If someone is bidding your spray foam home hvac systems please ask them if they are doing a Manual J, D, S on the building. It is in national energy code and 42 of 50 states code enforcement however, Texas has not yet started enforcing it at a state level. DFW, Austin, Houston, and San Antonio all enforce it to different extents. Just because it is not enforced does not necessarily mean its unnecessary. With spray foam homes humidity will not leave the home as easily and the swing between a warm spring day and a hot summer day will be much more extreme. Ensuring that you have an hvac system that will operate properly at both levels is essential and the duct design will become crucial to your comfort as your system moves from high speed to low speed.
This section does not necessarily only apply to spray foam, but any tightly constructed home. Essentially the home becomes so tight that there is no natural air exchange with outdoor air, which for efficiency purposes is great! For health reasons this can pose some problems though. If you have say the flu going around your kids school and he breathes it into the air that pollutant is now in the home and can not leave it just continues circulating through your hvac system and home. If you have a bad smell in the home that smell can not leave it just keeps circulating. By having fresh air exchange the air in the home is recirculated with outdoor air multiple times a day to ensure that the air you are breathing is healthy, fresh air.
The reason it is more important with spray foam is that the air seal provided by spray foam all but ensures its necessity in the home. The issue that we run into as hvac service contractors is that not every insulation contractor asks the homeowner if there is ventilation plan in place. Technically ventilation is the job of the hvac contractor so but if you have never gone through insulation training you may not know that the spray foam requires it, or how to properly do it. There are a few methods discussed in our IAQ section under home ventilation. There are codes set aside for figuring out how much ventilation you need based on the tightness that the insulation will achieve. A Manual J, D, S will include this in its calculations.
Attics in spray foam homes are generally a nice cool place compared to the traditional attic you are used to. This is great for home efficiency, heat load, and any of equipment you have in it. It does bring about another problem though. As the attic space becomes its own environment essentially with the sheetrock from the ceiling providing a floor and the spray foam ceil providing an upper barrier they can become susceptible to building humidity. In smaller attics there is often enough transfer around grilles, lights, etc... to keep it from being an issue but in larger attics you should be installing a dehumidifier to prevent this humidity from creating problems such as mold in your home.