My previous blog was about a system for a cabin deck structure. Observant readers may have noted the absence of any additional vapor barrier. This was intentional – and deserves an explanation.
In my part of the country (New England) it used to be standard procedure to incorporate a plastic polyethylene vapor barrier directly behind all interior surfaces, be they walls, floors or ceilings. This was to prevent moisture moving from an insulated and therefore warmer space (interior) towards the outside colder space and encountering the dew point along the way in the middle of the wall/floor/ceiling and condensing inside that structure. When condensation occurs inside the building structure all kinds of problems can result, ranging from such things as peeling exterior paint to rot and mold inside and throughout the wall. With the present emphasis on energy efficiency in buildings, the problem of moisture control has become more complex, and the old standards, like always using a vapor barrier, are no longer taken for granted. Builders may be constrained by building codes which dictate required structural components, insulation, etc., all primarily intended for year-round residences but overkill and inappropriate in some ways for seasonal cabins. For example, the design criteria for energy efficiency presents as the ideal a very tight building with all interior air ‘conditioned’ year-round. If you can build to your own design it will pay to understand moisture management, as the best system for a cabin may actually be less tight and more open.
There is a wealth of published material on this topic, one source being Joseph Lstiburek of Building Science Inc. The Journal of Light Construction published a practical and thorough book called The JLC Guide to Moisture Control. Reading these can produce some confusion until certain basic elements are grasped. The problem is partially that terms have changed – the addition of the new concept of an ‘air barrier’ as distinct from a ‘vapor barrier’ being an example. The problem is also partly because moisture movement depends on temperature differences between spaces – so one set of procedures applies to Northern climates, whereas a a different structural procedure may apply in Southern climates, all based on the same principles. So – what are the principles?
The first is that moisture moves from warm spaces towards cooler spaces. If, along the way, moisture encounters the dew point inside a structure, it will condense. The second is that you must always provide a path for moisture to move out of a structure once it gets in. This means ‘no double-sided vapor barriers’. You have to decide which surface – interior or exterior – you want to be the drying surface. Thirdly, most moisture moves in the air, with very little by comparison by direct diffusion. This is where the concept of the air barrier is useful.
One can easily find examples of building systems used in the past that created their own fiasco. The use of Tyvek house wrap with red cedar clapboards applied directly on top (no air space) is one. The tannins in the cedar degraded the water resistance of the Tyvek, turning it into a sponge that absorbed moisture, especially on warm days in the sun following a rain – driving the moisture into the sheathing and eventually bringing rot to the entire wall. There remains value in my mind in tried-and-true methods that a cabin builder can use, and avoid some of the experimentation with new systems and products which may turn out to be their own problems after a few years. For example, some of the products now in use, like the ZIP system of plywood sheathing with a pre-applied waterproof surface, call for special attention to the ‘no-double-sided vapor barrier’ rule. I think, done wrong, these will lead to problems. Understanding the principles will help do them right.
Going back to my floor structure. The main moisture drive will be from inside the building to the outside. There are small air spaces between the foam insulation and the subfloor, but most air infiltration into that space will be outside air because the subfloor is glued tongue-and-groove acting as a pretty good air barrier. During construction the subfloor will get wet and some moisture will make it through the subfloor to the insulation. The main object then is to allow a way for the floor to dry out. It will dry to the inside. Any moisture that gets into the air space from outside will only condense if the floor structure is cooler than the outside air, which is not likely unless the cabin is heavily air conditioned, which is not in the plans for this building. The poly vapor barrier would only get in the way and was therefore omitted.