Concentration of CO2 in the Atmosphere

Installing Heat Pumps in New Houses or Converting Existing Houses is a Good Idea, but…

This stone cellar wall can be insulated with spray foam. (Images: Bob Irving)

Robert H. Irving

Air source heat pumps (ASHP) are now in widespread demand for both new construction and older homes. Many with older homes are making the decision to toss their old oil or gas burner and install air source heat pumps. ASHPs provide air conditioning and dehumidification as well as heat for your home in one source, without fossil fuels.

This is an excellent step we can take towards stopping the planet from overheating by reducing reliance on fossil fuels.

The most common ASHPs are “minisplit” systems where indoor wall units are installed to deliver warm or cool air everywhere in the house. The systems are built by many different manufacturers, notably Mitsubishi, Fujitsu, and Bryant.

But there are drawbacks to understand before or while changing to an ASHP.

Most oil-gas-wood furnaces and boilers are installed in the basement and typically maintain a decent temperature there as well as livable temperatures in the rest of the house. It is easy to fail to appreciate how much of that old unit’s heat is absorbed into the concrete or stone walls and floors and how cool the basement would become without the boiler or furnace.

Without the old heaters the solution is to air-seal and “condition” that space to maintain a decent environment when the ASHP is installed. Remember that you were wasting much of your hard-earned money to pay for the fuel for the heat that leaked out of your dwelling.

The importance of Energy Audits

The stone cellar wall with closed cell spray foam covered with a fire-resistant paint.

An energy audit will quickly show you what has been going on and what to do about it, saving a homeowner a lot of money! Audits are done by “energy auditors” and need to include a blower door test. Make sure they identify and show you where your air leaks are coming in. It is best if you go around the house and check while the fan is going so you know what needs to be fixed. Bring some colored masking tape so you can identify exactly where the air is coming in.

It all starts in the basement

My company air seals and conditions basements in all the new homes we build and all of the net-zero renovations. Even without adding a heating unit in the basement of new homes, basement temperatures typically stay in the 50s and 60s year-round.

You can also retrofit a basement in an old house. Air-seal the whole basement space.

Doors. Make sure you have an insulated, weather-stripped exterior door, if there is one.

Sill. Seal all of the air leaks around the sill area of the entire basement, where the concrete or masonry meets the wood sill. Spray foam gives the best result.

Floor. Insulate your concrete or dirt floor: First place a layer of plastic (typically thick polyethylene) to keep out moisture. Then cover the floor with two inches or more of either white expanded polystyrene (EPS*) or colored extruded polystyrene (XPS* is the same as Styrofoam.) These foams do not absorb moisture and, in conjunction with the sheet plastic, will help keep your basement dryer. Then add two layers of plywood on top to walk on. (Install the plywood layers changing the orientation between them.) Screw the two layers together to create a solid floating floor.

Thermax used on concrete cellar walls.

*EPS is manufactured with water to expand it. XPS and polyisocyanurate insulation (polyiso) use chemicals to expand the foam making these options less environmentally friendly.

Walls. Calling in a professional to apply spray foam to the basement walls might be the easiest and best way to insulate them. Or you could rent a sprayer and do it yourself.

For an alternative do-it-yourself for walls, you can apply a similar technique to the one used for the floor on the concrete or stone walls.

Use a minimum of two-inch aluminum-foil-covered “polyiso” foam sheeting. The “rmax” brand polyiso is fire-rated for exposure and can be left bare without covering it. Not all aluminum foil covered polyiso is fire rated.

Keep a half-inch gap above concrete floors so the polyiso does not absorb moisture. Seal the resulting gap at the floor with caulk.

Attach the polyiso to the concrete with plastic pins such as IDP insulation fasteners at Holes for the pins will need to be drilled with a concrete drill. Be sure to cover bare foam with sheetrock, plywood or a coating of fire-retardant paint because the foam can be flammable.

Consult a professional for information and guidance on these D-I-Y approaches and other guidance.

Never use fibrous insulation against concrete because it will absorb moisture and can create mold.

In my old house, my now unheated basement gets no lower than the mid 50s in winter, and the whole house is far more comfortable. If you seal your basement well, you should have the same results. If you don’t, it is definitely time for that energy auditor assessment to find the culprit leaks!

And what about the rest of the house?

An audit will also show you the importance of attic insulation. And don’t forget about the walls of your home and air sealing around windows and any penetrations in the whole envelope of the building.

The point is to keep any heated aif inside once you have provided an efficient heating system without fossil fuels.

Bob Irving is the CEO of R.H. Irving Homebuilders, a high-performance contractor with years of experience in Passive House as well as net-zero and net-zero ready construction. He is based in Salisbury, NH. He is semi-retired, but you can reach him at He will be happy to answer your questions.

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