Renovation & Design

Use air conditioner to reduce humidity in home and crawlspace

Question: I built a 1,500 square-foot home at the lake on a rock. I used ICF (insulated concrete forms) blocks for the crawlspace foundation, setting them on the rock and filling with concrete. Even though no leaks occur under the ICF walls, the crawl space is extremely damp and the relative humidity (RH) in the home is very high in the summer. The home is very well insulated, has electric forced-air heat, air conditioning (A/C) and a heat recovery ventilator (HRV). We never use A/C because the building is extremely cool in the summer. I am going to put heavy poly on the rock floor to see if this will help lower the RH, but was also thinking spray foam. Any suggestions?

Thanks, Bill.

Answer: High relative humidity, condensation, and moisture issues are common to many homes with crawlspaces, but especially in lake country. Improved ventilation and use of the air conditioning should be the two most effective methods of managing this issue, while sealing the bedrock floor inside may not make much of a difference.

Seeing signs of moisture in a crawlspace on grade, at different times of the year, is quite normal but still should be managed. Excessive humidity can cause premature rot, mould growth, rusting metal components, and other issues inside the confined space. Sweating water supply and drain pipes can also lead to small puddles, but the most serious problems normally occur due to accumulations of water from the surrounding soil. Since you don’t have that issue, as you are ideally situated on bedrock, the main concern is how to prevent or remove moisture from the crawlspace air.

There are two main approaches to dealing with humidity-related issues in an enclosed space, like the crawlspace below the floor in your lakefront home. The simplest and most effective way to prevent excessive moisture buildup in this area is by adequate ventilation. This method also has two main possibilities, passive vs. mechanical means. The passive ventilation approach is the easiest, by installation of several screened vents on each side of the crawlspace grade beam foundation. If you have included these in the insulated concrete forms used for your foundation, it may simply be a matter of installing insulated covers for the winter and replacing them with screens for use when the weather is warmer. The screens should be made durable enough to prevent rodents chewing through the mesh, to stop them from gaining easy access to the crawlspace. If you have not done this at the time of construction, it may require hiring professional concrete cutting contractors to install these openings after the fact. Either way, ensuring good air movement through the vents will really help prevent condensation related problems, even with very high humidity in the summer months.

The second way to improve ventilation in the crawlspace will be to use the heating, ventilation, and air conditioning (HVAC) systems already in the home. The initial checklist should include ensuring there are enough duct openings or registers in the crawlspace from the furnace and HRV. If these have been omitted, or minimally installed, the mechanical systems may not be able to adequately help. There should be at least one intake for the HRV and two or more heating registers/open ducts. If there are areas where the floor joists are closer to grade than others, or multiple foundation angles, more duct openings will be required to increase the air movement and prevent stagnant air pockets.

Once enough duct openings are ensured, running the furnace fan continuously on low speed, from a control on the thermostat, should be done. This will use the blower to continuously move the air through the crawlspace, as well as the rest of the home. The second key to the success of this will be to use your air conditioner in hot weather, which will significantly reduce the RH in the entire building and crawlspace. While you may not think this is necessary for temperature control, the A/C system substantially reduces air moisture during operation. One caveat, if you have adequate passive vents in the crawlspace and open windows in the home, mechanical summer ventilation may not be necessary. Especially if you have a consistent breeze off the lake, using this to your advantage may make using the furnace in the summer a non-starter.

Finally, ensuring your HRV is properly set up, with the dehumidistat function operating properly, will help dry out the crawlspace. This should not necessarily be used in the summer, but will be crucial for use in the heating season, when the crawlspace is fully enclosed. While this will only dehumidify the area in the winter, preventing condensation at that time of year will minimize the chances of significant moisture damage to the wooden components, even if some does occur in the hot summer months. This influx of dryer winter air will help remove any residual moisture that may be trapped in the crawlspace from the previous summer months.

Having high RH in your crawlspace in a home located adjacent to a lake is not in itself a problem, unless the area is poorly ventilated and prone to condensation. Ensuring it has enough passive ventilation, or using the air conditioning system to mechanically dry out the air, are the two approaches to prevention of serious moisture-related issues and damage in that area. Installation of 6MIL polyethylene sheathing over the bedrock floor is a good idea, but may not help much with your high RH issue.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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