Insulating Under a Mobile Home with Foam Board (DIY Project)
Insulating under a mobile home isn’t the most exciting remodeling project but it does make a huge difference!
We’ll go over the steps one couple took to install foam insulation board under their home, learn about the different types of foam insulation, and cover underbelly repair. If you’re looking to insulate your home this is going to be a very helpful article for you.
Meet Laurel and Bryan Adams
When Laurel and Bryan Adams moved into their 1974 Olympic single wide mobile home they were excited to start remodeling.
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.
It was a sound, affordable home that had loads of potential!
The ‘Death Zone’ of Cold Air
The first year in a new home, especially an older mobile home, is always a learning experience.
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.
Insulating your Mobile Home on a Budget
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 have an issue in our home. For us, it’s usually either DIY or not at all!
How to Insulate a Mobile Home with Foam Board
Laurel and Bryan figured out a great way to insulate under their 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?
- 4′ x 8′ Foam Insulation Board
- Table saw and hand saw
- 3″ screws with 1″ washers
- Screwdriver (electric)
Details of the DIY Project – Insulating Under a Mobile Home
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 middle man whenever possible!
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.
It’s this 14.5″ section that received the insulated foam board. 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 an 1″ wide fender washer. Since the home was built on 2 steel beams at an 8′ interval he simply 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 use a couple different options to secure the foam. A thin wooden lathe could have been used as a bridge to hold the foam into place. He could have also went the more expensive route and enclosed the entire bottom of the home with 1/4″ plywood.
The results of the Adam’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.
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 manufactures 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 depend 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 it’s 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.
Other Types of Insulation
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:
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 down side 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.
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.
Insulating Under a Mobile Home
Here’s a few videos that may help you learn more about insulating your mobile home using the three main types of insulation, foam board, fiberglass, and blown insulation.
Closed-cell Foam Used as Shed Flooring Insulation
This video shows closed-cell foam board being used as flooring insulation for a shed. They speak about the vapor barrier aspects and the R-value of the material:
The second video shows how fiberglass insulation is added to a mobile home. The second is a informative resource on auditing a mobile home for energy use and various methods of adding insulation.
Blowing Insulation into the Belly of your Mobile Home
Adding Fiberglass Insulation Under an Older Mobile Home
Related: Useful Mobile Home Repair Manuals
Dealing with a Mobile Home’s Underbelly
While installing any type of insulation under a mobile home you will be likely be dealing with the underbelly.
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.
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.
The ground barrier combined with a vapor barrier (whether via a separate belly wrap or foam installation) will protect your 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.
More Mobile Home Foam Insulation Projects
If you Google foam insulation under mobile homes you’ll find a variety of photos and informative articles. Below are three that can help you understand more about using foam as an insulation under and on your mobile home.
MobileHomeRepair.com has a post in their forum that show the home before and after the foam installation:
The Walden Effect is a great blog that journals a couple’s experiences and lessons of living off-the-grid in an older mobile home. They have an informative article about insulating mobile home walls that you can read here:
Build a Cheap House (.com) has lots of information about insulating a mobile home. They added foam all around their older mobile home:
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
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 to Laurel for the awesome drawings!
Thank you so much for reading Mobile and Manufactured Home Living!
If you have a mobile home remodel or DIY project you’d like to share please contact me at firstname.lastname@example.org.