The Hidden Cost of Killing Radiant Load – Short #260

In this short podcast, Bryan talks about the hidden cost of killing radiant load, especially when it comes to installing radiant barriers in attics.

We sometimes get service calls about ducts sweating when they never used to sweat. If there are no problems with the system (i.e., the refrigerant charge, static pressure, airflow, supply air temperature, and blower speeds are all good), this type of situation can be a head-scratcher. Ask a customer if they’ve made any changes to the attic or equipment since the last service call, and there’s a great chance they’ll say something about adding a radiant barrier to keep the attic cooler.

Ducts pick up heat from the surfaces in the attic. Radiant heat can come from the sun, but visible light isn’t required for radiation to happen, and it can occur between objects. A radiant barrier prevents solar heat absorbed by the roof decking from radiating onto attic surfaces, but it doesn’t change the dew point. When attic surfaces are cooler and the cool air passes through, the surface temperature drops below the dew point. Condensation happens on the surface; when moisture accumulates, it can lead to other problems, like the dirty M word. Moisture issues happen with low sensible load and high latent load, especially in times like hurricane season in Florida.

Adding heat to a space also makes it drier because it acts as a reheat source and drops the relative humidity. It also adds to the sensible load, leading to longer runtimes (better for comfort but more expensive in terms of energy). When we remove that radiant heat, we increase the risk of condensation and short runtimes. If moisture problems are caused by a radiant barrier, then the barrier will need to come out or the attic will need to be encapsulated and dehumidified.

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