Top 3 Methods to Insulate Under a Mobile Home

If you insulate under a mobile home you can save money. Over the last 3 decades, the National Renewable Energy Laboratory (NREL) has partnered with the US Department of Energy to test various methods of energy efficiency for old and new manufactured homes. Adding better insulation to the walls reduced heat loss by 13%. Coming in a close second was adding better insulation to the underbelly, under the sub-floor but above the belly wrap. It reduced heat loss by 11%. Installing storm windows also decreased heat-loss by 11%.

As with most everything involving mobile homes, there is confusion surrounding the mobile home underbelly or crawlspace under the home. One reason for the confusion is the numerous names used to describe the same thing. Mobile home belly board is also called a mobile home belly wrap, underbelly, bottom wrap, bottom board, belly, and belly barrier. Regardless of the word you use, it needs to be well-insulated.

In this article, we cover the best materials and method to insulate under a mobile home. We also share how one couple insulated and repaired their own mobile home underbelly using foam board.

Blowing Insulation into the Belly of your Mobile Home

As we shared in our complete mobile home insulation guide, blowing insulation into the belly and the ceiling of a mobile home is the most recommended method to insulate under a mobile home.





However, this method of insulating a mobile home underbelly requires specialized equipment and advanced knowledge of a mobile home’s construction. Therefore, blowing insulation can be expensive. Fortunately, there are many state and local government programs that may help mobile home owners to add insulation via energy grants and low-interest loans.

Depending on your location you may qualify for a program or financing help to get your mobile home insulated better, especially if it was built before 1976.

Blowing insulation through the rim joist of a mobile home lengthwise and crosswise 2

Top Materials to Insulate Under a Mobile Home

While blown insulation is the favorite method of insulating a mobile home underbelly it is also expensive because you have to hire it out. Using foam, fiberglass, or Rockwool panels gives the homeowner the opportunity to DIY which is usually always a cheaper option.

Spray Foam

Spray foam is most recommended for already enclosed areas such as the walls or ceiling/roofing of a mobile home. It’s fairly expensive because you have to have special equipment.

Spray insulation can be made of a variety of materials. Most common is cellulose (newspaper), foam, and fiberglass. Learn more about spray foam here and here.

Advantages of Foam Insulation 

Foam insulation has many advantages. Here are just a few:

  • easy to cut with a utility knife, hand saw, or table saw
  • lightweight and easy to carry
  • flexible enough to easily maneuver into tight spaces – DIY friendly
  • doesn’t promote mold growth
  • water resistant
  • long-lasting – won’t decay
  • insecticide is used on most foam making

Since foam board can also act as both an insulator and a vapor barrier for your mobile home it’s ideal to install under your flooring.





Disadvantages of Foam 

Fire is probably the biggest disadvantage of foam board insulation and meeting your local fire protection codes may be bothersome. Foam is fairly hard to ignite, but once it catches it emits a dense smoke and toxic gases. However, some manufacturers spray the foam with fire retardant. You’ll want to research your local codes.

Three Common Types of Rigid Foam Insulation

Types of foam board insulation

Image Source

The three most common types of foam board are made of polystyrene, polyisocyanurate, and polyurethane. All are petroleum-based products.

Rigid foam can be used in walls, roofs, and foundations, for retrofits or new construction. Most varieties of foam have a higher R-value per inch than fiberglass, cotton, or cellulose. (Green Building Advisor)

Here are the three main types of rigid foam insulation:





Molded Expanded polystyrene (EPS or MEPS)

Types of foam insulation expanded polystyrene foam 1

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Expanded polystyrene is the most common type of foam insulation and the most affordable.

It is made from the same closed cell foam beads that your coffee cups and shipping peanuts are made from. It is molded, or extruded, into large sheets that can be used in roofing, flooring, siding, and other construction needs.

The R-value of EPS sheets depends on the density, with a single sheet of foam ranging from 3.6 to 4.2 per inch.

Extruded polystyrene (XPS)

Types of foam insulation - extruded polystyrene foam

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Extruded polystyrene is very similar to expanded polystyrene but it uses pellets instead of beads, making it smoother and denser. It also has better water resistance. You’ll recognize this product by its colors – it comes in blue, pink, and green depending on the manufacturer.





