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Léa Noémie Plourde-Archer
Léa Noémie


How to Insulate a Crawl Space : Tips and Guidelines

Last modified: 2017/09/06 | 4 mins

More often than not, crawl spaces are found in older houses. However, in the past few years they’ve been making a comeback in newer constructions. These underground spaces must be well insulated, well ventilated and completely waterproof. has collected a few guidelines on how to insulate a crawl space.

Insulating a crawl space

A crawl space that is poorly insulated could lead to various significant problems: frozen plumbing, humidity, mould, flooding, heat loss, etc… On a day-to-day basis, this also represents increased heating bills. Crawl space insulation problems often stem from the fact that stone and concrete walls are very bad at retaining heat. In order to properly insulate a crawl space, other materials that fill out the role of the insulating layer must be employed.

Two main crawl space insulation techniques exist :

- The insulating layer is installed on the outside walls. The crawl space is then turned into  fully heated space, like the rest of the house. - On the ground floor level: the product can be applied on the floor joists that are found in the crawl space, underneath said floor. It can also be applied on top of the flooring surface itself. In most cases, the first option is easier to execute.

Depending on the size of the crawl space, it’s not always easy to move around for complex renovation tasks. The joists can also make things more complicated, as they can vary in shape and size. Moreover, if your crawl space is humid, you should prioritize wall insulation, instead of working on the floors.

A few things to know about crawl space insulation:

- Before you start redoing or adding insulation material, remove all the elements that have been affected by water and/or by mould. Make sure all traces  of decay are removed and finish by applying an antifungal treatment that you will let stand for a certain period of time, according to the instructions that are written on the bottle.

- Mineral wool is not a recommended insulation material for crawl spaces. Instead, contractors prefer to use polystyrene (expanded or extruded), polyurethane foam or cellulosic fiber.

- For proper insulation between floor joists, always use a rot-proof product.

- Even when a crawl space is heated, pipes and drains can freeze if they’re not being used during the winter. As a solution for this problem, use automatic heating electric tape (found in hardware stores).

Photo: Islandbasementsystems

Photo: Islandbasementsystems

Improving crawl space ventilation

Beyond humidity problems, another major problem that can arise from poorly insulated and ventilated crawl spaces is radon ingress. Radon gas tends to accumulate in crawl spaces and if the area isn’t properly ventilated, it can seep into the house and settle in the air. According to the Quebec Lung Association, 16% of all deaths related to lung cancer are caused by radon exposition.

Therefore, this problem has to be taken into account. Despite what one might imagine, installing ventilation hatches in the crawl space will not have a positive effect in reducing the degree of humidity. Instead of being evacuated, the air will enter the hatches and will stay stuck in the crawl space, going on to produce condensation.

However, starting in the fall and up to the spring, you can install a small fan or a de-humidifier with a timer switch that you will set for 15-20 periods per hour. This solution is fairly affordable and easy to operate. Beware: this practice is not recommended during the summer because the fan would send warm air into the crawl space, thus nullifying the desired effect.

Waterproof construction and insulation materials

Employing waterproof materials is very important. Numerous severe complications could result from having a humidity problem within a crawl space. This aspect is especially important if you wish to use this space for storage. In order to reduce the humidity level and to protect the elements (in case of a sudden increase in humidity), materials that are waterproof should be prioritized.

For example: a vapour barrier that would be installed on the floor of the crawl space. The sheets that make up this vapour barrier must be properly sealed with sealing tape. Before you install the vapour-barrier, check that the floor is leveled and straight. If not, the material will not work.

Have you decided to install insulation materials on the ground floor?

Your floor covering might already have the proper qualities to act as a vapour-barrier. For example, if the floor is covered in linoleum, this material is already waterproof. If not, you could install a polyethylene sheet on the floor surface. Crawl space humidity levels can be verified using a hygrometer. If the instrument indicates 65-70%, especially during the winter, this might indicate a problem. In this case, contact a specialized contractor who will be able to give a proper diagnosis of the situation.

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