installing under floor insulation

Under floors over ventilated basements and other crawl spaces are easy to insulate. Installing under floor insulation, in such cases, is a DIY job as long as the access to the under-floor isn't obstructed and there isn't messy electrical cables and pipes.

Under floors over ventilated crawl spaces

Older homes often have ventilated crawl spaces beneath them, to fight moisture. However, these ventilated spaces are also a source of cold floors, drafts and consequently of energy-inefficiency and discomfort. To avoid them, you should insulate the under-floors over such crawl spaces.

This page deals with installing insulation in suspended floors, that is, floors with rooms, a basement or other crawl-space under them. The main goal of these insulation is energy-savings.

Note: There are other types of floors - pre-cast, beam and block, hollow-core floors, insulated beam floors...- involving other methods.

Insulation Materials

Under floor insulation over crawl spaces involves rigid insulation board (extruded polystyrene foam, polyurethane foam…), fiberglass batt or a similar insulation material. Foam insulation boards provide a better insulation value and also some extra structural strength.

1st step: Caulking

The first step of slab floors and suspended floor insulation is to... caulk possible voids and gaps, namely the gaps and voids around the top of the wall and where wiring runs through the floor joists...

Be careful when executing these sealing tasks: safe work practices are important when dealing with water pipes and electrical wiring and boxes, that should also be carefully insulated.

2nd step: Applying the insulation material

When using rigid insulation materials, if possible, buy the pieces of insulation with the exact lengths (16 inches or 24 inches batts and boards will fit exactly to standard situations of joists 16 inches or 24 inches apart...).

Alternatively, you must cut the pieces of insulation to length carefully. The pieces should fit tightly between the joists, for good performance.

When applying batts and other rigid materials, do not compress them; that is a cause of lower effectiveness.

The insulation material should be installed in order to touch the underside of the floor, without any space between them.

3rd step: Securing the floor insulation

The floor insulation should be held in place by one of several possible methods: wire insulation supports, wood lath, twine, garden netting laid over the joist or stapled to the underside of the joists, or other similar method.

Condensation cares

To avoid condensation in suspended timber floors, the vents associated to the floors should be kept clear. Never block them with the insulation.

Air vents provide paths to airflow through the sub-floor void, preventing condensation and rotting problems.

Be also aware of the implications of faced batts and boards. They are a common source of problems, because their vapor barrier can trap the moisture coming from indoors or from the ground. Unfaced insulation materials is, in such cases, the best option.

Solid Slab ground-floors insulation

Installing insulation on concrete slab floors is obviously different from that of the under-floors considered above. In slab floors, the insulation is usually incorporated between the slab and the flat board (screed), and often laid in two layers, with their joints conveniently taped and staggered - to prevent cold bridges coming from the concrete.

An overall thickness of 4 inches (10 cm) is required. This is a very labor intensive task, though also a simple one, easily achieved by an average DIY builder.