Conventional wisdom used to say floors of New Orleans homes shouldn’t be insulated. As people rebuild, conventional wisdom is changing.
This pier foundation supports a raised floor framed with pressure-treated lumber and insulation between the joists.
It’s an article of faith among many homebuilders and inspectors in these parts that you should never insulate the floor of a raised home in New Orleans. “You get moisture problems,” they say.
Tell it to my cold feet on the hardwood floor on a winter morn. Tell it to my glistening brow on a July afternoon, when the air conditioning is rapidly losing territory to the assault of summer heat. In those moments, a man wants his house to be hermetically sealed.
And tell it to the devotee of the Cult of Green as he strokes the soul patch that graces his smug chin. Surely he will “tsk-tsk” at the barbarity of your wasteful, breathable floors.
So how to insulate your floors without creating moisture problems? For local homeowners, it’s a question for the age –– an age when energy supplies may be on the verge of dwindling, carbon emissions cloud the skies, the developed world expands its footprint and the growing ranks of consumers gobble up resources like so many giant locusts descending on the earth.
Given the law of supply and demand, that makes it a pocketbook issue. And, with new building standards in Louisiana that require higher elevations on new construction and better insulation on homes being substantially rebuilt, it’s an issue even the willfully wasteful may have to wrestle.
The LSU AgCenter is on the case. With her colleagues, housing specialist and professor Claudette Reichel began studying the issue last year. “It is a huge problem and an issue for which there is not a consensus,” Reichel says.
The inspector grumbling about “moisture problems” has a point when it comes to traditional insulation on a raised house. “It can and often does result in moisture problems underneath the wood subfloor,” Reichel says.
Chief among those problems are nasty mold and rot. Moisture also produces “cupping” on the top surface of hardwood floors. That’s a sight that greeted a lot of flood victims when they returned to their homes: the edges of the floorboards coming up.
Don’t assume you’re in the clear if you have carpet, vinyl or tile, Reichel says. Those floor coverings likely rest on a bed of plywood, which is susceptible to mold and decay.
Air conditioning is a big part of the problem. Picture your house as a giant glass sitting in the summer sun. When you crank up the central AC, it’s like filling the glass with ice. What happens to the outside of the glass? It gets wet. Because warm air holds more moisture than cold air, the moisture in the outside air condenses on cold surfaces. If you don’t have insulation under your floors, this isn’t much of a problem because the subfloor is exposed and doesn’t get very cold from the AC. But if you have traditional insulation, close to the ground, especially with sagging, water vapor in the air can migrate to the cool subfloor, condense and get trapped.
Reichel and the AgCenter have been looking at alternative ways to insulate raised floors in South Louisiana homes without trapping moisture underneath them. Although their research remains under way, Reichel has some ideas based on what they think now.
There are four basic options for insulating beneath the floor, she says.
The most traditional and least expensive is to run fiberglass insulation between the joists. This method, however, does not stop airflow completely –– nor does it stop moisture. The closer your house hovers to the ground, the more moisture problems you’ll face. “If the house is high, you can get away with it,” Reichel says. “But if it gets wet, it’s toast.”
Another approach is to encapsulate the entire subfloor with a rigid foam board, attached at the bottom of the joists and sealed tight. The materials are more expensive than fiberglass insulation, and in a small crawlspace it’s difficult to execute properly. It depends heavily on the quality of the installation. It must be sealed tight as a drum and secured in such a fashion that the seal will hold up over time. In short, this is a risky approach.
The other two approaches deal with spray-foam insulation squirted up against the subfloor. This is more flexible than foam board and “more goof-proof,” Reichel says.
There are two kinds of spray foam: open cell (low density) and closed cell (high density).
Closed-cell spray foam gives the better performance at a lower risk of failure. It is nonabsorbent and will stand up to flooding. But it’s also an expensive product.
Open cell is less expensive but can let some water vapor through. Reichel says it may be a good compromise between cost and performance –– “promising but not yet proven.” To reduce moisture penetration, Reichel recommends coating the underside with a vapor-barrier paint.
But no matter what approach you take, she says, start with the basics. Deal with drainage issues underneath the house, making sure the ground is higher than around the house and covering it with plastic sheeting to stop the vapor drive from ground moisture. “That alone can prevent a lot of problems,” Reichel says.
And it will please your inspector.