Q: I have 2-by-12 floor joists, and I plan to use R-38 batts to insulate my floor. Is it OK to go with un-faced batts, or do I need faced insulation with the Kraft paper face-up towards the floor? I do have 6-millimeter black plastic on the ground. –Rod G.
A: The purpose of the vapor barrier is to prevent moisture from inside the house from migrating through the floor and into the insulation, where it can reduce the R-value (the measure of resistance to heat flow) of the insulation and, in extreme cases, cause structural damage.
Today’s plywood subfloors utilize exterior-grade resins in their construction that will act as a vapor barrier in most normal vapor situations. So if you have a subfloor made from plywood, oriented strand board (OSB, also called waferboard), or other sheet materials, you should be fine with un-faced insulation. If your subfloor is individual boards of any kind, I would recommend that you use insulation with a vapor barrier face such as Kraft paper, and install it with the vapor barrier facing up toward the living space.
Q: Just after moving into our new home, the builder came around and told us we were going to be having freezing temperatures, and that we should close our crawlspace vents to prevent freezing the pipes, which we did. Several months later, we had a home inspector examine the house to be sure everything was OK. He said we should never close the vents because the crawlspace needs ventilation, and the pipes were all up in the floor joists and well protected from freezing. I’m confused–whose advice should we follow when the weather gets frigid? –Bill H.
A: I’m going to side with the builder’s opinion on this one. The vents are placed in the foundation to allow natural airflow under the house, which helps move any water vapor from ground moisture safely to the outside. Along with the plastic vapor barrier on the ground, this is typically enough to keep any moisture-related problems from occurring under the house.
In homes with insulated floors, where heat loss from the house is cut off, the crawlspace stays colder and there is an increased danger of freezing pipes. To help prevent this, plumbers will try and place pipes up between the floor joists where they will be covered by insulation. Where the pipes are not between the joists, the insulation contractors will wrap them with a couple of inches of fiberglass insulation for freeze protection.
So, in the normal course of things it’s OK to leave the vents open in all but the worst weather. However, during frigid temperatures it only takes one small section of pipe that didn’t get insulated properly for you to have a freeze problem, and having the vents open greatly increases that possibility, so I like to opt for a middle ground. Leave the vents open for most of the year for maximum ventilation, but close them during that part of the winter when freezing is an issue. In milder, damper climates, you may only have the vents covered for one or two months in the heart of winter, while in colder, drier climates they may be covered for four to six months. Assuming there are no water leaks or ground saturation problems stemming from poor grading, I have never seen a moisture problem in a crawlspace that has had the vents covered part of the year.
Q: Should the foil or paper on the insulation be toward or away from the floor? It seems the foil or paper would be easy to staple to the floor joists. –Gerry C.
A: This is actually a pretty common question, since those flanges that are built into the paper or foil face of the insulation batts look awfully tempting for attaching it to the floor joists. However, the facing is actually a vapor barrier, designed to prevent water vapor from the house from reaching the insulation. A vapor barrier always needs to be installed facing the warm surface–which is the inside of the house–so floor insulation always needs to be installed with the paper or foil face-up, against the underside of the subfloor. Whether you use faced or un-faced insulation, it needs to be supported from below with wood lath or rot-proof string attached to the underside of the joists, or with metal rods placed between the joists.
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