What Is a Crawl Space?

The crawl space is a type of foundation commonly used in Swedish houses, especially in older buildings but also in modern villas. It acts as an intermediate space between the ground and the floor joists of the house and is sometimes ventilated with outside air, which can lead to moisture problems mainly during spring and summer.

This construction has both advantages and disadvantages, and understanding how a crawl space works is important to protect the house from moisture and mould problems.

First of all, what does a crawl space look like?

A crawl space is a low space, usually between 60 and 100 cm high, that extends under all or part of the house. It is not intended for storage or habitation, but serves as a technical space. The crawl space is often accessed via an inspection hatch from inside the house.

The soil in the crawl space often consists of soil or sand, covered with building plastic that acts as a vapour barrier. Above the crawl space is blind bottoms, the insulation and the floor joists of the house, allowing the foundation to act as a buffer zone between the ground and the living spaces.

Why build with a crawl space?

There are several reasons to build with a crawl space:

• Simple and cheap construction: Crawl spaces are often cheaper and quicker to build than a slab-on-ground and provide easier access to installations such as pipes and ventilation.
• Protection against ground movements: In areas of frost heave or subsidence, the crawl space can provide a degree of flexibility that reduces the risk of damage to the house.
• Good insulation from ground cold: When properly ventilated and insulated, the crawl space acts as a shelter from the cold of the ground.

But, despite this, there is a major problem with an unventilated crawl space.

Moisture - the big challenge of an unventilated crawl space

The crawl space is also associated with moisture, and an outside-ventilated crawl space is considered a hazardous structure.

Because the space is ventilated with outside air, warm, moist air can condense against cold surfaces in the crawl space during spring and summer. This can create a damp environment in which mould, rot and bad odours can take hold and spread if the conditions are right. Rain, surface water and groundwater also contribute to increasing the humidity in the space.

If humidity levels are too high, as they are likely to be in summer, there is a risk that the indoor environment of the house will be affected. Bad odour can spread up into the home, and there is also a risk of microbial fouling on the blind floor, insulation and building structure.

How can you protect your crawl space from high humidity?

There are a number of solutions for keeping a crawl space dry and clean. By far the easiest is to install a desiccant dehumidifier, which simply dehumidifies the air coming in.

By lowering the humidity to an optimal level and then allowing the desiccant dehumidifier to automatically regulate the climate when needed, you get a humidity-controlled space. But then it is important to prepare well before installing it.

This is how the installation of an Acetec EvoDry RCF dehumidifier can be done in the crawl space:

• Clean the soil of organic material, such as plants and construction debris.
• For the best performance and energy efficiency, the space should be carefully sealed.
• Cover the ground with age-resistant plastic sheeting in batches, overlapping by about 50 cm. For optimal tightness, the plastic should be attached to the bottom edge of the foundation beam with insulating washers or studs and nail plugs. The plastic is folded double against the beam/foundation wall and stretched slightly between the attachment points.
• Seal all valves except the one where the wet gas hose exits.
• Place the dehumidifier in a high position on, for example, a leca block, insulating board or the packaging supplied. Make sure there is at least 50 cm between the filter and the wall.
• Mount the wet gas hose on the outlet plate with hose clamps. Insert the wet gas hose through the valve from the outside and seal the back of the plate with silicone or equivalent. Secure the outlet plate with a screw plug in the base beam.
• Fit the hose to the dehumidifier. For better distribution of dry air, dry air ducts can be mounted on the connection nozzles. Make sure that the hoses are stretched, in soft bends and not folded or creased.
• Plug the modular cable into the dehumidifier and pull the cable up to the dwelling. Install the control panel highly visible and plug the cable into it.
• Connect the dehumidifier to a grounded electrical outlet (230V, 10A, slow blow fuse).
• Check that the control panel shows contact and reasonable values.

Read more about install a desiccant dehumidifier here.

ATTENTION! The installation instructions above only apply to Acetec EvoDry RCF dehumidifiers. Detailed and up-to-date instructions are always included in the manual for each model. Follow the accompanying documentation during installation.

💡 Do you need help sizing a dehumidifier for, say, a crawl space? Feel free to try our proprietary tools which suggests appropriate models.