Due to the variety of building insulation materials available and the various building elements that may require insulation, there are a number of ways of installing building insulation.
Where to insulate
Where to insulate depends on where your living or conditioned space (the space that you heat and air-condition) ends and where your unconditioned space begins. Treat unconditioned space as if it were outdoors, minus the rain and snow. Insulate the living space as if you were insulating from the outdoors. For example, if your crawlspace is unheated, and you want it to stay that way, then make sure it has adequate ventilation, and insulate the floor above. If your attic is unheated, and you want it to stay that way, also make sure it has adequate ventilation, and insulate between and over the floor joists.
If you occasionally want to heat only some sections of the living space, you should insulate the walls between the sections you want to heat and the sections you don’t want to heat.
If the basement space is unheated, it may be best to insulate between floor joists (basement ceiling) instead of around the foundation (basement floor and walls). There is no harm done in insulating both the ceiling, and the floor and walls.
Generally, you should insulate:
- Attic, especially the attic door hatch.
- Doors and windows.
- Floors over unheated spaces.
- Ceilings with unconditioned spaces above.
- Knee walls and rafters of a finished or conditioned attic.
- All exterior walls.
- Walls between conditioned spaces (such as living room) and unconditioned spaces (such as unheated garage or storage area).
- Floors over unconditioned or outside spaces.
- Around the perimeter of a concrete floor.
- Around the slab (if present), close to grade level on the outside.
- Walls of finished, conditioned basement.
- Foundation walls above ground area.
- Foundation walls in heated basements
- At top of foundation, where foundation meets mud-sill.
- Around perimeter of house at band joist.
- Between rafters, but leave an air space for ventilation between the insulation and the roof deck.
- Floors above cold spaces, such as vented crawl spaces and unheated garages.
- Any floor section that is cantilevered beyond the exterior wall below.
- Around slab floors built directly on the ground.
- Foundation walls of crawl spaces (people often insulate crawl spaces so poorly that the insulation is ineffective).
If you are curious what kind of insulation already exists, here are some ways to inspect your walls for insulation:
- Remove electrical cover plates and look through gap on side of electrical box.
- Remove piece of siding and sheathing.
- Drill hole in interior or exterior wall and extract sample.
Location of structural insulation
Thermal insulation works best on the outside of the structure, as this allows walls, floors and ceilings to stay closer to room temperature, thus preventing condensation in the living area of the house and increasing comfort by the use of the building's structure as thermal mass to dampen temperature swings.
A well-insulated house requires a vapor barrier because of the risk of condensation on cold parts of the structure with resulting damage, such as mold and rot. The vapour barrier is usually a sealed plastic film inside the wall and should go on the warm side of the insulation.
A vapor barrier must be continuous to be effective. Seams must be closed between sheets or panels.
- In a heated house, the vapour barrier or air barrier goes close to the warm inside of the wall.
- In very warm climates, on a well-insulated air conditioned house, the vapour barrier should go on the outside so that the insulation is between the vapour barrier and colder, air conditioned parts of the house.
- In some places, vapor barriers are controversial. Some people believe that in temperate, humid climates, you should leave out the vapor barrier entirely.
The highest R-values per inch are provided by spray foam and rigid panel insulation. These are still only conductive thermal insulators, not radiant barriers, except in the case of rigid panels that have a reflective metal facing.
Insulating ducts and pipes
Insulate all ducts and water supply pipes where they pass through unconditioned spaces, such as through an attic or crawlspace that is not heated or air-conditioned. This includes heating, ventilation, air-conditioning, and return ducts, and both cold and hot water supply pipes.
Before insulating ducts and pipes:
- Cement duct joints.
- Check the ductwork for leaks, and repair any that you find. Do not use duct tape - it is not designed for permanent applications, and will dry out and fall off over time.
Why to insulate ducts and pipes:
- Preserve desired temperature of air or water, and save energy and money.
- Prevent condensation on ducts or pipes when cool duct or pipe runs through hot, non-conditioned space in the warmer months.
Insulating materials for ducts and pipes:
- Foil-faced fiberglass.
- Foil-backed bubble wrap.
- Rock and slag wool, fiberglass, or another non-flammable loose-fill (not cellulose!) around stove and furnace pipes where they pass through floors and walls. If the stovepipe reaches high temperatures, you should not pack any insulation around the pipe.
