Basement Exterior Wall Insulation


Many moisture problems can’t be remedied or lessened with ventilation, or by fixing internal sources of moisture.

When the sources of moisture are at the foundations (concrete basements, but also crawl spaces and slabs), solving the moisture problem demands drainage, waterproofing and insulation of the external faces of the foundation walls. Insulating the interior walls doesn’t fix structural problems associated to exterior moisture sources.

Partial Depth Waterproofing And Insulation

Digging a trench around the foundations is costly and difficult (it poses problems involving the weather, paved carports, trees, fences, electric lines, gas pipes, telephone and cable TV hook-ups…).

To minimize these costs and difficulties, if the basements do not present cracks, water leakage and dampness at its bottom, and if the ground isn’t saturated with water, you may try a partial depth waterproofing and insulation. It’s mainly a good method in dry and moderate-rain climates.

This method involves waterproofing and insulating at least the first 1 feet/30 cm of the underground wall of the basement, and the installation of a second piece of waterproofing and insulation materials at the bottom of the first piece, with a convenient (20º/30º) oblique angle and length (2 feet/60 cm, at least).

Full-Depth Waterproofing And Insulation

Exterior basement walls waterproofing and insulation involve:

1) digging a trench around the foundation;
2) sealing and damp-proofing the walls;
3) installing a rigid insulation;
4) installing a flashing;
5) installing a protective covering on the exposed parts of insulation and…
6) backfilling the excavated area with soil.

1 – Excavating Around The Foundation

The digging should go down to the limit of the house footing and no more. Be careful with pipes, electric lines, telephone hook-ups, etc. The width of the trench should be sufficient to work in it, and not bigger than the strictly necessary.

2 – Sealing And Damp-Proofing The Walls

Clean the surface of the foundation. Repair holes, cracks and other damages, and allow repairs to dry. Then, apply waterproofing – typically two coats of a waterproofing compound – all over the wall. All penetrations should be sealed and removed.

3 – Installing The Rigid Insulation

Measure and cut the insulation board to the desired height (typically, from the top of the footings to the flashing…). Use extrude expanded polystyrene (higher density type), high density polystyrene, foil-faced polyisocyanurate or rigid fiberglass (unsuitable for horizontal uses and oblique uses).

Start at one corner and keep the insulation boards tightly glued to the wall. Use the soil to held insulation in place, and also the flashing, fasteners and washers, at the top.

4 – Attaching A Flashing To Keep Water From Getting Behind The Insulation

The flashing allows the insulation to extend beyond the line of ground, protecting the insulation and the basement foundation from water. It also helps keep the insulation in place and provides a neat junction. Discuss its location and the type of flashing with an expert.

5 – Installing A Protective Covering On The Exposed Sections Of Insulation

The insulation should be protected from sunlight, lawn or garden tools. A number of materials can provide that protection, including cement brick, grade plywood or treated siding of fiberglass panels…

6 – Backfilling The Hole

Cover the drain tiles (made of perforated plastic pipe) with adequate clean gravel.

If possible, use free-draining soil whenever the old one is a poorly draining type. After backfilling the hole, ensure a proper run-off of rainwater.


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