Why You Should Avoid Batt Insulation

One of the most common things I get asked after presenting an estimate to a potential client is “Why do you have to remove the insulation in the crawlspace?” It’s typically followed up with, “Don’t we need that?”

I then go into detail on how we have to remove the batt insulation to treat the mold that could be growing on the sub-floor behind it. I also explain that builders are moving away from sub-floor insulation in crawlspaces these days for a number of reasons.

Why does the sub-floor insulation need to be removed?

In order to properly remediate a crawlspace or unfinished basement, we need to be able to see and treat every square inch of wood. This includes the joists, as well as the sub-floor area. Sub-floor insulation usually covers most of this space. (I say “usually” because I’ve been in many crawlspaces where over half of the sub-floor insulation had fallen to the ground, but we will get to that in a minute.) The sub-floor insulation, when present, must be removed so we can address any mold growing behind it. 

Aside from removing the insulation for purposes of mold remediation, removing that batt insulation also removes potential homes for pests, excess moisture, and mold spores held by the insulation itself!

contaminated batt insulation

Contaminated Insulation must be pulled to assess the damage that it could be covering.

“Do you put the insulation back?” 

This one gives us a little chuckle. No, the insulation cannot be reinstalled. More often than not, the insulation has already fallen in many spots, and has degraded on the ground. Removing the insulation  also prevents the contaminated insulation from shedding viable mold spores back onto wood surfaces in the crawlspace. 

You may also wonder if the insulation can just be replaced. This is costly, even though batt insulation is the cheapest insulation option, and almost never done to a standard that actually creates a thermal barrier. 

Why don’t you need the insulation?

There are two main reasons that, here in Middle Tennessee, you do not need to insulate your sub-floor. 

As we mentioned above, we often see insulation falling down when in crawlspaces (and even unfinished basements). Gravity is relentless, and over time pieces of insulation are going to sag or fall completely. 

There’s also common mistakes made when installing the insulation. Cutting the insulation strips too wide causes them to compress, diminishing any hope of a thermal barrier. There can also be gaps between strips, and voids spaces between insulation and the sub-floor or joists. Sometimes the kraft side (paper side) is installed facing the wrong direction, or the insulation isn’t cut around wires, pipes, or HVAC vent boots. Not only that, but the wire rods, also called Tiger Teeth, cause compression when installed to support to insulation.

I’ve even seen fiberglass batt insulation installed with two layers of it! It’s still unclear if the homeowner had another company install new insulation and not remove the old, or if the original installer thought two layers would provide a double thermal barrier. Which…yeah, it doesn’t really work like that.

But what if it’s installed perfectly?

Unfortunately, I’ve never been in a crawlspace or unfinished basement where one of these issues wasn’t present. It’s nearly impossible to achieve a perfect installation of batt insulation. 

Even this batt insulation which was recently installed is already sagging in parts and compressed in others.

Aside from the common mistakes made when installing, the climate in Tennessee makes it so that sub-floor insulation isn’t necessary. Tennessee’s climate zone is Cfa, categorized in the Köppen-Geiger Climate Subdivisions. The Cfa climate zone is a humid, subtropical zone. This basically means that we have hot, humid summers with mild winters and year round precipitation. It doesn’t get so extremely cold in this climate zone that sub-floor insulation makes a noticeable difference.  

Why is it installed in the first place?

Fiberglass batt insulation is installed in sub-floors for a few different reasons. Above all, it’s the least expensive insulation option. Also, it is easy to procure, being available at most hardware stores.  It also allows a builder to say “Look, your home is insulated!” Though it likely offers little thermal barrier and will fail over time, leaving a homeowner with a nasty mess in their basement or crawlspace. 

For the DIY types out there, it is also a tempting project for those looking to improve their home’s value and/or HVAC efficiency. The thinking here is that if the home is well insulated, the HVAC will run less, causing energy bills to be a little lower each month. However, even those who have been well trained struggle to properly install the batts. 

What if I still want my sub-floor insulated?

Those of you out there that still aren’t convinced that you do not need to insulate your sub-floors (this applies to our humid, subtropical climate zone that Tennessee is located in, not colder climate zones), there are other batt insulation alternatives. 

Option 1

The first option is to opt for spray foam insulation. There are two types of spray foam: closed-cell and open-cell spray foam. Both options are great for getting into all the little nooks and crannies that are easily missed with batt insulation. The main difference between the two types of spray foam insulation is that closed-cell is much more dense than open-cell, and closed-cell has a much higher R-Value because of this. Closed-cell is generally regarded as the better option for crawlspace applications.

However, even spray foam insulation can come with its own possible issues. Installers can miss spots where it should have been applied or apply too thin of a layer. In very rare cases you can get a bad batch of chemicals that don’t set properly and end up contracting instead of expanding. 

Option 2

If you don’t want to opt for spray foam insulation, the other option is to insulate with a rigid foam board. Like with batt insulation, rigid foam board has to be cut to fit between floor joists and mechanically attached to the sub-floor or joists so that it can’t fall victim to gravity over time. Rigid foam board can produce an adequate thermal barrier, however the installation process is very labor intensive. Foam board can also get pretty expensive when purchasing enough to cover the square footage of the sub-floor. 

What else can be done?

Even though batt insulation is capable of providing a thermal barrier, it rarely ever does. Installation mistakes and gravity seem to be the biggest culprits in preventing it from being an effective insulation option. Since it’s the least expensive option many DIYers and builders still turn to it. Homeowners in Middle Tennessee’s climate zone would be better off turning to spray foam insulation, or even no sub-floor insulation at all. 

When a client calls us for a mold remediation in the crawlspace, we will require that the insulation is removed before we can address removing the mold. In place of insulation, our recommendation is typically two steps once the mold is remediated.

  1. Encapsulate the crawlspace
  2. Install a dehumidifier

The idea with this process is to treat the crawlspace as a separate building envelope. Seal it off from the ground (encapsulate), which prevents ground moisture from creating a humid environment. Then control the humidity with a dehumidifier. We have had good success with this process, and have never had a client call us back to say their home’s comfort level changed for the worse.