Heat Pump Sizing - Factor 2: Insulation & Air Sealing
Heat pump systems are highly efficient and cost effective, and they are marketed as an efficient and clean form of heating and cooling. Throughout the last ten or so years this statement has been proven to be true. With this said, you will come across all kinds of complaints online mixed into raving reviews, and it makes it hard to know what the truth is. As someone that has been involved in system sizing and installation for much of the last ten years, I have seen the same trends when it comes to homeowner satisfaction. The three things that determine if the heat pump system succeeds or fails is an accurate Manual J calculation, insulation and air sealing, and proper installation. This article will discuss the second factor, insulation and air sealing,
Insulation
Insulation is absolutely necessary for a heat pump system to perform well. If a house loses heat as fast as the system can generate heat, the heat pump will run continuously at max capacity, leading to high electrical costs and an uncomfortable house. You can do a Manual J for a home with minimal to no insulation and correctly size the system, but it doesn’t mean the system will be efficient and you will have low energy costs.
Generally speaking, in an uninsulated home, a heat pump will be most efficient while the ambient temperature is about 30 degrees to about 50 degrees. Once you get below 30 degrees, the rate of heat loss increases, and the heat pump system needs to work harder to keep the house warm.
In a well-insulated home, heat pumps can efficiently produce heat during single digit and negative degree days. New construction homes often have R-21 walls, R-50 attics, and R-30 floors over unconditioned space, and sometimes greater. These high insulation levels are perfect for a heat pump system, but existing homes don’t need to meet today’s code to install an efficient heat pump system. An existing home with no insulation can install R-11 to R-13 blown cellulose into the exterior walls, and add R-38 to R-50 into the attic, and the difference in the homes efficiency will increase dramatically. Floors over unconditioned space will benefit from any R-value batt insulation, and you will notice a big difference from insulation as low as R-11.
Insulation is extremely important and fortunately, many states around the US have programs to insulate existing homes at a reduced cost. I am most familiar with the Weatherization Program in Massachusetts, which is a part of the Mass Save program, offering 75%-100% off of approved insulation and air sealing upgrades. States like New York, South Carolina, Arizona, and Maine, also offer discounted insulation work. If you are considering installing heat pumps I would recommend checking for a program within your state.
The point of insulation is to slow heat loss to the outside in the winter, and to slow heat gain into the house during summer. Insulation can be installed in nearly all existing homes, and is one of the most important factors in home efficiency.
Air Sealing
Air sealing is done to prevent air transfer between conditioned and unconditioned space, influencing building envelope tightness. Existing homes built in the 1990s and prior lack air sealing because air sealing was not done regularly in construction until HERS Ratings became a requirement. Before HERS Ratings, it was the common belief that “a house needs to breathe.” This means that air needs to freely flow through the structure, in theory preventing mold growth. Building envelope tightness is highly debated by contractors that have been around well before HERS Ratings, but the importance in air sealing has been proven since the start of the 2000s.
It is easy to air seal new construction house at rough when the house is framed and the sheathing is installed. The whole building envelope is accessible so any and every gap can be sealed with foam or caulk, eliminating air transfer, and making the house “tight.” A tight house has a lower heating and cooling load because less conditioned air is going to escape from the building envelope.
It is harder to air seal an existing home, but there are plenty of areas that can be sealed and you will notice a big difference in energy costs and comfort after air sealing. The two most imporant areas to air seal are between the first floor and the basement, and the top floor and the attic.
There are many penetrations between an unconditioned basement and the first floor, with gaps around pipes, electrical wires, chases, etc. You can also seal gaps in the floor boards or subfloor, the rim, and any other area that would allow airflow. Air can transfer through tiny gaps that may seem insignificant, but when these gaps start to add up, you are looking at a lot of building envelope leakage.
Attic spaces can be harder to seal, but sealing in the attic can have a greater impact in many cases. The key locations are gaps between ceiling drywall, gaps around piping, electrical, and vents, the point where the drywall meets the top plate, and attic hatches without weatherstripping.
The exterior walls can be hard to seal because the house is finished, but taking care of the areas you can access, such as the basement and attic, will make a big difference.
Conclusion
Insulation and air sealing will tighten the building envelope, slow heat loss and gain, and create a more stable indoor environment that is separated from the outdoors. No matter the age of your house, you can make improvements to it, and if you plan to install a heat pump system it is highly recommended that you do. A heat pump system is only as efficient as the house allows it to be, so creating a more energy efficient building envelope is necessary for the heat pump system. If you plan to insulate and air seal your home, check for state programs that will do the work at a reduced rate.
