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Air changes needed in a conditioned attic and crawl space

Dustin_7022224 | Posted in General Questions on

I previously asked about insulation in an conditioned attic/”unvented” roof assembly – will be built in Climate zone 5A.

I figure that we should have some sort of air changes in the attic (or is that crazy)?  Should we have ERV exhaust and supply vents in the attic and plan for a certain number of ACH? 

On a similar note, any recommendations for ACH in the crawl space, which will also be sealed within the building envelope.  I read somewhere (that claimed reference to IRC) to plan on 1 cfm per 50 sq ft of crawl.  If the crawl spaces are open to the mechanical room in the unfinished basement, which communicates with the living space, that I can place an exhaust port for the ERV at the far end of the crawl.

If you are wondering why I am asking, or what we’re doing in a nutshell, there will be insulation sandwiching the roof sheathing.  We are using radiant hydronic heating and cooling – no forced air, but will have an ERV and integrated dehumidifier.  Given that, the attic will be conditioned by the fact that it is included within the air sealed/insulated building envelope, but not because we will have HVAC ducts supplying otherwise conditioned air to the space.  The foundation and exterior walls will be ICF.

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  1. Expert Member
    Peter Yost | | #1

    Hi Dustin -

    I can't remember the Building Science Corp resource on this off the top of my head, but I have stuck in my head that the recommendation is for 100 cfm mechanical ventilation in conditioned attics.


  2. GBA Editor
    Martin Holladay | | #2

    Building codes require some type of air exchange in a crawl space amounting to "1 cfm for each 50 square feet of crawl space floor area," as you correctly note. For details of ways to comply with this code requirement, see "Building an Unvented Crawl Space."

    The recommendation for unvented conditioned attics (especially ones insulated with open-cell spray foam) is to supply 50 cfm of supply air from the HVAC system per 1,000 square foot of attic. (This recommendation comes from building scientist Joe Lstiburek.) For more information, see "High Humidity in Unvented Conditioned Attics."

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