Why We Sometimes Don’t Recommend Batt Insulation In Exterior Walls

Fiberglass batts are one of the most popular insulations. Contractors use them throughout home builds. Homeowners opt for fiberglass for many DIY projects. Historically, fiberglass batts were used everywhere and for everything.

South Central Services does not install fiberglass batt insulation in exterior walls when performance is a priority. We use fiberglass batts for certain areas of the home, but we often recommend different insulation for exterior walls . By the end of this article, you will understand what performance you can expect from fiberglass batts in your exterior walls.

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Fiberglass Batt Insulation Offers Mediocre Performance.

One of the best things about fiberglass is what an affordable investment it is. When estimating the insulation cost upfront, fiberglass cannot be beat.

Unfortunately, fiberglass batts deliver the same level of performance that you pay for. As a budget insulation, they are mediocre. There are three reasons for the budget performance of fiberglass batts.

1. Varying batt sizes
2. Lack of air barrier
3. Moisture absorption

Let’s examine each of these in more detail.

1. Fiberglass Batt Sizes Vary.

Walking through the insulation aisle at a Lowe’s or Home Depot, you’ll see many types of fiberglass batts for sale. Not only do these batts vary by R-value, but they also vary by size.

Purchasing a batt that meets necessary R-values according to building codes is important. However, it is equally important, if not more so, to purchase a batt that fits snugly into a cavity.

If your exterior walls need an R-value of R-21, you’ll purchase a batt at that value or higher. However, if your wall cavities are 18 inches wide, you may not find a batt that can fill that cavity without a minimum of a 2 inch gap.

Matching fiberglass batts by R-value and size can be quite tricky. If you have the wrong R-value or size, the insulation performance will be compromised.

2. Fiberglass Batts Cannot Stop Air Infiltration.

Let’s assume you can match the R-value and size of your exterior wall cavities. Even in these ideal conditions, fiberglass batts still offer mediocre performance.

Fiberglass, as a material, allows air to pass through it. The insulating capability of fiberglass is only measured by conductive heat transfer. In reality, heat travels through your home via the air. These forms of heat transfer are known as convection and radiation.

Fiberglass is defenseless against radiant and convective heat. The material allows air to pass through. This allows air to exchange between your home and the outside. In these air exchanges, your expensive conditioned air is replaced with air from outside.

In the winter, heat leeches through fiberglass insulation barriers to meet the cold outside. In the summer, heat presses through fiberglass insulation to come inside your air-conditioned home.

3. Fiberglass Batts Can Absorb Moisture.

Fiberglass batts have no vapor barrier. At best, faced fiberglass insulation can offer some vapor retardation. The facing on the insulation helps to repel some humidity, but the facing is only on one side of the batt.

When fiberglass is placed in an environment with excessive moisture, it absorbs that moisture. The vapor in the air condenses into liquid water, weighing down the insulation. If fiberglass batt stays in place, that moisture can contribute to mildew, mold, or wood rot.

This fact alone indicates that fiberglass batts do not belong in areas of the home that face excessive moisture. Basements and crawl spaces often receive fiberglass insulation despite the vapor transfer in these areas.

Exterior Walls Often Need Premium Insulating Performance.

Your exterior walls are not located below the ground. For all you know, there is a type of batt that will meet your necessary R-value and dimensions. So, why should you reconsider installing fiberglass in your exterior walls?

There are three reasons why exterior walls usually need high-performance insulation.

1. Thermal envelope
2. Air leakage
3. Wintertime vapor transfer

Let’s examine each of these reasons in more detail.

1. Exterior Walls Are Part Of The Thermal Envelope.

Your home’s thermal envelope is the barrier between your interior environment and the world outside. The thermal envelope is generally made up of your attic, exterior walls, basement, and crawl space.

All of these areas have the outside world on one side and the home interior on the other. You can think of your thermal envelope as your first line of defense. If the thermal envelope is poorly insulated, regulating your home environment becomes a catch-up game.

Because fiberglass batts only defend against one of three types of heat transfer, their performance may not enough for this important area of the home. If you want to keep your energy bills low, fiberglass performance may not be enough.

