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Thermal Performance of Building Envelopes

Posted December 22nd, 2010 in building science and tagged , , by TitanWall
Thermographic Image of TitanWall Research Facility
Courtesy Prof. Tang Lee

Hello everyone. Today we continue our series on the building envelope with a discussion of thermal performance, one of the key factors in determining the comfort of the indoor environment and the energy efficiency of a building.

These days every home builder and construction material salesperson you talk to will spout off terms like “R-value,” “U-value,” and “thermal mass” in an effort to make you believe that their product is not going to cost you a fortune in heating bills and ensure a comfy space. We’re no exception at TitanWall, but hopefully by the end of this series, we’ll help you understand a bit better what these terms really mean. In addition, we want to give you an understanding of the bigger picture — of how the thermal performance of your envelope can interact with other systems in your building to provide options for more efficient and cost effective design.
Its hard to believe that just over a century ago buildings were rarely insulated at all. Central heating wasn’t common until the 1800′s and fibreglass insulation wasn’t even around until the 1930′s. How quickly things have changed! Now there are so many different materials and methods to choose from, it can be overwhelming to try and figure out what will work best for your project. From fibreglass batts, to spray foam, to blown in cellulose or our favorite, MgO insulated panel systems, there are more choices becoming available almost every week as new innovations are brought into the marketplace.
So why is thermal performance important? For Canada, in 2008, 60% of all commercial energy consumption and 82% of all residential energy consumption was for space conditioning and water heating, according to Natural Resources Canada. By increasing the thermal performance of a building envelope, there is an opportunity to significantly reduce the heating energy use of a building. This often has the biggest ROI for any green building investment.
There are several factors that affect the thermal performance of a building:

  • thermal resistance
  • thermal mass
  • air permeability
  • building orientation and form
  • mechanical system design
For an effective thermal strategy for your building, all of these factors must be addressed together, and we will try to do so in this series, but for today’s post, we’re going to takle factor number 1, thermal resistance.

Thermal Resistance:

Thermal Resistance is the degree to which a material resists heat flow through itself. It is the opposite of thermal conductivity, or the degree a material conducts heat. There are several measures of thermal resistance including RSI (SI units) and R-value (imperial units). For all you geeks out there like me, RSI is measured in Kelvin square meters per Watt (K m^2/W) and R value is measured in square feet degrees fahrenheit hours per british thermal unit (ft^2 ºF h / Btu). They are easily confused but also easily converted: 1 RSI = 5.68 R.

The important thing to remember about thermal resistance is that the higher the value, the better insulated your wall is.
Wikipedia has a great chart of R and RSI values per inch of various materials. From the chart you can see that EPS, like what we use in our panels ranges from R 3.85 to 4.2 per inch, depending on density. Although there are other materials that have higher values, if we were to look at cost and environmental footprint as well as r-value, EPS is one of the best choices.
A wall isn’t just comprised of highly insulative materials though; it has other components that need to be taken into account when thinking about thermal resistance. Things like studs, bolts, plates, sheathing and anything else in your wall. Depending on the make up of these wall components and their thermal resistivity, a wall can have a true R-value significantly lower than the R-value of the insulation. This is due to a concept known as thermal bridging.

Thermal bridging is when a material with a lower R-value provides a conduit for heat to flow faster through a material of higher R-value, thereby lowering the total thermal resistance of the assembly. In a typical stud wall, up to 25% of the wall area is comprised of wood studs, with an R-value of about 1 per inch.
This can cut the R-value of a wall by more than a third in some cases. Windows also can contribute to lower r-values so it is important to select the most efficient windows your budget will allow.
Another material characteristic that affects your wall assembly is that of thermal mass. Be sure to subscribe to our feed to you can see our next update where we’ll explain the benefits of thermal mass in building materials.

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