Moisture issues in floor-on-pier foundation?
I’m building a cabin (12′ x 20′) on a pier foundation in climate zone 6 (Northern Vermont) and had some questions regarding potential moisture issues. From what I read it is advisable to add 2″ or more of rigid foam underneath floors filled with “fluffy” insulation. I was hoping to use batt insulation and rigid foam once it is in it’s final location. Unfortunately the building will live at least temporarily on a metal trailer until I find a permanent home for it. My concern is that I will not be able to add rigid foam underneath the entire floor due to the metal trailer framing and was wondering if that will create moisture issues in the floor assembly. The building will likely spend 1 or 2 winters on the trailer.
我应该避免绝缘地板上,毛茸茸的吗sulation given my situation? Alternatively would a flash and batt system work in the floor?
Thanks in advance for any help!
Cheers,
Nils
Replies
Nils,
It's possible to use 100% fluffy insulation to insulate a floor system, although there is likely to be more thermal bridging through the floor joists than an approach that uses a continuous layer of rigid foam. That said, a properly detailed floor assembly with 100% fluffy insulation won't have any moisture problems.
The exact details depend on the trailer configuration. Buildability issues also arise.
Ideally, this is what you'll have:
1. A flat trailer deck on which to assemble your house.
2. At the bottom of your floor assembly, you'll have a continuous layer of OSB or plywood with sealed seams, fastened to the underside of your floor joists. One way to accomplish this is to build your floor assembly upside down near the trailer, and then invite all of your friends over on a weekend to pick up the floor assembly, flip it over, and place it on your trailer.
3. The joist bays can be filled with a fluffy insulation like fiberglass, mineral wool, or cellulose.
4. Above your floor joists, you'll install OSB or plywood subflooring with ring-shank nails and construction adhesive.
Needless to say, you're going to want to keep this fluffy insulation dry during the construction process. It rains in Vermont, so the best way to accomplish this is to build your house in an old barn or a commercial garage.
Many Thanks Martin. The process you describe is similar to what I had in mind.
I'm still curious to why this layup would work in a floor while a similarly detailed wall would not with regard to moisture. Perhaps I'm incorrect, though wouldn't a wall filled with fluffy insulation, sheathed on both sides, and without a vapor barrier be prone to moisture buildup and damage in this climate?
Nils,
In fact, most walls without interior vapor retarders work fine, as long as they are built with attention to airtightness. It's rare for vapor diffusion to cause problems in walls, except in unusual applications like indoor swimming pools or greenhouses.
The number-one cause of moisture problems in walls is water entry from the exterior (rain). The number-two cause of moisture problems in walls is condensation of moisture piggybacking on exfiltrating interior air. (This problem can be nipped in the bud with attention to airtight construction methods.)
Vapor diffusion is way, way down on the list of potential problems.
One more point: Floor assemblies tend to have far fewer moisture problems than wall assemblies or ceiling assemblies, due to the stack effect. The stack effect tends to introduce warm, humid interior air into ceiling assemblies and the upper half of wall assemblies, and this humid air causes problems.
The stack effect tends to pull cold, dry outdoor air into floor assemblies, and this dry air is beneficial to the long-term durability of floor assemblies.
Thanks for allaying my fear of sodden insulation in the floor. Sounds like the trick is to pay extra close attention to air tightness.
Cheers,
Nils