You’ll pay more for extruded polystyrene but you’ll get better energy efficiency. The R-value of extruded polystyrene is around 5 per inch.

Extruded polystyrene is ideal for basements and slabs and makes an ideal choice for insulating under a mobile home if the added cost isn’t an issue.

Polyisocyanurate (ISO) and Polyurethane

Types of foam insulation - polisocyanurate and polyurethane foam




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Polyurethane and Polyisocyanurate have advantages and disadvantages.

They have higher R-Value per square inch but they are also the most expensive of the three types of rigid foam.

Foil-faced polyisocyanurate is more resistant to ignition than unprotected XPS or EPS. For this reason, some (but not all) building inspectors allow foil-faced polyisocyanurate to be left exposed on crawlspace walls or in attics without requiring a layer of drywall as a thermal barrier.(Source)

An advantage of polyurethane is the foil that covers the sides. The foil can act as a vapor barrier and is used most often in roofing and foundations.

Laurel and Bryan’s ‘Death Zone’ of Cold Air

The first year in a new home, especially an older mobile home, is always a learning experience. That was certainly true for Laurel and Bryan Adams when they moved into a 1974 Olympic single wide mobile home.

The couple’s new vintage mobile home had served Laurel’s aunt very well until she passed last year at the age of 93. While the mobile home’s decor may not have made it to the cover of Home and Garden, auntie had lovingly updated it over the years. She had heroically painted the walls herself with a three-inch paintbrush in bright yellow and aqua.

During their first winter in the home, they learned how much their utilities cost and where all the cold spots were located in the home. Needless to say, their 1972 mobile home had a few cold spots and their heating costs were too high for such a small home. The following Spring they began researching how to best insulate their home.

Laurel and Bryan decided to solve their problem energy loss using foam board. They were kind enough to share the details of their project with us, complete with hand-drawn images. How cool is that?





It was a sound, affordable home that had loads of potential!

The Adam’s vintage mobile home sits in Edmonton, Alberta, and since older mobile homes are notoriously under insulated, they experienced wild temperature fluctuations during their first winter.

How to Insulate Under a Mobile Home with Foam

Once Spring arrived, the couple set out to find a way to increase the home’s energy efficiency, especially in the ‘death zone’, as Laurel playfully called it. The death zone was an area  2 to 3 feet above the floor that remained much colder than the rest of the home.

Materials Used:

  • 4′ x 8′ Foam Insulation Board
  • Table saw and hand saw
  • 3″ screws with 1″ washers
  • Screwdriver (electric)

The Adam’s were able to buy their 4′ x 8′ insulation foam board from the manufacturers directly. That saved them almost half off the retail price. It pays to cut out the middleman whenever possible!

Insulating Under a Mobile Home with 2″ Foam Board

Insulating under a mobile home - cutaway of mobile home

Related: 8 Energy Efficiency Ideas for Your Manufactured Home

The mobile home’s floor joists are on 16″ centers, which means the wood floor joists are placed 16″ apart from each joist’s center point. Since the joists themselves are roughly 1.5″ wide the section between each joist was approximately 14.5″ wide.

Bryan bought 2″ thick Styrofoam insulation board and used a table saw to cut the boards 14.5″ so that the foam was easy to push between the joists and stay put (there are 4″ foam boards available but the 2″ is easier to handle).

Bryan concluded that layering 4 boards together to create an 8″ total thickness of insulating foam would work best for their area’s average winter temperatures.

Each stack was made of three 14.5″ wide by 2″ thick foam boards. The fourth foam board was left full-size (4′ x 8′) and attached to the bottom of the wood joists. This holds the 3 boards above it in place and creates a total of 8″ of insulation under the entire mobile home.

Insulating under a mobile home - hand drawn image

To secure the full-size sheet to the bottom of the home Bryan used 3″ screws with a 1″ wide fender washer. Since the home was built on 2 steel beams at an 8′ interval he used the lip of the steel beams to act as a shelf for the ends of the foam board. The wood joists are a bit shorter than the steel beams so this method worked well.

