Trained Eye

 
 
 

Log Cottage Crawlspace

Question:

Dear Mr Marantz

I have come across your Winnipeg Free Press articles in the Homes Section and find them very interesting. I am asking some questions below because I'm still not sure what would be best to do in my case.

I will be building a log cottage with a ground level crawl space within a 2-foot high concrete perimeter foundation with floor joists above. The 2-foot is the minimum considered by an engineer that I have consulted. The perimeter of the foundation is to be insulated fully on the outside with thick rigid foam sheets and the footings to be insulated above and away from the building with the same thick sheeting, with backfill to grade. The building will be 22 feet by 32 feet. I will have electrical service.

I understand I would need to heat this building year round, because of the limited depth of the footings, even though they are insulated. Otherwise, I would expect frost heave. I will be using a wood stove in the main floor area along with electrical baseboard heat.

What type of unit should I consider using to heat the crawl space? Would your choice have a de-humidifer feature? Is there any point to also insulate the floor above the crawlspace? Am I creating enough ventilation with 4 summer vents, having a combined total of 2 square foot?

Thanks in advance.

Answer:

The style of foundation for your cottage is fairly common in our area, but may be subject to some movement due to seasonal temperature changes and frost heaving, depending on the soil conditions in the area he is building. Heating the crawlspace may minimize movement, but may not prevent it completely. If the soil below the footing is primarily clay, movement may still occur. If the cottage is being built in the Canadian Shield area of South-Eastern Manitoba, where bedrock is closer to the surface, potential movement will be less of a concern.

Insulating the crawlspace and footing is certainly advised and may allow for fairly simple heating of this area in the winter. Electric baseboard heaters and thermostats may be installed in the insulated space below the floor. The thermostat may be set at a fairly low temperature for the heating season, just high enough to prevent freezing of the soil in the crawlspace. Care should be taken when building and insulating the crawlspace to install a polyethylene air-vapour barrier over the dirt floor of the crawlspace to prevent moisture intrusion from heating this area. This vapour barrier should be caulked at all overlapped seams and sealed to the wall vapour barrier of the foundation and home above.

The only drawback of this style of system is a build-up of moisture in the crawlspace during the heating season. For this reason, I would not insulate or install an air-vapour barrier in the floor of the cottage. Insulating this area would only trap moisture in the crawlspace and create conditions ideal for mould and rot growth. Allowing some of the heat and moisture to escape the crawlspace and rise to the heated cottage above will minimize this potential problem.

Dehumidification of this area will be difficult if the cottage is not occupied in the heating season. Most freestanding humidifiers have to be emptied regularly and can break down and stop functioning if not serviced regularly. Installing timers on bathroom and kitchen exhaust fans to allow periodic operation will help reduce moisture build-up in the cottage. Leaving the wood stove door and flue open for the winter may also create a natural vent for some moisture to escape the building.

Installing removable vents in the crawlspace for summer ventilation is critical to removing excess moisture beneath the cottage in the warm months. Four vents the size indicated may be adequate, but depend on location and relative humidity in the area the cottage is built. Most cottage areas have high Relative Humidity year round, due to nearby lakes. Installing as many vents as possible will ensure quick drying of moisture in the crawlspace in the warm months. It would be difficult to install too many vents, and not all have to be uncovered, if desired. Constructing and installing well insulated and sealed covers of these vents in the heating season will prevent heat loss and ice build-up around the vents.

 

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