On Envelope vs. Ventilation Energy Loads

Thanks for your article on “Establishing Priorities.”

Under saving energy, the first item: “In buildings with skin dominated loads,” incorporate high R-values. What about buildings with greater internal loads? Does it follow to use less insulation? Make it less airtight? I have a high school where the general contractor is saying, “How come you have so much roof insulation when we’re moving so much air around? We can cut it in half and save a bundle.”

We are disposed to build the best building possible: high R, airtight, thermal mass, etc.; but often seem to find ourselves in this gray area of uncertainty.

Thanks,

C. Stuart White, AIA

Norwich, Vermont

Editors’ response:After consulting with engineers (and Advisory Board members) Marc Rosenbaum and Mark Kelley, we have the following thoughts:

While it’s likely that the heating load due to ventilation dwarfs the skin load, that doesn’t mean that the skin load shouldn’t be addressed. The main point both engineers made is that careful building analysis is essential to determining how to minimize energy use with a minimum of added cost. If the building’s energy use is truly dominated by internal cooling loads (unlikely in this case), adding insulation can actually increase energy use for cooling—again, detailed simulations are the best way to find out.

Also, as Amory Lovins has pointed out, sometimes the combined effect of multiple conservation measures, which are not each cost-effective in themselves, can be the least-cost option. This happens when the combined savings add up to allow the elimination of an entire portion of the mechanical system, for example.

Finally, if the particular building you’re working on has decent solar access, it may be a good candidate for the Solarwall™ product to reduce ventilation-related heating loads (see Solarwall—Simple and Effective Solar Heating).