It is easy to look at the R value rating of a material and assume that our entire wall assembly matches it uniformly. But when you factor in the thermal bridges caused by framing materials and any discontinuities in the wall structure, you can also quickly deduce that this is not the case.
It doesn't take expensive equipment or an engineer to identify a thermal bridge in a home, run your hand along your walls on a really cold day and they will find you.
This isn't only an issue of heat loss and the higher bills that come with it, it is about comfort and quality of life. On really cold days it can make being too close to walls quite unpleasant, rendering part of your home somewhat unusable at times.
Wood, metal and concrete in exterior walls act as thermal bridges, conducting far more heat than the insulation on either side of them. But with most types of wall assemblies we have little choice but to use some kind of heat conducting material as wall frames; foam studs wouldn't carry a Canadian snow load.
What you can do is factor that in during the design phase so you achieve the actual wall performance you want. Installing R19 batts of insulation with R5 wood studs on either side results in a wall that performs closer to R13 or even lower, depending on the amount of studs. The true or 'effective' R value of a wall is further impacted by the quality and amount of windows you install.
If you ran an energy simulation model to find out how well a wall would perform but you neglected to calculate thermal bridges, you would be trying to solve a math equation without using all the variables and your conclusions would simply be wrong.
Thermal bridge-free construction:
1- Be conscious of the use and type of exterior framing members. Some of the wood used in exterior framing can be safely eliminated - headers on non-load bearing walls, unnecessary cripples, redundant studs in corners etc. See details material efficient framing in Fine Home Building
2- Avoid metal fasteners of any kind that span the entire wall assembly.
3- Design your basement wall so it is better protected against moisture and water damage. This can allow you to use wood instead of switching to metal as a durability precaution.
4- Leave metal stud cavities on exterior walls empty, and instead use the money you would have spent on batts to thicken up a seamless layer of board insulation. You may lose a couple of inches of interior space, but that space would be more comfortable and you will get a much better return on your insulation investment.
Insulation between metal studs:
Metal studs are becoming more commonplace in residential construction, and not always with great results. Some builders love them, and for interior walls that's fine, but the energy performance of buildings that trade out wood for metal on exterior walls can take a serious hit when we ignore thermal bridging.
Metal conducts heat so easily that there is little point in putting insulation on either side of it. It is estimated that metal studs with batts in the cavities can reduces the overall performance of that insulation by 60 to 70%, or more.
According to Building Science Corporation, "The R-value of 6 inch deep steel studs installed at 16 inch centers with cavity insulation of R-21 is reduced to R-7.4, a value only 35% of the nominal".
If putting heat conduits on either side of batt insulation reduces their performance even by half, that means you essentially paid double the price for the R value you think you purchased. So your money may be better invested by employing alternatives.
Building codes and thermal bridges:
It is not only sensible to address thermal bridging in construction, it is becoming law. Recent revisions to building codes have increased the thermal requirements of building enclosures in many regions, including requiring a thermal break.
Contrary to common assumptions, the insulation values in building codes are not about promoting the most efficient and cost effective wall systems, they exist to protect home buyers by ensuring there is a bottom rung of performance we don't fall below.
So if breaking thermal bridges has now made it into code as a 'must do' practice, then you know the effects are significant. Thermal bridges affect a home's energy performance but they also have health and durability implications by causing cold spots that will increase the risk of condensation. So beyond heat loss and comfort, air quality and structural integrity can be a factor as well in terms of mold, mildew and rot.
Insulating exterior wall stud bays is not something we should stop doing, but we should not ignore the overall effect of thermal bridges when we add up all that wood, and we definitely shouldn't replace insulated wood frames with metal if it can be avoided.
Using metal studs as a means of holding insulation was called an 'abomination' by Joe Lstiburek of Building Science Corporation who eloquently explained it like this:
''Putting insulation between metal studs is like eating a sweater to try and stay warm.''
Thermal bridges that will cause significant heat loss are not limited to just studs in a wall. They include discontinuities in the insulation, corner junctions, badly installed insulation (convective thermal bridges), concrete slab junctions at walls and balconies, service openings (structural thermal bridges), and 'penetration' thermal bridges such as metal I-beams that pass through wall assemblies.
Calculating heat flow through building enclosures is much more complex than calculating it through individual materials, particularly when highly conductive components such as steel or concrete are in play. So the R value written on your insulation indicates what it is capable of, how well it actually performs is up to the designer and builder.