Refer to Health and safety considerations, for general advice on safe working procedures.

For every underground service that enters your home (e.g. gas, electricity, telephone, water and sewage), find out where each is located before you begin digging. Utilities provide this service at no cost.

How to insulate outside the basement

The work may require several weeks of effort. Plan for extra time if you need to excavate, repair cracks, waterproof the exterior foundation walls and install a drainage system.

Insulating the outside of the basement includes the following steps:

i) Dig the trench

The excavation should ideally go down to the footings but never below. The trench should be wide enough to work in. It is a big job, so do not make it even bigger with an oversize trench. Digging may be done by hand or using appropriate machinery. The excavated dirt can be stored on a tarp or sheet of polyethylene at least 610 mm (24 in.) away from the excavation.

In some cases it may only be practical to install insulation to a minimum of 610 mm (24 in.) below grade, especially when the foundation walls and drainage system are in good condition. This will reduce the excavating required while still offering substantial thermal protection, and can be coupled with further interior insulating. An optional horizontal rigid insulation skirt can also be added to reduce heat loss to the surface, which can mitigate frost-related problems (see Figure 6-4).

SAFETY WARNING:

In particular, follow safe trenching practices to avoid accidental burial and trench collapse. Protect the trench from running water and the elements, and ensure that people and animals cannot fall in. Some soils are not stable and may require bracing to prevent collapse. If you are doing this yourself, consult local building authorities for advice on proper shoring and trenching practices.

ii) Prepare the surface and site

First clean the surface of the foundation with a wire brush and scraper or use a pressure-washer if you can easily remove the water. Inspect and repair any major holes, cracks or damage and then seal all penetrations. Smooth or replace deteriorated surfaces and old parging with an appropriate type of new parging. Allow repairs to dry.

Check the condition of the drainage tiles and repair as needed. Install a drainage system if one does not exist but only if it can be done properly (i.e. one that can drain to an appropriate discharge). It is best to do this task after completing work on the foundation wall. Consult a drainage systems expert before proceeding.

Have a contractor apply waterproofing material from grade level down to and over the top of the footings, and then seal all penetrations and overlaps. Sheet materials, sprays and roll-on compounds are available. Follow all manufacturers' directions.

TECHNICAL NOTE:

Some experts suggest using two layers of insulation with overlapping joints. The insulation is held in place at the top edge by the flashing and by using corrosion-resistant fasteners and washers. The below-grade portion of the insulation will be held in place by the backfill, but may require some fasteners to hold it in place during the process.

iii) Apply the insulation

Three types of insulation are used on the exterior of basement walls: rigid mineral wool boards, high-density polystyrene (Type IV) and polyurethane/polyisocyanurate boards. (See Caulking and other air sealing materials, for more information.)

Type IV polystyrene board is the material most commonly used in exterior below-grade applications (see Figure 6-3 and Figure 6-4).

Insulation with drainage capability (see Figure 6-5), such as mineral wool board, must be used only if:

• It is applied to the full depth of the foundation wall.
• There is a drainage system.
• The insulation has no horizontal seams that break the drainage path.

Measure and cut the insulation to the desired height (generally from the top of the footings to the exterior wall flashing). Start installing the insulation at one corner, overlapping at the corners and keep the insulation sheets as tight to the wall as possible (see Figure 6-6).

It may be convenient (although more expensive) to purchase a special interlocking system of grooved polystyrene boards with steel channels. These should be used in the above-grade portion only to a depth of 305 mm (12 in.). There are also special clips and fasteners for applying the rigid board to the wall; check building-supply stores.

Apply parging to rubble and brick foundations to smooth the wall after all repairs have been completed. The type and flexibility of the insulation will determine the smoothness of the parging required. The correct parging material will also act as a sacrificial surface material to help protect the mortar in the foundation wall. Waterproof the parging, insulate and then proceed as noted above.

iv) Apply the flashing

Flashing helps keep the insulation in place, prevents water from getting behind it and provides a clean, neat junction. There are two major considerations: the location of the flashing, which defines how far up the wall the insulation extends, and the type of flashing used.

If the siding can be partially removed or pried up, use standard Z-flashing inserted at least 50 mm (2 in.) behind the siding and building paper (i.e. behind the drainage plane). If you cannot insert flashing behind the siding (such as with brick), then either a metal J-channel must be installed prior to the insulation or a wood flashing installed after the insulation.

Flashing should accommodate the width of the insulation and protective coat. Wood flashing should be sloped with an overhang of at least 20 mm (¾ in.) and have a drip edge on the underside.

For wood flashing or a J-channel, seal the joint between the flashing and the house with a suitable caulking.

For brick siding, keep the weep holes open that allow water to escape from behind the brick.

Ideally, carry the insulation up past the header area by at least 150 mm (6 in.) as shown in Figure 6-8. This often cannot be done because of practical or aesthetic reasons. If the insulation is carried up only to the header area or lower, then air seal and insulate the header area from the interior. This is described later in this chapter.

v) Protect the exterior by covering the exposed insulation

Protect the insulation from sunlight and physical damage with a covering applied from the top of the insulation to a point about 300 mm (12 in.) below ground level.

Covering options include the following:

• expanded metal lath with cement parging
• polymer-modified pargings, which go directly on some types of insulation without metal lath - check manufacturer's literature
• pressure-treated plywood installed using stainless-steel fasteners vinyl or other siding that matches the house siding

vi) Refill the excavation

First, cover the drain tiles (i.e. the perforated plastic pipe) with 150 mm (6 in.) of clean gravel - 4 mm ? in.) or larger - and a strip of filter fabric. If drainage insulation is used, the gravel should extend at least 100 mm (4 in.) up the side of the insulation.

Backfill the excavated area in stages by removing large objects from the backfill and then compacting or tamping the ground. In poor draining soils such as expansive clay, it would be better to bring in free-draining backfill. When the excavation is filled, make sure the ground slopes away from the house. Usually a slope of 10 percent (i.e. 200 mm [8 in.] for the first 2 m [6 ft.]) is provided to allow for settling. This will encourage drainage away from the insulation, as will the addition of eavestroughs and downspouts that direct excess surface water away from the foundation by at least 2 m (6 ft.).

Cover the filled excavation with any type of surface - patio stones, grass or a garden. Some additional settling may take place, so it is better to wait before undertaking any expensive treatments such as paving.

vii) Complete the finishing details

Windows can usually be finished by wrapping the insulation around the foundation to meet the window frame. Apply lath and parging on top of the insulation to the edge of the window frame. Caulk the joint between the frame and parging, and inspect it periodically to make sure it is still sealed.

Doors should be outlined with a J-channel or equivalent flashing. You may have to extend the doorsill to protect the flashing beneath the door.

Seal penetrations through the insulation and covering to prevent wind, water and vermin entry. Some penetrations (gas lines, electrical conduits) should be sealed with a compatible and flexible sealant.

viii) Insulate the exterior joist header area

If the exterior insulation does not extend above the header area by at least 150 mm (6 in.), air seal and insulate the header area from inside the basement. (See Insulating the basement from the inside.)