Log home construction

Interim changes to the National Building Code of Canada 2010 (NBC) that were published in December 2012 included incorporation of energy efficiency requirements for housing and small buildings. Some of the new requirements affect the construction of log homes. To help Code users understand these changes, the following questions and answers have been prepared.

Question 1:

Will the energy efficiency requirements eliminate log homes from the Canadian landscape?

Answer to question 1:

No. The Canadian Commission on Building and Fire Codes (CCBFC) was keenly aware of the issues related to energy efficiency in log home construction. From the beginning, the committees working on these changes were concerned that the new energy requirements might eliminate log home construction, and they discussed the matter with the log home construction industry. The president of the BC Log and Timber Building Industry Association also attended a committee meeting, and two consultants working for the log home industry provided technical data to the committees. As a result, the lumber material data and calculation methods for log walls were made consistent with those used by the log home industry.

Question 2:

Why isn’t there a simple exemption for log walls?

Answer to question 2:

The requirements address the energy used by the building with no exemption within the prescriptive requirements for any specific type of assembly construction. This approach levels the playing field for energy use by the building, regardless of the type of construction used. Unlike the Model National Energy Code of Canada for Houses 1997, the new requirements therefore do not include a single, specified RSI value for all log walls.

Question 3:

How is the RSI value for log walls calculated?

Answer to question 3:

The requirements include a reference to the thermal envelope section of the ICC 400 standard “Design and Construction of Log Structures”. This standard establishes the minimum requirements for log structures to safeguard the public health, safety and welfare through structural, thermal, and settling provisions. The ICC 400 document defines the wall thickness as the “average cross sectional area divided by the stack height”.
This approach puts all log profiles on a level playing field regardless of their size or shape. It eliminates any variation, averaging or rounding necessary to determine profile factors for particular log shapes. Once the log wall thickness is established, the RSI‐value for a log wall can be looked up in a table. This table shows the appropriate effective thermal resistance based on wall thickness and on the specific gravity of the appropriate wood species – including the exterior and interior air film coefficients.

Question 4:

Are log walls considered an air barrier system?

Answer to question 4:

Yes. The air permeance of solid lumber is negligible and timber logs can therefore be treated as an air barrier material. To become an air barrier system, log walls must comply with prescriptive requirements for airtightness. This means that the joints between logs need to be designed to receive and maintain the sealants through in-service conditions such as shrinkage and settling. Joints also need to resist air and moisture infiltration and sealant materials must be compatible with all materials in contact with the sealant. Many of these requirements are required in the thermal section of the ICC 400 standard “Design and Construction of Log Structures”

Question 5:

Are log walls considered to have a vapour barrier?

Answer to question 5:

Yes. Solid timber has a low enough vapour permeance so that it meets the NBC requirement of 60 ng/(Pa•s•m2), which technically makes it a vapour barrier. For example, test results indicate that a 2x6 wood stud (SPF) has a vapour permeance between 20 and 30 ng/(Pa•s•m2) depending on the wood’s orientation and the testing direction (thickness). Unlike for air barrier systems, there is no NBC requirement that a vapour barrier must be continuous. This requirement is not being added.

Question 6:

Can log homes comply with the prescriptive requirements?

Answer to question 6:

Log homes can comply with most requirements in the prescriptive path. The main issue with log wall construction is that pure log walls generally are not able to meet the required prescriptive RSI-values for above-grade walls – especially in the colder climate zones. In all other prescriptive energy requirement aspects (i.e. roofs; floors; basements; heating, ventilating and air-conditioning equipment efficiency; water heating equipment efficiency), log home construction could be considered roughly equal to conventional construction. Log home builders should therefore be able to comply with the same adjustments that the rest of the industry has to make. There are prescriptive requirements specifically for calculating log wall RSI values and for construction of air barrier systems using log walls.
A trade-off method for walls (including log walls) is available that could help log home builders demonstrate compliance with the requirements. For example, the trade-off method allows the installation of additional attic insulation to make up the difference between the required RSI-value for walls and the actual effective RSI-value achieved by the log construction.

Question 7:

Which log types are affected most in having to use alternative compliance routes?

Answer to question 7:

Some log types, for example large-diameter rectangular logs or hybrid (insulated) log wall construction, would meet the prescriptive requirements in many climate zones. In the Northern climate zones, and for wall construction with thinner or round logs that have lower thermal resistances, a trade-off with additional attic insulation is needed to make up the difference between the required RSI-value for walls and the actual effective RSI-values achieved by the log walls.

Question 8:

Is there a limit to how much insulation can be traded within the trade-off path?

Answer to question 8:

In the simple trade-off method, the RSI-value of walls cannot fall below 55% of the required value. Most, if not all, log wall constructions meet this limit. Other limitations are that only above-grade assemblies can be traded, and opaque assemblies can only be traded against other opaque assemblies (e.g. wall against attics). These limitations are necessary because the differences between how heat is transferred through above-grade and below-grade, or through windows, are significant. The limitations apply to all buildings using the trade-off path.

Question 9:

What if the trade-off method does not allow sufficient flexibility, say in the North?

Answer to question 9:

If the difference between the required and actual log wall R-value is too large to be made up by additional attic insulation, the builder can use the performance path. In the performance path, improved basement or floor insulation, heating and ventilation efficiencies or other energy efficient construction features can be used to trade off the missing resistance in walls. The performance path can be completed through computer simulation using, for example, NRCan’s HOT2000 software, which is the same tool used for NRCan’s Energuide Program for New Houses. The performance path would then compare the log home (proposed house) to a house built with wood-frame construction according to the required prescriptive values (reference house). During the development of the prescriptive requirements, an evaluation of eleven wood-frame house types across all six Canadian climate zones resulted in an average Energuide for New Homes Rating of 78.

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