Why R-value is Important for Foam Board Insulation.
R-values are a key metric used to assess insulation products. But what exactly do they measure? And why are they important?
In this post, we’ll find out what R-values are, why they’re crucial to the homes we build, and how various insulation products compare in R-values.
What are R-values?
R-values measure how well a 2-dimensional barrier can resist conductive heat flow. The higher the R-value, the lower the heat flow, the better a product at curbing a building’s heat loss and energy use.
How are R-values Measured?
To get a fair comparison between different foam board insulation (FBI) products, the initial R-value is measured at the time of production, with a standard temperature of 75°F. That said, some FBI panels keep more of their R-values as time goes on, and fare better in extreme temperatures, than others. Let’s see how 4 common FBI products stack up in long-term R-values and thermal performance.
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Graphite Polystyrene (GPS)
GPS boards are made of expanded polystyrene foam (EPS) that is infused with graphite particles. The graphite in these foam boards reflects radiant heat, complementing the foam air bubbles’ ability to block conductive heat. This unique function gives them a relatively high initial R-value of 4.7 per inch.
GPS’ special feature is its R-value’s longevity. Over time, the boards’ R-values experience little-to-no thermal drift, so you can count on them retaining R-4.7 years down the road.
Not all insulation products behave the same way in extreme cold weather. GPS, for instance, receives a 10% boost to its R-values when temperatures plummet to 25°F.
Another handy feature of the GPS is its permeability. When used as continuous exterior insulation with a vapor barrier on the inside, GPS boards let the wall cavity dry to the outside.
GPS panels pack way less embodied carbon than its peers of similar thermal performance.
Extruded Polystyrene (XPS)
A popular rigid insulation choice, XPS features an initial R-value of R-5 per inch. That said, the long term R-values of this foam board decline over time. For example, after just 5 years, an XPS board will see its R-value fall by 10%, and even more going forward.
In severe cold, XPS panels do see an increase to their R-values. In temperatures of 25°F and below, for instance, these panels gain an extra 10% of thermal resistance.
Unlike GPS, and by default EPS, XPS boards are impermeable. This means that they do not allow moisture to pass through. Because of this drawback, XPS boards should not be used as exterior continuous insulation where there’s a vapor barrier on the interior. Such a composition could cause the “double vapor barrier effect”, where a minor imperfection can lead to moisture build-up inside the wall cavity.
Because of the blowing agents used in its manufacturing process, XPS boards come with vast amounts of embodied carbon.
Short for polyisocyanurate, polyiso is a thermoset plastic, meaning it’s cured to rigidity using heat from a liquid form.
Polyiso offers solid initial R-values of R-5.6 to R-6 per inch. But there’s a catch: you’ve got to keep these boards warm, preferably above 50°F if you don’t want them to lose these R-values. Unlike most rigid foam boards, they ironically lose their thermal resistance as temperatures fall.
The other flaw of polyiso is that its R-values diminish over time. Similar to XPS, polyiso is known to lose 10% of its R-value in the first 5 years of service.
Impermeable just like XPS, polyiso should not be used as continuous exterior insulation when the interior has a vapor barrier.
On the upside, polyiso is produced with low embodied carbon, certainly making it a green alternative to XPS.
Mineral wool is made with recycled stone dust. It’s initial R-value is R-4, and it doesn’t abate over time. In cold weather, mineral wool performs well, gaining 10% more R-value in 25°F.
Highly permeable, polyiso blocks can not be used as an air or moisture barrier. That said, it can serve as continuous exterior insulation when a vapor barrier is applied on the inside.
In another upside, stone-based mineral wool contains low embodied carbon.
An R-value is a key indicator of our buildings’ thermal performance. Higher R-values mean less heat loss, more comfort, and a reduced load on heating and cooling systems. As you’ve seen, different rigid insulation products have distinct R-values, and not all of these last as time goes by. What’s more, properties like permeability and embodied carbon also vary based on the type of foam and its production. Be sure to take these factors into account as you choose an insulation product for your next project.