Mobile Home Weatherization – Underbelly

Posted on Posted in General Article

By weatherizing your mobile home, you have the largest opportunity to save on heating and cooling bills by addressing the underbelly of the home. Due to the factory construction process by which they are built, they have a large open cavity between the bottom board insulation and the underside of the sub-floor. This cavity is referred to as the “underbelly”. Some homes have larger than others ‘underbellys’. Typically when you are performing air sealing measures on the home, you will get the greatest reduction of air infiltration by filling the void that is the underbelly. There is little to no insulation directly behind the rim joist, which gives the home a broken thermal envelope. Blowing insulation into the underbelly will not only give the home a true thermal boundary but drastically reduce air infiltration by closing the void. This is done by using an insulation blowing machine coupled with approx 100′ of hose. Most big box stores offer rentals of the machines. Blown fiberglass insulation is to be used instead of cellulose. Fiberglass is naturally water repellant which makes it a much better choice by comparison.

  • Patchwork – The first thing to address before insulating the underbelly is any tears or holes that the bottom board might have. If these are not fixed then there is nothing to hold the insulation up. The most cost-effective way to accomplish this is by using a “Tyvek or Typar” house wrap as your closing material. You can staple this up to the existing material or you can use screws and washers aimed at the bottom of the floor joists. For smaller holes or for sealing up the patches, two-part spray foam insulation works well.
  • Blowing Insulation in the Center of the Mobile Home – The key to blowing insulation is knowing where to put the material. Mobile homes were designed to have the plumbing lines run through the floor near the heat duct so as not to freeze in winter months. The center of the home, where plumbing lines exist have to be blown with a low density. Do not over-insulate, otherwise the heat from the heat duct cannot get to the pipes, and they will freeze most definitely. You want to install the insulation to where you are filling the cavity but the material still has some fluff or buoncy to it.
  • Blowing Insulation at the Perimeter – Blowing insulation around the house, including the ends only if they do not have plumbing near, is where you get the most bang for your buck. The perimeter is defined as the outside of the I-Beam sections. Again watch out for plumbing, there is usually a couple of these sections that have plumbing. Blow the insulation to a maximum density, filling the void completely and packing the material in. This is where you stop the air infiltration and radiant heat loss/gain from the bare rim joist. Treat the ends of the center the same, if there is no plumbing.
  • Entrance Point – Gaining access to the cavities can be done one of two ways. Removing skirting or siding on the exterior and drilling holes through the rim joist is an option. An aluminum pole attached to the insulation blowing hose is typically used to enter the cavity. The other is crawling under the home and cutting holes in the bottom board, inserting the hose directly and patching holes when complete. The both have their pros and cons.

By keeping these ideas in mind, you can easily and confidently weatherize your mobile home’s underbelly.