Insulation Basics

If you only have time and money to do one thing to your house, you’ll want to make sure it’s well insulated. Inadequate insulation and air leakage are key factors that waste energy in homes—so it pays to be smart about your insulation choices.

Top 3 reasons to install insulation:
  • It will save you money on energy costs.
  • You will be more comfortable in your home, no matter what the season.
  • It will probably cost you less than you think; many states offer free or low-cost home energy assessments, so you can find out from a pro the places your home is well-insulated—and where there’s room for improvement. Even better, many states offer rebates and tax incentives when you install insulation.

Insulation and Air Sealing

When you are insulating your home, you are actually fighting two battles. The first is to keep your expensive, conditioned (heated or cooled) air inside the building envelope (the barrier between the air inside and the air outside of your building). This process is called air sealing. The second is to reduce the amount of heating or cooling that migrates out of the building through the building shell with insulation.

An analogy: you are sleeping in a cold room with an open window. What do you do? The first thing is you close the window (air seal) and then you use a blanket (insulate).

Some forms of insulation also do a great job of sealing out the air flows – but many do not. So when you start insulating your building, you need to be sure you are also air sealing.

Air sealing can be done in a few different ways. The first is manually: you could simply seal the holes yourself. This includes caulking and weather stripping. You would then insulate using a packed material like fiberglass, any type of blown material (like cellulose) and a few others.

Then there are types of insulation that both insulate and air seal. Generally, this means a foam insulation such as Icynene or numerous others. Typically, foam does a better job of both air sealing and insulating, while packed material is a lot less expensive (though you still need to air seal which costs money).

So which to use? Where you can air seal easily and cheaply, like the floor of an attic, a packed insulation is very cost effective. Where air sealing is a hard or big job, like the underside of a roof (the ceiling in an unfinished attic) foam is the thing. Also, you can only insulate finished spaces with a packed material.

Can I Air Seal and Insulate My Home to the Point Where it is Too Tight?

Yes. This is a critical health and safety issue. A leaky home may cost a lot to heat and cool, and it may be uncomfortable to live in during periods of extreme hot and cold weather, but it does allow toxic gases and moisture to ventilate outdoors. But there is a simple fix; be sure to use a qualified energy auditor and contractor to check your building envelope.

Where to Insulate in your Home

For energy efficiency, your home should be properly insulated from the roof down to its foundation. A better-insulated home lasts longer, and costs less to operate.

Insulation defines the thermal barrier of a building, (the boundary between heated/cooled space and “unconditioned” space), so wherever that thermal barrier is, that’s where the insulation should go (as shown in the diagram below).


Four of the best places to insulate are:

  1. Attic
  2. Basement
  3. Ducts/Pipes
  4. Walls
Types of Insulation Materials

Insulation comes in a variety of materials and forms, such as fiberglass batts and loose spray foam. Download our Type-of-Insulation-Materials guide.

Does a “Green” Insulation Option Exist?

Insulation isn’t just pink fiberglass anymore. Newer (and greener) options include insulation that contains recycled materials, and can be made of Rock Wool, Fiberglass, Cellulose, Plastic Rigid Foam, and something called Polystyrene Polyisocyanurate.

Of course, each type has its merits; you and your contractor will want to think carefully about which insulation will work best in your particular situation.

Behind the Walls
Innovation Center
One Broadway
Cambridge, MA 02142