Mobile homes (and some modular homes) have a different structural design than a normal house. Mobile homes are built in a factory setting and physically transported to a location for installation.

The underbelly of a mobile home is often built from steel beams and girders. The underbelly is also home to some of the plumbing and wiring, in addition to axles and wheels.

Insulating the underbelly of your mobile home will add comfort, improve sound deadening, and  energy dollars to your wallet by reducing the amount of energy you use.

One of the most cost effective methods to insulate the underbelly of your mobile home is to install expanding spray foam insulation. Spray foam does a great job, but you need to understand where to apply it and areas you should avoid.

Here we will discuss the simplest, most effective methods for installing spray foam on a mobile home underbelly and offer advice for doing it yourself.

How Do I Know Which Areas Of My Mobile Home Can Be Insulated With Spray Foam?

You are generally safe to install spray foam insulation to any part of the underbelly that doesn’t move or have replaceable parts. For example, the underbelly of a typical mobile home will consist of a steel frame, plumbing pipes and drains, and the axles that allow it to be transported.

Avoid Plumbing Drains

Mobile homes have restrictions that limit how close the frame can be to the road during transport. Plumbing drains, for example, could extend up to 24” below a shower in a normal home. In a mobile home however, adjustments must be made because the clearance to the road cannot be violated.

Plumbing drains and other components are often made especially for mobile homes to help reduce the vertical footprint. Plumbing vents, for example, are often located inside the home to avoid extending a pipe through the roof, increasing the height.

Avoid Plumbing Supply Lines

You’ll want to avoid spraying foam insulation onto components you might need to access later, like plumbing joints and junction boxes. Keep spray foam at least an inch away from these joints and stay two inches away if the joint has a door or access panel.

Avoid the Axles

Of course, no spray foam is strong enough to stop an axle from turning. However, most axles need periodic lubrication, especially just before a relocation. Spray foam can block the stems used to grease the axles, preventing the axle from being lubricated.

If you are insulating your mobile home for the first time with spray foam, mark the locations of any junction boxes, cleanouts, or filters. After the foam cures (cure time will vary, depending on the foam), use acrylic paint to mark these locations for easier identification later.

Pro Tip:

Be sure and use acrylic paint only to mark the joint locations. Paint from a spray can will instantly melt the foam and you’ll have to start over.

How Do I Know Which Spray Foam to Use For a Mobile Home?

Spray foam insulation is available in two versions. Open cell spray foam is used to seal gaps around windows, doors, and foundations. Most open cell foam expands to about three times its normal size as it dries, making it great for sealing both large tiny gaps and cracks.

Closed cell is primarily used for large installations, and is now available in either a can or as a kit. For the underbelly of a mobile home, closed cell spray foam is often the best choice because it is more dense than open cell, meaning you’ll need less of it.

For convenience, you can use both types of spray foam in different locations. If you are installing the spray foam over a weekend, it may be convenient to use open cell foam for larger gaps and hard to access spaces on Saturday and closed cell foam for larger areas on Sunday.

One common rule among spray foam users is to empty a can or tank before stopping. Some versions require periodic cleanups to prevent the material from hardening and clogging up the tip. If you work with one container at a time, you can’t accidentally ruin one while working with the other.

How Much Does It Cost to Insulate a Mobile Home With Spray Foam?

Do-it-yourself spray foam kits like this one usually cost about $2.00/sq.ft. Of course, the thickness of the foam will really dictate the R value and thus, how far your kit will go. 

Most mobile homes contain about 800-1200 square feet, so a typical installation will cost about $2000.00 dollars. The cost could be significantly lower or higher, depending on the R value you need.

For example, the kit mentioned earlier provides an R value of 6.8 per inch of foam. In this case, the kit includes enough foam to insulate 200 square feet to an R value of 6.8. If you need closer to R-13, you will need to install two inches of foam, netting about 100 sq. ft. of coverage.

Do I Need Skirting If I Install Spray Foam On the Underbelly Of My Mobile Home?

Local codes will vary, but generally you will need to install skirting after installing spray foam on the underbelly of your mobile home. Spray foam needs to be protected from animals and insects, and works best when the drafts under the mobile home can be controlled.

