Sealing behind baseboards

Another question that I need a right answer on.

If you caulk behind baseboards to minimize air leakage does that hold moisture in at the bottom of the wall and start rotting the house? The water I would be talking about is from humidity in the house that hits the vapor barrier and runs down (in theory).

Your comments Please



I have worked in energy conservation and retrofit in the field, regulation and training since 1977 which included running an energy auditing, airsealing, and insulation company from 1980-90 (and still do a bit of retrofit now…10-15 houses/year). During and after that period, I was a site advisor/inspector/researcher for the R2000 program for 9-10 years. Presently, I’m just concluding a gov’t contract/ pilot project overseeing the retrofit of low income homes here.

I find that from stories heard “on the street”, people have concocted so many off base visions (in theory) in their heads about wet insulation, rotting houses* and calamitous failures. It’s not that these things don’t occur but they are rare and caused by people who don’t have the proper training and experience in building science and construction…something that will be hard to get on these boards. I find that some of the posts,especially in the IR/building science threads lack the in depth knowledge needed to truely understand some phenomena.

  • I’ve been consulting on a major failure (rot…but there are other issues also…so far on site: 2 engineers, a Phd mycologist, an energy auditor, home warranty rep and myself) in a 16 month old $800,000 house (the cheapest in the subdivision is $625,000… this is in the Maritimes where land is really cheap) locally as the builder/developer (with an MBA…see the untrained aspect I mentioned) was trying to do a better job but royally screwed up!!!

Back to your question: Your vision is very rare in a properly built home but can happen on site during winter construction due to wet lumber, tracked in snow/ice, use of unvented gas construction heaters (In 1989, I consulted on a project of 23 starter bungalows where the OSB sheathing was buckled due to gas heaters…the builder went bankrupt and the homes sold at an average of $23,000). If you have water running on the inner face of a VB during construction, (1) you don’t have a proper airtight exterior sheathing membrane and cold air is getting to the VB to cool it (if there are no insulation gaps), and/or (2) there is not enough ventilation being used to remove moisture/dry the interior air and/or (3) the house is not being heated during the cold nights, cooling is allowing the interior air to reach its dewpoint…the windows would have lakes of water at the sills in the morning.

If the moisture is at the outside face of the VB, that’s a problem with using wet lumber…when you seal the walls with the polyethylene AVB and then heat the house, the studs start to dry into the cavity and then with the cooling at night, condensation forms on the back of the plastic. If the thermal shell of the house is built with an outer surface that is 10 times more permeable than the inner surface, this moisture will diffuse outward over time and cause no longlasting problems although it scares the hell out of people when it occurs. I can’t count the # of times I have been called on this.

By airsealing the gap at the bottom plate/subfloor gap, depending on wind direction, stack effect and mechanical ventilation/heating air movement, you may stop warm air from exfiltrating at this point one day and the next day may be stopping cold air from infiltrating at the same location. If the cold air is cooling the back of the baseboard, condensation/ice may form, especially if the house is “wet/damp” (this may be more of a ventilation issue rather than a serious air leakage one but working on both is good). If damp air is exfiltrating, you may be adding water to a hidden cavity. Both of these situations are undesirable so sealing the gap is a positive move. With a warm VB and the house run at 35-45% rh in southern Canada during the winter, moisture should not form and your vision will not be fulfilled!!

By the way, the moisture that is constantly hitting the VB behind drywall in winter is virtually a molecule at a time as vapour diffusion is a very slow process. Now air leakage is a totally other thing…read as much as you can on this and then read more and more…it’s the biggest culprit causing condensation along with not enough ventilation of the house interior.


Thanks for the info. It is good to hear this from somebody with lots of experience in this field.


Air transported moisture has a much greater effect in moisture issues than vapor permeance.

Air leakage can also reduce your heating/cooling efficiency by as much as a third.

Stopping the air movement is more important than if you seal it inside vs. outside the plate. Which is easier (thus more effective) inside or out?

Seal outside to stop the weather,(snow, rain, dust …and insects etc) from getting in.

For air leakage control, if at all possible, seal at the inside layer. This is the last point of entry or first point of exit for air. If you seal at the sill outdoors, air can get to the inside baseboard by getting in through any of the 10’s of other outer cracks/openings and moving to the inside opening by passing through insulation, wall cavities, etc.