Jun. 14, 2011
Insulation is a key player in the creation of the thermal envelope, and will be the focus of discussion for part two of this series.
For half log construction, the thermal envelope does not differ from the thermal envelope of a conventionally framed home. Notice in graphic “A”, the continuous outline created by the insulation. It is crucial that this outline surrounds the house with no breaks to achieve the best result in the control of heat flow. The majority of the R-value in a wall or roof assembly is created by the addition of insulation.
Some areas of the country have code minimum requirements for R-values. Let’s use Minnesota as an example. The state is broken into the southern and northern areas and the requirements are different for both. The current requirements for the northern areas are as follows:
• Fenestration U-factor – .35
• Skylight U-factor – .6
• Ceiling R-value – 44
• Wood frame wall R-value – 19
• Mass wall R-value – 15*
• Floor R-value – 30
• Foundation wall and rim joist R-value – 10
• Slab R-value and depth – 10, 5 ft.
• Crawlspace wall R-value – 10
*Mass wall R-value does not apply to full log structures. A minimum diameter log size of 7” has been determined by the state provided that the U-factor of the windows is .31 overall on average or better.
When we get into full log construction (graphic “B”) it gets increasingly more difficult to comply with energy codes using a performance based method (i.e. energy compliance software such as ResCheck). The good thing about the thermal envelope is that it performs as a whole. Granted, it is only as good as the weakest member of its sum, but increases in other areas of the envelope will help it to perform better. Full log structures can comply using the performance based method when tradeoffs are allowed.
In some areas of the country, a full log structure may actually perform better given the introduction of thermal mass. Thermal mass is a concept that is simply defined as absorbing heat from a source, and then releasing it slowly as the heat source is removed. A full log wall is a thermal mass conductor. While we know that thermal mass makes a difference in the performance of the thermal envelope, there is no good way to prove this using a performance based method of energy compliance. A prescriptive based method of energy compliance can lend itself better to a thermal mass wall system.
In conclusion, whether your new log home is going to be half log or full log, it is best to have a design professional complete your plans, as well as evaluate the energy requirements in the your area, to ensure that the best possible efficiency is achieved.
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