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This past weekend I recently had to fix the flooring of my family cottage. In a heavy-traffic side entryway, the plyboard floor was bending when you walked over it.

We opened up the floor to see what the issue was. The ends of the crossbeams in that section were rotted right through (6 in total). We had to cut them back to a another beam where they were not rotten and replace them.

While we were replacing them I had a look in the crawlspace and noticed significant moisture specifically under the washroom section (e.g. crossbeams sweating and dripping). Not sure why it's just that specific section, but the entryway to the washroom suffers the same issue when you walk over the flooring (bending) so we will most likely have to replace that part as well.

I'm wondering what can be done to fix the humidity issue. The cottage has small crawlspace vents all around. My father used to open them up every year and close them in the winter until a contractor told him not to a couple years back.

I have read a lot of articles and also some questions on this site about closing off the crawlspace vents and putting up vapor barrier on the dirt floor even on the walls up to the floor boards. Some people say that this is the best solution, others say that it is location and climate specific and may also be a bit overkill.

I was wondering if there were some knowledgeable people out there that can shine some light on a possible solution.

To give some context the cottage is located on the Bruce Peninsula, Ontario, Canada. Cold winters, hot summers, lots of insects/pests. The concrete foundation goes a couple feet above and below ground level with a dirt floor crawlspace. Cottage is probably over 40 years old with the crawlspace vents being open in Spring/Summer.

  • Is the cottage air conditioned? Only when you visit, or all the time? – Harper Aug 19 at 20:38
  • I'm kind of thinking your dad had the right idea by opening the vents. It seems likely to me that this problem is relatively recent for a 40+ year old cottage. Air should circulate through there to dry it out. Can a water leak from the washroom (or kitchen/air conditioning system) be ruled out? – Greg Nickoloff Aug 19 at 20:54
  • @Harper the cottage is not air conditioned, only heated during the winter by a wood fireplace. – Matt K Aug 19 at 22:18
  • @GregNickoloff No leaks can be seen. The entryway flooring that we replaced is also not near the washroom where condensation is visible. It's not near any plumbing either. The two sections might be two different issues but I think it's fair to say there's a general humidity issue down there causing boards to rot and attracting carpenter ants but that's another question altogether. – Matt K Aug 19 at 22:25
  • @MattK - My bad. I was thinking it was both ends of the same boards. Not sure about the carpenter ants (I think an exterminator may in order) but I still think the vents are your solution once the rotten boards are replaced. – Greg Nickoloff Aug 19 at 23:14
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Venting the crawlspace is important. It helps to reduce moisture by allowing air to circulate and by equalizing the temperatures inside and out and thereby preventing condensation. I don't know about Ontario, but I'm in Ohio and I believe the building codes here (about the same climate) specify venting crawlspaces at specific ratios

This is a good article which explains vented and non-vented crawlspaces and how they are supposed to work better than I can..

Open up the vents when it gets warm in the spring and close them back up in the fall when the air cools down. There are even automatic vents available that will do this on their own at appropriate temperatures without using electricity.

Automatic crawlspace vent

Additionally, you should probably install the plastic vapor barrier on the dirt floor and walls to keep out additional moisture (we do it here) and some screens over the vents would be good to keep insects and other uninvited guests out of there.

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I too think the problem is condensation.

Remember, warm air holds more water vapor than cold air. As the vapor moves from a warm space to a cold space, it looses some of its vapor as it turns into water (condensation).

So, when the air temperature is colder in the living space than in the crawl space, then condensation will occur when the vapor reaches its “dew point” as the vapor travels from the crawl space to the living space. (I doubt it occurs in the opposite direction, as heat rises...which may affect the attic space/joists.)

One way that moisture can be reduced in the crawl space is to install a vapor barrier on the ground and then add crawl space vents. In the U.S., the Code requires 1 square foot of vent opening for each 150 square feet of floor area. This area can be reduced to 1 square foot of vent opening for each 1,500 square feet of crawl space if a Class 1 vapor ground barrier is installed. (See ICC Section 1203.3.2)

However, the vent openings should be arranged for cross-ventilation. Also, I live where the wind blows so hard (and often) that a pin-hole could dry my crawl space out. You may need to rearrange the vent openings depending on the direction of the prevailing wind, location of surrounding barriers, (I.e.: adjacent houses, fences, bushes, etc.).

Keeping the crawl space dry is key...and the size and location of the vents is most important...and kept open year-round.

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