May. 03, 2011
You have probably heard much talk lately regarding the energy performance of homes across the nation. Designers and builders alike face increasing challenges achieving compliance with the ever changing energy codes.
We currently have the technology and knowledge at out disposal to not only conform to the energy codes for new construction, but in most cases, to exceed the necessary requirements for code compliance. Returning to the basics of building science is key in achieving this feat. Let’s start with what we already know from research done, and expand upon that.
Thermal Building Envelope
The first thing we need to do is establish a well sealed and well insulated thermal envelope. Webster’s defines the thermal envelope as “the building’s exterior shell”. This can be a common misconception. Webster’s definition actually applies to the term “building envelope”, or the physical separation between the interior and exterior of a structure. Thermal envelope is better defined as a “heat flow control layer”. A good way to visualize this difference is to imagine an insulated attic floor. The roof of the home acts as the shell of the building envelope, but the insulated attic floor acts as the shell of the thermal envelope.
With that being said, what makes up a thermal envelope? It can be broken down into five essential parts:
For this blog entry, I will focus on wall and roof assemblies.
Wall and roof assemblies are a crucial part of the thermal envelope. Their performance is typically rated by using an R-value, or a measure or thermal resistance. As a rule, the higher the R-value, the better energy performance of the assembly. In full log structures, showing energy compliance is becoming increasing difficult. A full log wall assembly is excessively lacking in R-value compared to a wall that has insulation in its assembly. This comes from the fact that R-values are usually measured at so many units of thermal resistance per inch. Comparing a 10” pine full log wall assembly (R-7), which is truly nothing but wood, to the same 10” half log wall assembly that contains high density batt insulation in a 2×6 stud wall (R-26), would make anyone do a double take.
Does this mean that a full log structure is not a viable method for home construction? Not necessarily. The thing that energy compliance software doesn’t take into consideration is the way the log performs. It is widely known that solid mass wall assemblies offer a thermal mass benefit that you can’t get with a stud framed, a SIP (structurally insulated panel), or an ICF (insulated concrete form) wall, and depending upon the location of the build, this aspect could benefit a home owner more or less.
Roof assemblies act very similar to the wall assemblies. They are typically made up of rafters or trusses, insulation, sheathing, and the actual roofing material. Another roof assembly option is SIPs. No matter which way a roof is constructed, it is generally insulated well to achieve the highest R-value possible and in most locations must meet a code required minimum R-value.
If a full log structure is your dream, we can certainly meet or exceed energy compliance with the help of the other pieces of the thermal envelope which we will get into in our next discussion.
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