Questioning the Need for Mechanical Ventilation
After reviewing the informative articles on the site, I’m still struggling with the necessity of mechanical ventilation in our circumstance.
We are building a 1500 ft2 home and 500 ft2 guest house in the arid high desert of southern Utah. In the coldest part of the winter, daytime temperatures are in the low 40s and in the hottest part of the summer, daytime temperatures are in the 90s. Average annual rainfall is just shy of 11” and snowfall is around 30”. Relative humidity caps out at 60% in the coldest part of the winter and bottoms out in the summer at 20%. The buildings are well-insulated: 2” Zip R sheathing on the walls with 2×6 framing and 14” TGIs (no exterior rigid foam). We plan on 1-2” of closed cell foam in the walls plus blown in fiberglass. In the un-vented roof, 2-3” of closed cell foam plus blown-in fiberglass. Heating and cooling will be mini-split, plus a wood stove in the home. The buildings will be occupied more or less continuously from mid-March through September and intermittently the rest of the year. In the summer, it gets cool enough at night that windows are typically left open.
Our HVAC contractor doesn’t think mechanical ventilation is necessary. What do you think? If you recommend it, HRV or ERV, and how big?
As an aside, given the tight seal provided by the Zip R, would you skip or reduce the closed cell foam in favor of more blown-in insulation?
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