2. Exterior Walls Face Significant Air Leakage.

Because your exterior walls are part of your thermal envelope, their air leakage matters. Air leaks in your thermal envelope mean higher energy bills, disrupted living environment, and poor indoor air quality.

Let’s think of a home in the summertime. It’s hot outside. In southern PA, it could also be very humid. To stay comfortable indoors, you keep your windows shut. You set the thermostat to a reasonable temperature.

If your exterior walls have fiberglass, air will still exchange between inside and outside.

The cool air that your AC produces swaps with the sticky air outside. When your HVAC system realizes the temperature is wrong, it kicks on and chases the temperature you want.

The air conditioning is expensive. You pay more because your AC keeps running. The energy is wasted because it keeps exchanging straight through your walls. And all day, you cycle through being overheated and sweating.

To prevent this air leakage, we usually recommend an airtight seal in your exterior walls and other areas of the thermal envelope. Exterior walls with an air-seal offer more comfort and lower energy costs for a home.

3. Exterior Walls May Need A Vapor Barrier.

Air transfer through your exterior walls is a universal concern for energy efficiency. However, vapor transfer is also a worry for certain climate zones.

In our climate zone, Climate Zone 5, temperatures are more extreme in the summer and winter. Some winter days dip below freezing.

The reason that temperature differences matter so much is because of how vapor and air interact. Warm air holds significantly more water vapor than cold air can. When cold air reaches the maximum amount of vapor it can hold, it condenses into liquid water. This is known as dew point.

Even a vapor retarder facing on a fiberglass batt is no match for vapor transfer in the winter. If the warm inside air passes through cold fiberglass insulation, the vapor content could condense inside the walls. Since fiberglass absorbs moisture, that liquid water would be held inside the walls.

When vapor transfer is a serious concern due to weather patterns, we recommend closed cell spray foam insulation for exterior walls. Closed cell has a built-in vapor barrier at a depth of 2 inches of insulation. For new construction homes, there are some other options for vapor barriers in exterior walls.

Discuss vapor barriers and vapor transfer with your insulation contractor. If you live in a climate with extreme temperatures, fiberglass in your exterior walls could present some unique concerns.

Fiberglass Batts Are Best For Interior Walls.

Although fiberglass is not a high-performing choice for exterior walls, fiberglass batts can still be used in homes for other projects. When energy performance is less of a concern, fiberglass is often an affordable and effective insulator.

At South Central Services, we commonly use fiberglass batts for interior walls. Let’s look at the three reasons why fiberglass can be used successfully in interior walls.

1. Interior Walls Aren’t Part Of The Thermal Envelope.

Your interior walls are the division between two areas of the home. There are no significant environmental differences from one side of the wall to the other.

In fact, the biggest concern for interior walls is usually preventing sound transfer. While fiberglass cannot stop air or vapor transfer, it does absorb sound waves well. Fiberglass can be used for sound attenuation purposes in interior walls.

2. Interior Walls Can Have Air Exchange.

Air can exchange between your hallway and your dining room. There should not be any significant differences in temperature or humidity within the rooms of your home.

3. Interior Walls Are Less Concerned With Vapor Transfer.

The only interior walls where we may caution against using fiberglass are kitchen and bathroom walls. Kitchens and bathrooms face excessive moisture compared to other rooms in the home. If the vent systems in these areas are not set up appropriately, the fiberglass within the walls could absorb and hold moisture.

Vapor transfer is not a concern for other walls like your bedrooms, living room, and dining area.

The Bottom Line About Why Fiberglass Batts May Not Be Best For Your Exterior Walls

Fiberglass batts are a budget insulation that delivers budget performance. For interior walls, fiberglass is an optimal choice. A low investment cost can provide some sound attenuation from room to room.

However, your exterior walls may need more performance than fiberglass batts can deliver. Exterior walls are part of the thermal envelope of a home. The thermal envelope deals with air infiltration and vapor transfer. When it comes to the barrier between your comfortable living environment and the world outside, you may want to invest in insulation that can air-seal.

Now that you understand why fiberglass batts aren’t best for exterior walls when performance is your main priority, your next step is to:

• Discover the best way to insulate a new construction home
• Learn whether spray foam can be used for your existing exterior walls
• Compare the costs of open cell and closed cell spray foam