- Preformed closed-cell foam pipe sleeves for water pipes.
You may want to wrap your water heater in a nonflammable thermal blanket, especially if you have an older, inefficient water heater that does not have much internal insulation.
Insulating around electrical fixtures
To prevent a house fire, keep insulation away from any electrical fixtures that generate great amounts of heat, such as ceiling fan motors, and older recessed lighting fixtures that are not IC-rated. Newer recessed lights contain a heat sensor to turn the lights off when they reach a threshold temperature. The newest recessed lights, known as IC-rated (Insulation Cover) fixtures, are designed so that they do not require any air clearance and will work safely buried deep in insulation. If you have non-IC-rated recessed lighting fixtures, you should install baffles around the fixtures to maintain at least 3 inches of clearance from insulation. This is a stop-gap measure to remedy the excessive heat. Ultimately, you should:
- Use low temperature compact fluorescent bulbs.
- Replace the old fixtures with new, IC-rated fixtures.
- Replace the recessed fixtures with non-recessed fixtures.
Insulating exterior of foundation
Ideally, a home should have poured concrete walls, waterproofing, and 2-inch rigid foam panels. Complete retrofit foundation insulation may be prohibitively expensive. Since most of the heat loss from a foundation occurs where the foundation is above grade and exposed, you can partially insulate the foundation wall and still have good results:
- Buy the thickest panels that you can find (2 inches is ideal).
- Dig around the foundation at least 6 inches down.
- Clean the foundation with water and let it dry out completely, then parge twice (that is, coat with two thin layers of Portland cement), for added protection and damp proofing.
- Attach rigid foam panels to the exposed part of the foundation. Make sure they are rated for use below grade, since part of them will be buried.
- Add a protective covering, such as cement plaster, fiberglass, plastic, or metal.
- Fill in and compact the soil.
The sheets of extruded polystyrene foam attached to exterior foundation walls before backfilling serve as insulation, but their main purpose is to protect the waterproof coatings or membranes applied to the foundation wall. You should protect them from backfilling, as well, since they will not function as effectively as possible if they are cracked or torn while backfilling.
Rigid foam panels applied to an external foundation wall:
- Protect both the foundation wall and its waterproof coating during backfilling.
- Reduce dampness.
- Some types of rigid panels provide drainage or direct groundwater to the footing drains.
How to apply insulation to an exterior foundation wall when building a house:
- Waterproof the foundation wall (usually consists of an elastomeric rubber membrane).
- While the waterproofing is still tacky, spread the appropriate type of adhesive on the back of a rigid foam panel. Each type of panel has a type of adhesive that is designed to work with that type of panel.
- Stick the rigid foam panel to the foundation.
- Carefully backfill, preferably after you frame the first floor. A thermal weak spot is the spot where the foundation touches the mudsill. While building a house, make sure the builder puts a rigid closed-cell foam gasket between the top of the foundation and the mudsill, around the entire perimeter of the house. You should use sheathing grade 5/8-inch plywood and 2 x 6 timbers when framing the exterior walls of a house, not only for structural strength, but also for the increased space for insulation.
Rigid foam panels do not have to stop where the siding begins. You can extend them up, underneath the siding, all the way up to the roof. When you secure them to the sheathing, make sure that you use galvanized nails or coated screws, and make sure they penetrate sufficiently into the studs, without cracking the panels. Unlike with the panels installed against the exterior foundation, with these panels, you should leave slight gaps between them to allow moisture to escape.
Exterior insulation and finish systems (EIFS) are a new type of home construction that uses “all-in-one” insulating and structural walls with a stucco-like finish. Face-sealed EIFS is susceptible to trapped moisture if it is not installed meticulously. Drainable EIFS allows moisture to escape. Some building experts think that EIFS should only be used in hot, dry climates because of its tendency to collect moisture. 
Some contractors pour insulation into concrete blocks while building the foundation. Turn this concept inside out, and you have an insulated concrete form (ICF). An ICF is a rigid foam block, usually polystyrene, that homebuilders can stack so their centers are aligned, insert reinforcing bars into, and fill with concrete. These foundations are both structural and insulating – insulation is incorporated directly into the foundation walls, rather than added as an afterthought. The tricky part is making sure that concrete fills all of the voids in the foam blocks.
Here is an example of an ICF home: Habitat for Humanity ICF Build Apr 2004