As with most everything involving mobile homes, there is confusion surrounding the mobile home belly board and the crawlspace under the home. If you insulate under your mobile home or just repair your mobile home underbelly you can save a lot of money.
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 board, belly wrap, underbelly, bottom wrap, bottom board, belly, and belly barrier. Regardless of the word you use, it needs to be healthy.
In this article, we’ll go over the best methods of repair you mobile home underbelly, the best materials to insulate under your mobile home, and finally, we’ll share first-hand examples of how mobile homeowners insulated and repaired the underside of their homes using foam board.
The NREL (National Renewable Energy Laboratory) states that insulating the underbelly and the HVAC ductwork in a mobile home are the two best projects a homeowner can do to increase energy efficiency and comfort.
How to Repair a Mobile Home Underbelly
If you’ve ever read the government published pamphlets on energy efficiency projects you’ll quickly notice that they aren’t particularly worried about cost. Every recommendation typically ends with ‘hire a professional.’
Out here in the real world, we have budgets and can’t just call up a pro every time we need to repair our mobile home. For us, it’s usually either DIY or not at all!
Does your mobile home belly look like this?
If a mobile home underbelly looks like this you should never buy it. If this is how your mobile home currently looks it isn’t good.
Four things are happening when a mobile home belly looks like this:
- there is no insulation in the one area that loses more heating and cooling than any other (the homeowner is literally heating and cooling the air outside of their home)
- critters can get direct access to the home
- plumbing lines are more susceptible to freezing
- the floor joists and subflooring has no protection and is likely being damaged by moisture which can cause damage to the rest of the home.
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 of insulating a mobile home per energy efficiency experts.
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 but there are many state and local government programs that may make this a possibility.
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. Click here to read our article on 18 ways to finance a
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.
Roxul Rock Wool
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 is a rigid mineral wool insulation sheathing board that is non-combustible, water-repellent, fire-resistant and sound absorbent
How to Insulate Under a Mobile Home with Foam
Laurel and Bryan figured out a great way to insulate under a mobile home and solve their problem energy loss at an affordable price. They were kind enough to share the details of their project with us, complete with hand-drawn images. How cool is that?
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
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)
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)
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
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.
While foam is a favorite material to insulate under a mobile home there are several other types of insulation that will work. Here are two more popular options for insulation:
Laurel and Bryan’s ‘Death Zone’ of Cold Air
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.
The first year in a new home, especially an older mobile home, is always a learning experience. When Laurel and Bryan Adams moved into their 1974 Olympic single wide mobile home they were excited to start remodeling.
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.
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.
- 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
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.
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:
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.
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.
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.’
Also, blowing insulation is the favorite method of insulating a mobile home underbelly but it’s also expensive because you have to hire it out. Using foam, batts, or Rockwool panels gives the homeowner the opportunity to DIY which is always a lot cheaper.
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
How to Repair a Mobile Home Underbelly
Repairing or replacing the mobile home belly wrap is a tough job, not gonna lie to you about it.
In most cases of older mobile homes, the plastic sheeting known as the belly wrap or underbelly has been damaged or even removed completely. In these cases, adding foam will help tremendously – it can act as an air barrier, vapor barrier (humidity guard), and insulation.
Before you begin you will want to lay down the ground moisture/vapor barrier. This helps protect you from getting nasty while under the home.
Steps to Repair Mobile Home Belly
First, everything has to be removed. The old insulation and the black belly wrap will need to be carefully cut out and taken away so that you are left with just the floor joists, HVAC ducts, plumbing pipes, and the subfloor.
Step 1: Remove the torn insulation and black belly wrap
First, you will need to remove and discard the old insulation and belly wrap. Once everything is removed you can start repairing and replacing each element one by one.
Step 2: Reseal or Replace the Ductwork
Next, you will reseal or replace the duct work. Make sure all the corners and seams have been taped together or replaced.
Step 3: Inspect Plumbing
Next, you’ll want to make sure your plumbing is healthy. This is a great time to run new water lines. You can always cap off the old lines and run the new right beside the ductwork. If you live in a cold climate it’s a good idea to run your water supply lines beside the duct work so the heat can help them from freezing.
Step 4: Install Insulation
Next, you’ll want to install insulation. There is are a few different methods and materials available. Most people like to use batts or rolls of insulation but foam board insulation is a good choice too.
Step 5: Install the New Belly Wrap or Belly Board
Once the insulation is installed you will replace the belly wrap, or the black plastic-like material that encloses the entire home.
We’ll go over each step in-depth.
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:
Damage to your underbelly is most commonly caused by animals but repairmen are notorious for leaving the cut open. When hiring professionals to make repairs under your home (and above the underbelly) make sure the total cost includes repair to the plastic.
If your underbelly is present but has holes you should repair it after installing the foam (if possible). Here’s a great video showing how to repair holes in your plastic underbelly:
The University of Wyoming released a government-funded booklet titled Mobile Home Energy. Following is their steps to repair a mobile home’s underbelly:
To repair large holes in the belly wrap (generally in the central part of the belly between the two main steel beams) use the following procedure:
First, measure the space needed to be covered. Cut a piece of Tyvek that is at least 1 foot larger than the hole on all sides. If the hole is really large, wrap one edge of the Tyvek around a long 1×2 inch furring strip.
Screw the 1×2 to the bottom of the floor joists next to one of the steel I-beams. Drape the fabric under the main duct trunk line so that space for insulation remains. Wrap the opposite edge of the replacement fabric to another 1×2 furring strip, then screw it to the joists next to the other metal frame.
Glue and stitch the other two sides of the replacement fabric to the existing belly.
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.
Say No to Typek
One well-respected blog recommends using Tyvek on as
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!
Thank you so much for reading Mobile Home Living!