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Ensuring the right ventilation approach

I'm getting ready to build a home near Indianapolis, Indiana, and I've been referring to the Building Science material for guidance on best practices for construction. When I look through the Building Profile for a Mixed-Humid Climate: Louisville it mentions the use of rigid insulation sheathing. It also notes that in mixed-humid climates (like Indianapolis), roof and wall assemblies are best designed to dry to both the exterior and interior, but the is not always possible when rigid exterior insulating sheathings are used. It states that with insulating sheathings only inward drying is possible. For mechanical ventilation, it recommends the use of an intermittent central-fan-integrated supply.

Here's where I'm confused... If an intermittent central-fan-integrated supply is used for mechanical ventilation, won't that slightly increase the pressure of the home (relative to the exterior of the house). And won't this increase in pressure make the general flow of air to go from the interior (the low pressure zone) to the exterior (high pressure zone). If that is the general air flow through the house, won't it be harder for moister to move with the air to the interior for inward drying?

For my new home I'm planning to use rigid insulation sheathing, and I was planning to install an intermittent central-fan-integrated supply for mechanical ventilation, but I want to ensure that the system works will for proper drying of the structure and I don't create issues by using the two together in a mixed-humid climate.

Thanks for any guidance that can be offered on if these systems will work well together in my climate!

Asked by Tyson Clemmer
Posted Sun, 08/17/2014 - 15:43

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Tyson,
Several factors affect whether how air moves through cracks in your building envelope.

One is the stack effect. This factor tends to cause interior air near your ceiling to escape through ceiling cracks, and exterior air to enter your house through cracks near your floor -- especially in winter.

Another is wind. This factor tends to allow exterior air to enter your house through cracks on the windward side of your house, and tends to allow interior air to exit your house through cracks on the leeward side of your house. (Of course, the windward and the leeward sides are not fixed, but change daily with changes in wind direction.)

A third factor is the effect of fans in your house, including ventilation fans. A central-fan-integrated supply ventilation system (like the one you describe) slightly pressurizes the interior of your house, but this effect is usually overwhelmed by the stack effect and the wind effect, especially since the airflow rates of a ventilation system are relatively low (in the range of 50 cfm to 125 cfm). So this effect is minor, and shouldn't lead to any worries. For more information on this issue, see Designing a Good Ventilation System.

Finally, the inward drying discussed by articles on the Building Science Corporation website is diffusion drying (evaporation). It has nothing to do with air leakage. Wall assemblies can dry inward by diffusion even when there is no air movement, or even when some air is leaking outward through the wall assembly. Diffusion is a different drying mechanism from ventilation drying.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 08/18/2014 - 03:54
Edited Mon, 08/18/2014 - 03:56.

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