ICF -Insulated Concrete Form – aka – homes with poured insulated concrete walls fit for a bunker. Not only are these homes stronger than conventionally built homes, but they are considered super efficient because the solid, insulated walls keep outside air from leaking into your home. Conventionally frames homes have cracks at seams and are more prone to air leaking in. Air leaking into your home is called infiltration and it can account for up to 30% of the total cooling load of your a/c unit in conventionally built homes. ICF wall are solid and leak tight and reduce infiltration through wall by as much as two-thirds – which has a major impact on your utility bill, but also an impact on the size of the a/c unit tonnage required for the ICF home.
Typical rules of thumb for sizing ICF homes for a/c tonnage do not apply, but a/c considerations go further than just tonnage sizing. There’s a real need to examine the home for proper air flow and thermal mass effects of heavy concrete walls on the comfort level of the occupants. Unfortunately, the inexperienced will rely simply on the Manual J load calculations to size the air conditioning, but the Manual J software is not refined enough to consider effects of ICF thermal mass. ICF walls in the Florida sun are like rocks around a campfire, and when the fire goes out the rocks continue to pour our the stored heat – so do the ICF walls – long after the sun goes down. Some of the heat get poured into the home after the sun goes down. The Manual J software does not have the computational depth to model these effects and this can lead to trouble. ICF homes need to be modeled using a more robust energy analysis software which runs a 8760 hour analysis of the ICF home, and meets ASHRAE standard 90.1, to ensure that the thermal mass effects of ICF walls are properly accounted for continuously for every hour of the day, every day of the year.
Air distribution is another area that needs careful consideration with ICF homes. Because a/c equipment comes in standard sizes, the blower fan is sized to match the tonnage. Smaller tonnage unit gets you a smaller blower fan – no choice. Although the tonnage can be reduced in an ICF home, this generally is not true of air flow. High ceilings and large open spaced demand sufficient airflow to circulate air though out the space, otherwise you can end up with hot spots or cold spots. So here is where it can get tricky. Because supply flows will be lower out of the supply grilles (and consequently the throw distance of the air), location of supply and return grilles are critically important in ICF homes to ensure proper air distribution. Go the distance with the design of a/c for ICF homes – make sure your using a HVAC professional with design experience in ICF homes and not relying on a Manual J energy calc or conventional HVAC ‘rules of thumb’ to give you the answers.