R-value Does Not Tell the Whole Story

Last week we discussed K-factor, R-value and thermal performance, here it is, No More “Effective R-value”, if you missed it. 

So, let’s pick up where we left off… 

R-value alone does not tell the whole story.

R-value of an insulation can be derated, based on poor application, this is where effective R-value comes into play. Insulation is designed to be installed so that it is in contact with an air barrier at all six faces of the insulation. So, if fiberglass or cellulose is not installed properly – is not uniform and has gaps and voids – it can be derated. Meaning its effective R-value would only be a fraction of the listed, tested value. 

This is a little different for spray foam insulation, because the insulation is an air barrier. In a stud-wall cavity application, the insulation does not have to contact the inner face of the cavity, because the insulation is already on the same plane as, and is already in contact with, the air barrier because the insulation and the air barrier are the same material. And by implementing an air barrier solution, in contact with the insulation, spray foam can reduce a structure’s convection heat transfer load and provide a thermal benefit that is not captured in R-value.

Additionally, spray foam insulation can also have an impact on radiant heat transfer and the associated radiant loads. Have you ever wondered why a traditional, vented attic temperature could easily be over 120°F when it is only 90°F outside? This is primarily due to heat radiating off of the superheated roof deck to the inside of the attic.

Under the hot sun, a black shingle roof could easily exceed 150°F surface temperature and as the heat conducts through the roof deck, the interior roof deck surface might be close to 140°F. This heated surface acts like a heating element in your toaster and radiates heat into the attic heating the air up to 120°F. 

By installing spray foam insulation to the underside of the roof deck, just like turning off your toaster, you change the temperature of the interior radiant surface. The new radiant surface is the exposed surface of the spray foam and rather than being 140°F, like the exposed roof deck, it will be closer to the interior conditioned temperature, around 80 to 90°F. Thus having a tremendous effect on radiant heat transfer.

Overall spray polyurethane foam can address all three forms of heat transfer: 

  1. Conduction, of course, with it’s tested and documented K-factor and R-value
  2. Convection because of its air barrier qualities
  3. And, radiation because the temperature of the exposed radiating surface is significantly lowered.

This is why spray foam outperforms other insulation materials, not because of some magic “effective R-value” that trumps the material to up to be more than what it is, but because spray foam can fundamentally, scientifically, have an impact on all three forms of heat transfer. 

So, what do you do to show a difference between spray foam and other insulations?

Talk overall thermal performance, talk “effective thermal performance”, hell, walk them into an attic that you have insulated with spray foam and show them the difference; now you have something you can hang your hat on .

When it comes to analysis and building codes, consider energy modeling software that allows moving the air handler and ductwork into the building envelope, reducing duct leakage to a minimum (now that it is within conditioned space) and has an input for building leakage rates (natural air changes per hour).

Stay tuned as we continue to share more about spray foam insulation every week.

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