Raised Floors & Crawl Spaces for New Orleans Homes

Our hot humid climate provides many challenges for homeowners who want to insulate the underside of their raised home floors. Major concerns when insulating our floors are the durability and the expected lifetime of the insulation, the over-all thermal ability of it to actually insulate and, as always, moisture concerns. Insulating with fiberglass batting does very little for wind and moisture flow. In fact, it allows wind to flow right through it and may actually hold on to moisture in some cases, depending on how it was installed, resulting in a considerable decrease of R-value.

Without more expensive installation methods that protect the potential lifetime it is expected to last a few years at best… but the cats, rats, and other crawling creatures surely love it while its there and before they accidentally tear it down. Radiant barrier materials are an option but they do very little for insulating, the expected lifetime is short, and if not installed properly can create moisture issues or simply provide minimal gains in efficiency at best.

When insulating under the floor of our homes we want to accomplish three main objectives:

  1. Provide R-Value which is most important during cold season.
  2. Eliminate air-flow which is important all year-round.
  3. Stop or greatly retard moisture flow which is a major concern during our exceptionally humid spring and summer months.

Whether your house is raised by piers or it conforms more to a vented crawl space all three concerns above apply

Open Cell VS. Closed Cell Under The House

There are two general spray foam options available for insulating under our raised houses: open cell (which includes .5lb and 1.0 to 1.2lb hybrid) and closed-cell. To find out which option is better for you lets consider the facts on how vapor travels.

Water vapor is water in an invisible gaseous state. Water vapor travels through a process call vapor diffusion. Vapor naturally travels (1) from higher vapor pressure to lower vapor pressure and (2) from higher temperature to lower temperature. Consider the typical scenario of a house in a hot and humid climate such as Louisiana and the Gulf Coast. As we air-condition our homes we are (1) dehumidifying and (2) cooling. Therefore, through vapor pressure differences and temperature differences water vapor is in a constant state of migration into our homes from the outside. This natural phenomenon of vapor travel is actually more pronounced from underneath our homes do to the greatly increased relative humidity, especially during period of excessive rainfall.

Air pressure differences will also result in water vapor travel. Houses typically exist in a state of depressurization and are pulling moisture-laden air inward through leaks in its envelope. Leaky duct-work and stack affect (which is more pronounced with raised houses) are two major reasons for contributing to these pressure differences which make up a great deal of moisture coming into the home from the crawl space.

Why should we NOT install open cell foam under our raised homes?

Although open cell spray foam is a very effective insulation and a more economical alternative to closed-cell spray foam in many residential cases the fact is it is not to be installed in any exterior application and in areas with high vapor pressure, especially under our homes. Why should we NOT install open cell foam under our raised homes?

  1. Not manufacturer approved for exterior use. This is spelled out in all manufacturer specifications and data sheets.
  2. Because of its high permeable characteristics…it DOES allow moisture to pass.
    It will NOT dry out if it gets wet, especially under a house in shaded damp conditions.
  3. No structural strength and those cute crawly things can easily bury themselves in it.

First, here is what almost every other spray foam contractor is not going to tell you. Locally, it has only been since the inception of EnviroGreen that other foam contractors began to realize that they’ve been doing it wrong or that homeowners were beginning to realize that they were being misinformed. When you purchase insulation from a spray foam contractor you are not buying a product that “we” make. We purchase the material we spray from a manufacturer or from a distributing company that sells for the manufacturer. All the colorful names that we come up with to market ourselves can really confuse homeowners at times. All spray foam manufacturer data sheets and specification literature disclose the fact that open cell is not to be used for exterior application or make indirect reference to this fact by stating international building codes.

Why is open cell not an exterior-approved and rated insulation product?

This question leads us to explain numbers 2 and 3 above. Let’s consider building codes. We require a water plain and vapor barrier (vapor diffuser retarder to be exact which has a perm rating <1) over our exterior walls and roofing. Of course there should be vapor control under our homes, especially considering the high relative humidity under these structures versus the area around the house. But, as usual, building codes walk slowly to catch up with building science and laws of physics. If we were to put open cell under our homes we would first have no protection against it from getting wet. Is this a big deal? Yes, ask our local competing spray foam company sales people if you should put open cell in an exterior wall cavity of an old New Orleans wood-sided house that has no water barrier. Most will have at least enough knowledge of building science and potentially-occurring water problems to say that this is a “no no.” So, why under the floor? Just as open cell should not go in an exterior wall without a water plain and vapor barrier it must not go under the floor.

insulating-raised-floor1_thNow let’s consider the vapor problems, especially in our hot humid Climate Zone 2 and underneath our homes. Reading the above statements and the scientific literature that we provide we know that vapor is constantly moving into our raised homes from below. Relative humidity increases by 2.2% for every 1 degree in temperature drop. A typical scenario of our pier and beam homes is illustrated below courtesy of our Louisiana Department of Natural Resources 2009 Builders Guide.

Not only does the ground act as a sponge for water but it also remains at a relatively constant temperature equal to that of the average outside temperature. As we vent our open crawl spaces or simply leave our sub-floors exposed to these high relative humidity conditions we are increasing the rate of vapor travel through our beautiful polyurethane coated solid wood floors and into our homes. One problem with this is that without the vapor protection the central air-conditioning system is being forced to “work” harder attributing so many of its btu’s of cooling to simply trying to keep up with the additional latent heat load (moisture) from below. Building science already recognizes that even our most energy efficient homes located in the hot/humid south usually require additional de-humidification that our traditional single stage AC systems just can’t provide. The second problem is that if we use a permeable insulation, such as open cell spray foam, this vapor passes right through it on its way to trying to get into the house where simple laws of vapor travel state it shall go… its cooler inside and the relative humidity is much less than underneath the house. With a perm rating of approximately 16 per 3 inches of open cell foam those little gaseous water molecules have no problem moving right through the insulation and into our beautiful polyurethane-coated wood flooring.

“Into our polyurethane-coated wood flooring,” is where we can really mess up the house now. One, if we really want to create an energy efficient floor than stopping or retarding vapor movement into our homes is obviously just as important as is stopping it from coming in anywhere else. Secondly, and most importantly, spraying open cell under this type of flooring is putting the “health” of our home at a greater risk of wood rot, mold, and certainly buckling! The polyurethane coatings typically applied to our hardwood flooring become a vapor barrier. These water molecules, in a gaseous form, move right through the open cell and into the wood flooring. Having no where else to go this vapor may therefore accumulate and condense in our polyurethane-coated wood flooring (like condensation on a glass of iced tea), resulting in buckling.

And finally, open cell provides no structural integrity to the under-side of the home and does very little for keeping out the critters. Although the most vicious rats and other creatures do have the ability to eat through closed-cell the fact is that they can not create a nest in it as they easily can with open cell.

So what is the solution?

Closed cell foam at a 2″ inch average thickness creates a code approved vapor barrier (vapor diffuser retarder, Perm <1) which retards the vapor flow and potential condensing of moisture between the sub-floor and the insulation or in the actual wood flooring itself. So many companies quote 1 inch or “1.5 to 2 inches” to display lower prices on bids and/or because they can get more coverage area out of it. This means its right for the contractor (if a quick profit margin is the motive) but may not be right for the homeowner. Few closed-cell foams do create a vapor barrier at less than two inches, most do not. To be on the safe side it may be worth simply assuming that 2″ is what is required to create a perm rating of

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