With more and more homeowners showing an interest in using spray foam to insulate their homes, it’s no wonder that I get calls every day from potential clients who have questions about ventilation, especially if the client plans on using spray foam in their attic or if the HVAC unit is located in the attic.
Every house is different, so those old heating and cooling load calculation charts are simply out of date, just like naturally vented appliance are out of date! It’s far more efficient to consider where the spray foam will be applied so we can consider how the foam will change the performance of the structure, then use an HVAC software package to calculate heating and cooling loads.

After Spray Foam is Applied, Your Home Will be an Energy Sipper, not a Guzzler
Everyone knows that applying spray foam insulation will greatly reduce your heating and cooling needs, which ends up saving you tons of money.
How does spray foam accomplish this? When you consider that spray foam is both an insulating material, as well as an air barrier, you can see how this happens.

When it comes to your current HVAC unit, chances are that an oversized unit was chosen due to the typical leaks that come with constructed housing.
If you’re in the market for a new HVAC unit and you think that adding spray foam first means you can purchase a smaller unit, you are correct, however, without proper ventilation, you will find that the HVAC unit doesn’t need to operate as long, which means it does not remove condensation from the air inside the home.

This can result in not only uncomfortable humidity levels, but it can also allow mold to develop inside the building.

HVAC in a Conditioned Attic
Many new housing designs place the HVAC unit in the unconditioned attic or even a crawlspace. Converting an unvented attic or crawlspace will lead to huge reductions in challenges with creating comfortable spaces during the hottest/ coldest days of the year.
Depending on local laws, these spaces can be called conditioned space or indirectly conditioned space, or you might even hear it called buffered unconditioned space.

Whatever you call it, when Furnaces and Ducts are in unconditioned/ uninsulated spaces, special provisions will need to be met, including:
Ducts and other equipment should be “within the conditioned area” and duct losses should be into conditioned space.
The volume of the attic is added to the overall volume of the conditioned space within the house or building. More space to heat and cool but dramatically less loss of heat/ cooling.

Combustion appliances need to be sealed out outfitted with an outdoor air intake, with exhaust also being directed to outdoor space. Two pipe installs actually require both pipes be properly terminated OUTSIDE!

Vents that were originally designed to exhaust into the attic will be continued until they have reached outdoor space.
Ventilation must be provided according to the 2018 IRC, Section R408.3.

Mechanical/Natural Ventilation Systems
You may have noticed that many new buildings are excluded from the requirement of a dedicated mechanical ventilation system since natural infiltration rates exceeded the delivery volume that a mechanical ventilation system would have required.
The problem here is that natural ventilation is not only unreliable but it often delivers humidity into areas where this extra moisture can create problems.
Homes and buildings that are insulated and air sealed using spray foam no longer rely on natural infiltration for ventilation. This means that a mechanical ventilation system will need to be put in place to meet building codes.