Mobile homes can be moved because they are not permanently attached to the ground. Once set however, a mobile home should have some form of facade that covers the axles. The skirting blocks much of the wind that would otherwise chill your floors and prevents vermin from using the spray foam for a nest.   

Are There Any Hidden Costs to Watch For?

You May Need to Rent Or Purchase Tools

There may be a couple of things you need to do before installing spray foam. Spray foam insulation will stick to nearly anything, but avoid grease and oil when possible. If your mobile home underbelly is dusty or oily, use an air compressor or pressure washer to clean it.

You May Find Damage 

Once you are under your mobile home, you may notice signs of a water leak or rot in the subfloor. You will need to make any repairs before installing the spray foam because the foam may seal in the moisture that caused the damage in the first place.

If possible, build a 10% financial cushion into your budget if you are a do-it-yourselfer, or 20% if you’ll need a contractor for small repairs.

Do I Need to Do Anything After Installing Spray Foam Insulation?

If the mobile home has effective skirting the spray foam will not require a covering, but it’s a great idea. Some mobile home owners attach foam panels or 6mm plastic over the spray foam to add an additional layer of protection.

Installing materials over the spray foam will help protect it from damage, but it could also trap moisture between the warm spray foam and the cold plastic. This could result in moisture build up and lead to mold and mildew. 

If you opt to add a plastic layer, make a few slits in it to allow expanding moisture to escape instead of condensing. If the mobile home is moved later, the plastic should be removed before transportation in case it detaches on the roadway.

What Tools Do I Need to Install Spray Foam Insulation Under a Mobile Home?

Aside from any carpentry that may be required, you will only need a few simple hand tools like a sharp blade and a putty knife to install spray foam. If you purchase a closed cell foam installation kit, you will also probably need a few wrenches.

Regardless of which spray foam you are using, appropriate safety gear is required. Wear eye and skin protection at all times. If working in an enclosed area, a respirator is also strongly recommended. 

How to Install Spray Foam Insulation On the Underbelly Of Your Mobile Home 

As mentioned earlier, you will need to make sure the underbelly is free of debris and fairly clean.

1. Prepare the Spray

You may need to do some prepwork under your mobile home like removing debris, locating gaps, and keeping your materials warm. You’ll want to keep both open cell and closed cell foam warm while you use them to ensure a smooth flow.

2. Map Out the Areas to Spray

Note the location of valves, access panels, and any other replaceable parts located on the underbelly. As mentioned earlier, be careful not to block or cover these as the spray foam will make them very difficult to access should an adjustment or repair become necessary.

3. Apply the Spray Foam

If you plan to insulate more than a few trouble spots, don’t hop around. Start on one end and move consistently to ensure each bead of foam will contact the prior one. This ensures that when the foam expands and cures, there will be no gaps in the foam.

Spray foam adheres to itself best when both foams are still wet. If you expect to need a certain thickness, apply all of it at the same time. Don’t wait for the foam to cure before applying another layer as this can weaken the bond.

4. Allow the Spray Foam to Cure

Most spray foam insulation will cure within 24 hours and tack up within an hour or two. Temperature and humidity can affect the curing process, so read the directions on your spray foam carefully. 

Work within the required temperature ranges for your foam. Spray foam usually requires a warm environment to cure, so don’t be tempted to install it outside the manufacturer’s requirements. 

Possible Alternatives to Spray Foam For a Mobile Home Underbelly

Foam boards are a good option for the underbelly of a mobile home because they can be cut for a perfect fit and stacked for additional R value. In most cases, foam boards are glued to the underside between the joists.

Depending on the design of the mobile home, the floor joists may run lengthwise or crosswise of the axles. Take very close measurements between each joist and transfer them to the panel. Inspect the foam boards prior to relocating the mobile home for damage and loose connections.

Foam panels are most easily cut with a razor knife and a straight edge guide. If the panel has a vapor barrier (as most do), place it towards the floor. Apply tile mastic or construction adhesive to the vapor barrier and press it firmly against the underbelly.


Evan has decades of experience as a project manager for large-scale commercial renovation home-building projects throughout the US. Currently, Evan runs a successful construction management company in Virginia.

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