Crawl Space Ventilation

[Roger A. Frost]

Had a question posed to me on ventilation of crawl space. This is a seasonal cottage that only has one half heated during winter, and this is minimal heat. Any thoughts on what is being proposed below? or if you have a better idea would be nice to hear it


I think we have given up on the idea of dehumidifying the crawl space. Aside from the difficulty of finding such a unit, it would need something to circulate the air and bring the humidity to the unit. We are focusing on enhancing the airflow through the crawlspace with a fan that vents the crawlspace air to the outside. One company wants to install a fan in the floor opening in that bedroom. I think there is a 10 foot run to the outside wall and that will reduce the fan efficacy, even if it is a 6" duct (though the CFU is more than enough to ventilate that small crawlspace of some 1,500 sqft. Another simpler alternative is to install simple exhaust fan (or fans) with humidistat and temperature control onto the outside wall. I am awaiting a call from a heating/ventilation contgractor who has a cottage up north and knows the exact problem. Mount fan either directly on lower outside wall or with an 'elephant' duct to a fan raised above the snow line. This will draw air from the crawlspace. The crawlspace will have negative pressure and will draw air from the house. There is always an issue of humid summer air being sucked in and the motors can be switched off during warm months. From fall through spring, the heated crawlspace will be warmer than the house (though both are just warm enough to prevent freezing) and this will facilitate humidity staying in the air and being blown out and replaced with dry colder air. Might need a plenem to direct airflow in the floor of the house, either in a cupboard or in the middle of that long, narrow hallway. In most cases, house air is cooler than outside air. And we will keep a couple of dehumidifiers going in the house so the air sucked into the crawlspace through cracks or plenum will be drier than outside. Will also fix the bathroom exhaust fans. In no case would we draw outside air straight into the crawlspace and this is in keeping with the concept of no longer using simple vents.

• Regards

[Jeffery L. Haynes]

If you have the money, then simply do an encapsulated system and be done with the problem FOREVER.

Crawlspaces are problematic in humid areas.....(we have the same issues in parts of the Carolina's)......many times with fans, as you know, they simply pull in more humid air from the outside............sure you can shut it down when its too humid but that accomplishes nothing.

It was costing me around $2.00 per square foot to encapsulate......if you do it yourself you can save quite a bit more.....just make sure its done right and leave at least a 6 inch gap perimeter below the sill plate in order to check for termites.

[Larry D. Kage]

Lots of good info here: http://www.crawlspaces.org/