral groove). Therefore, the results should be used with caution.
9 (Source:Wood Handbook-Wood as an Engineering Material, USDA, 1999). Here I want to share a story with you. Once I was approached to provide techni- cal information to a builder who was frame wall with walls rated R-12. It was determined questioned by the building official the window/door to-wall log interfaces that the log wall performed 17 percent regarding the R-values of log walls. The the log-corner interface. better than its calculated rating and it was builder sent me a slick computer-generat- It is very important to pay attention to concluded that the relatively large Thermal ed picture of a log house that I would put these structural details during design and mass of log wall is an energy-conserving in the category of a glass structure with a construction, as they make a large differ- feature.
10 Log infill. Beautiful home, but what kind ence in overall energy efficiency of a log The Model Energy Code (CABO 1992). of R-values are we talking about here? One structure. It was shown that gasketed walls gives log walls Thermal mass credits but half of the house was made of material with tightened through-bolts perform bet- these credits are dependent on the site (windows) that has Thermal Properties ter than those just using fiberglass, and specifics and regional climatic conditions, equivalent to 2" thick cedar blank. In that that exterior/interior chinking reduces air including Heating-Degree-Days (HDD).