Best Insulation For Crawl Spaces

If you have a crawl space, chances are it needs insulation. Most crawl spaces in southern Pennsylvania either have ineffective insulation or no insulation at all. But if you're going to insulate your crawl space, you want to do it right. What is the best insulation option for crawl spaces?

South Central Services has insulated hundreds of homes with spray foam. Many of those jobs have included encapsulating crawl spaces for better air quality and energy efficiency. If you want to get your insulation right the first time, we know from experience which insulations work well in crawl spaces.

By the end of this article, you will understand:

• Which insulation is bad for crawl spaces
• Which insulation is better for crawl spaces
• Which insulation is best for crawl spaces

Fiberglass Should Not Be Used In Crawl Spaces.

Fiberglass batt insulation is the most common insulation in crawl spaces in our service area. Most crawl spaces we are hired to insulate have batt insulation lying on the floor.

Why is the insulation lying on the floor? Fiberglass absorbs moisture. When installed in a crawl space, batt insulation sucks up the vapor it's surrounded by. Over time, the humidity makes the batt insulation heavy. Eventually, the insulation falls from the ceiling cavities it was stuffed into.

Even when batt insulation stays put in the ceiling cavity, it still offers underwhelming performance for homeowners.

Fiberglass batts do not prevent air passage between the crawl space and the living space. The air in your crawl space exchanges freely with the air in your living space. Musty, unclean air in exchange for conditioned, clean air is not a fair trade.

Another reason why fiberglass is bad for crawl spaces is the nesting nature of batt insulation. The average vented crawl space is often infested with some critters like spiders and mice. Batt insulation commonly has rodents nesting inside, making a permanent home for pests.

Fiberglass batt is not a high-performance insulation product. When placed in a musty and damp crawl space environment, it cannot deliver good results.

Fiberglass Batts Are Not Suitable For Below-Grade Use.

You may be wondering about faced batt insulation. If you install batt insulation with a vapor barrier facing, can you avoid these moisture problems?

While it is true that some batt insulation is sold with facing, these facings cannot solve the problems with fiberglass batts in the crawl space.

Fiberglass rarely fits snugly into a cavity, meaning there are gaps in the vapor barrier. Any gap in the vapor barrier renders the vapor barrier ineffective entirely. Moisture in the crawl space can bypass that facing and absorb into the batt itself.

Foam Board Is A Good Option For Crawl Spaces.

Foam board is a significant improvement compared to fiberglass products. Foam board doesn't absorb moisture and has the potential for a seamless application. Gaps created by batt insulation are challenging to solve, but foam board joints can be sealed with can foam or caulking.

While fiberglass insulation is commonly installed in crawl space ceilings, foam board is best used on crawl space walls. Many homeowners who DIY crawl space encapsulation opt for foam board as their crawl space insulation.

There are three shortcomings of foam board in crawl spaces. Let's examine these in more detail.

1. Foam Board Is Not Ideal For Walls With Variations.

Some crawl spaces have stone walls, while others have cinder blocks. The composition of your walls determines whether foam board is a reasonable option for your crawl space.

Foam board insulation works best on flat walls. When variations and texture are introduced, foam boards are more likely to have cracks and extra seams. Any cracks in your foam board mean vapor can leak.

Think of your vapor barrier like a swimming pool liner. The job of a pool liner is to hold water without any leaks. Similarly, a vapor barrier's job is to stop water vapor from passing through without any leaks.

A flat wall is ideal because the foam board can sit right against the substrate. Stone walls are not a good fit for foam board.

2. The Edges Of Foam Board Must Be Sealed.

Foam board insulation is not seamless on its own. For foam boards to be effective, the joints must be sealed with can foam or caulking.

Even on a flat wall, fastening the foam board side-by-side is not enough. A "crack" still exists in your vapor barrier until an air-sealing product is used.

3. Foam Board Cannot Wrap Floor Joists In Insulation.

When it comes to floor joists, foam board's rigidity is its most significant shortcoming. While rigidity is helpful for flat concrete walls, it is unhelpful for insulating floor joists.

When encapsulating a crawl space, it is ideal to wrap the floor joists in insulation. Otherwise, there are gaps in the insulation and the vapor barrier. Ironically, wrapping the floor joists is especially important for cinder block walls because of their open core.

Essentially, cold and musty air can bypass the foam board on the crawl space walls by exchanging between the floor joists.

Because of their rigidity, it is incredibly difficult to wrap floor joists with foam board. Any transition from masonry to wood framing is also tough to insulate with foam board.

Closed Cell Spray Foam Is The Best Option For Crawl Spaces.

While foam board is a significant step up from fiberglass batt insulation, the best option for crawl spaces is closed cell spray foam.

Like foam board, closed cell spray foam is most commonly used as wall insulation for a crawl space encapsulation. For crawl spaces facing standing water issues, closed cell can instead be installed in the ceiling.

Whether installed in the ceiling or on the walls, closed cell spray foam insulation is the best choice for crawl spaces.

Not only does closed cell spray foam outperform fiberglass batt insulation, but it also solves the shortcomings of foam board insulation. Let's examine these abilities in more detail.

1. Closed Cell Can Seal Any Contour Or Imperfection In A Wall.

While foam board insulation is best used on flat cinder block walls, closed cell spray foam can be installed on any crawl space wall.

Even bumpy masonry walls can be insulated with closed cell spray foam. Since closed cell is spray-applied and adheres to the substrate, the imperfections of the wall do not matter.

2. Spray Foam Creates A Seamless, Airtight Seal.

Foam board insulation needs additional can foam or caulking to give an airtight seal to its seams. Spray foam insulation does not have any seams that require seals.

Because spray foam is manufactured on-site and spray-applied, it can be sprayed on a wall of any size.

3. Closed Cell Wraps Floor Joists In Insulation.

Besides its versatility, closed cell spray foam also beats foam board in floor joist applications. While foam board is too rigid to insulate the transition from wall to floor properly, closed cell can easily wrap around floor joists.

Closed cell spray foam seamlessly envelops floor joists, preventing air leaks.

The Bottom Line About Insulating Your Crawl Space

Insulating your crawl space is an excellent step toward lower energy bills and better air quality in your home. If you want to insulate your crawl space properly, you should consider encapsulating with either foam board or closed cell spray foam.

No matter how you approach insulation in your crawl space, fiberglass batt insulation is not a good option. Your crawl space needs a seamless vapor barrier, which batt insulation cannot provide.

Now that you know why foam board and closed cell spray foam are best for crawl spaces, your next step is to:

• Learn what crawl space encapsulation is
• Compare vented and encapsulated crawl spaces
• Discover if DIY encapsulation is best for your crawl space