Log Cabin vs Moisture Build-up

by Cody Farmer

Air Tight Construction Colorado

I got that call again yesterday. The type us building geeks don’t want to hear about.  A brand new custom log cabin sweating to death from the ceiling. Looking at the pictures the owner emailed me, I could practically smell the fresh chink outlining the logs. I love Log Cabins!

Many Log cabins have full vaulted ceilings, however. On top of the exposed ceiling logs is a 2x12 frame, non-ventilated space filled with fiberglass batt insulation at R-38. Which doesn’t fill completely the 11.25” cavity of the 2x12.  So this particular phone call isn’t un-common in the Rocky Mountain Region.

 

If you live in the Rocky Mountain Region, you’re already aware of this, but if you’re not – our temperatures and humidity levels are constantly shifting. Even in May you could have a sunny morning, rainy afternoon, then snow in the evening. Locals often jokingly refer to our weather as “bi-polar”. Without a tightly sealed home (like that found in Passive House construction), your indoor temperatures and humidity levels can be just as shifty as those outside. Added to that “fun” fact is the fact that with shifty weather comes a constant pattern of moisture freezing, thawing, and evaporating. This constant freezing, thawing and evaporating can cause wood rot, mold, and cracks (which occur with a quick thaw). Combine that with all of the normal humidity we create within our homes and you just might have a recipe for disaster.

Log Cabins face moisture problems from both inside and outside sources.

I can explain what’s going on with the inside best by comparing it to the human body.

On a Freezing Cold Colorado Morning, put a shower cap on your head, then go for a run until you sweat real good. Look in the mirror and notice the steam stuck in the cap. Now, the important part, GO OUTSIDE and freeze that steam to ice and repeat the process. Some water will leak out of the cap down your face; but the majority of the water from your sweaty run will stay in the shower cap on your head.

Inside the Cabin, the non-vented cavity has trapped moisture that enters from with-in the cabin. The run you did that caused the sweat is a shower or cooking dinner to the cabin. The steam rises up (because heat rises) through the tongue and groove pine ceiling right through the Tyvek (yes the builder used Tyvek as a VB) through fiber glass batts to the underside of the roof deck. It collects there and forms drops that freeze and thaw and fall down off the bottom of the roof decking through the batts on the Tyvek. The Tyvek is routing the water inside through can lights and as you can see the outside is drying out where it can. 

Air Tight Construction Colorado

Log cabins also have outside moisture challenges (which can seep inside) for a few possible reasons:

If your cabin’s concrete base has more than a 12% moisture content at the time of construction, it’s possible for that moisture to evaporate into the cabin, causing moisture build-up on your floor boards. It’s also possible that if your cabin’s concrete base is larger than the cabin, when it rains outside, that moisture falls onto the concrete, which allows it to pool near your home and when the weather gets warmer and drier, that moisture evaporates up through your floorboards.

If that moisture pools near your home, there are a few different problems that creates:

  1. Sitting water will attack the floor bearers
  2. Water can evaporate and rise causing extra moisture to enter your log cabin
  3. If there is a plastic sheet or linoleum flooring moisture can accumulate in the air-tight gap, attacking your floorboards.

Whether you’re facing moisture problems caused by indoor or outdoor issues, there IS hope!

You can read more about this process and how we solve it HERE, but we’ll also talk about the way to correct this problem in next week’s blog.