The drawing below shows it better than I can explain:





Adding insulation under a mobile home drawing

Laurel also drew a schematic of the screw placement. She explained that had Bryan not been able to screw into the bottom of the wood joists that he could have used a couple of different options to secure the foam. A thin wooden lathe could have been used as a bridge to hold the foam into place.

Insulating under a mobile home - bottom view - hand drawn image

The results of the couple’s DIY insulation project has been a great success! Laurel states that the ‘dead zone’ is long gone and since the foam has insulated the duct-work under the home the forced air heat is reaching the end rooms better.

Using Fiberglass to Insulate Under a Mobile Home

Fiberglass is the most common insulating material. It is made of very fine strands of glass formed in rolls and loose fill batts that are installed between beams, joists, and studs.

Since it is made of glass it is an excellent insulator that is affordable, non-flammable, and water resistant. The downside is that it is fairly dangerous to handle. You’ll definitely want to use protection when working with fiberglass.

Depending on density you’ll get R-value ratings ranging from R-2.9 to R-3.8 per inch.

This is a great video that follow the whole project of removing and installing insulation under your mobile home:

Using Roxul Rock Wool to Insulate Under a Mobile Home

One expert recommended Roxul Rockwool for a few reasons. The following benefits are from the Rockwool website:

  • Non-combustible stone wool insulation with a melting point of approximately 1177°C (2150°F)
  • Fire-resistant due to its high melting temperature
  • Water and moisture resistant; does not absorb moisture to maintain insulating value
  • Chemically inert
  • Does not rot or promote the growth of mildew, fungi or bacteria
  • CFC- and HCFC- free product and process
  • Made from natural and recycled materials
  • ROCKWOOL® can contribute to earning LEED® points
Rockwool installed under floor

Rockwool is a rigid mineral wool insulation sheathing board that is non-combustible, water-repellent, fire-resistant and sound absorbent

Different Opinions About Insulations

Some experts may frown on the 4.5″ left over from each 48″ wide board after getting 3 cuts. That seems kind of silly of a reason to not like a project as the foam can be used on a multitude of other things. You may even be able to get 2 or 3 different areas of your home insulated using the ‘waste.’

In the construction world you have the wrong way and the really wrong way. Everyone has an their opinion and everyone else is wrong. Ask 3 different construction exerts the same question and I guarantee you will get 3 different answers.

Related: Useful Mobile Home Repair Manuals 

Installing a Ground Moisture Barrier

It is recommended that all mobile and manufactured homes have a ground moisture barrier. A barrier is usually just a polyethylene plastic sheeting placed directly on the ground and secured under the home to act as another shield between earth and home. it should extend 6″ past your mobile home skirting.

The ground barrier combined with a vapor barrier (whether via a separate belly wrap or foam installation) will protect your mobile home from moisture. It can also deter animals from going under your home and help you locate leaks and water issues as the water will sit on top of the plastic instead of soaking into the ground.

Check your Local Codes Before All DIY Projects

Inspections are a necessary evil in the construction world, regardless of what kind of DIY project you do. Every location, whether in Canada or America, has building codes that must be met for all update and remodeling projects. The location of your home will impact the codes that you will need to meet. Homes in Canada face different issues than homes in Florida so be sure to research your local codes.

Summary for Insulating Under a Mobile Home

Repairing and insulating under a mobile home (or the mobile home belly) isn’t the most exciting remodeling project but it can make a huge difference in your energy bills and your comfort.

Unfortunately, most mobile homeowners have no idea how important the belly of their mobile home is to the overall condition of their home.

If your home is losing heat or seems harder to keep heated or cooled you may want to consider insulating under your flooring as the first step in updating your home.

Insulating your mobile home with foam is proven to be effective. The foam is easy to use and fairly affordable for its high R-value and energy conservation properties.

Laurel states that she doesn’t think insulating the walls would have made nearly as much of a difference as the underfloor insulation has and she would know best, it hit -4 Fahrenheit last week and their home stayed warm and the death zone is long gone.

A HUGE thank you to Laurel and Bryan Adams for sharing their project and teaching us how to insulate and repair a mobile home underbelly. and to Laurel for the awesome drawings!

Learn more about a mobile home belly board and underbelly here.

Thank you so much for reading Mobile Home Living! 

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