Let’s Talk About Bottom Board

I know that all too often we take for granted the importance of that black, plastic-like material that stretches under the floor of manufactured homes. Some folks, (and the manufacturer’s installation instructions), call it bottom board. Others call it “underbelly” or “belly board” or some other name, but for today, lets just call it “bottom board”.

Obviously the bottom board is needed to secure and protect the floor insulation, and keep critters out of the floor cavity. But that is only half of the story.

I am certain that you have had your customers ask you why the holes cut in the floor decking for drain lines are often over cut. Why does the factory cut 2 ½” holes for a 2” pipe? What about the big hole for the bathtub “P” trap? We all have seen homeowners stuff insulation around these pipes in an effort to reduce heat loss. But is this necessary?  Not really!

Leaking “P” trap was fixed, but the installer failed to replace the insulation and repair the bottom board!

If you look under the floor decking, you will see cables, drain lines, water supply lines and most likely, un-insulated metal heat ducts. These un-insulated heat ducts keep the entire floor system warm in the winter, and prevent the water pipes from freezing. Since the floor cavity is in a sense “conditioned space” those holes in the floor decking are not a path of heat loss or air infiltration. So, what keeps the outside, unconditioned air from getting inside of the floor cavity? The Bottom Board!!

Water lines in the floor. Be sure to replace the insulation and access panels to keep from freezing!

The bottom board acts as the pressure envelope, which is the primary air barrier to limit air leakage. With that in mind, we should start thinking about the bottom board as less of a part of the transportation system, and more of a thermal component which is critical to the overall performance of the manufactured home’s energy efficiency!

The red line represents the pressure envelope that is intended to contain the conditioned air inside the home.

Consider the impact of every hole, slice, cut, tear or missing access panel of the bottom board.  They all allow unconditioned air to enter the floor cavity. This unconditioned air can cause significant damage to the home. In summer months, it can cause condensation to form on the heat ducts, and promote the growth of mold and mildew. In the winter, it can cause higher heating bills, and could impact consumer comfort.

The manufacturer’s installation instructions all require the installer to “Repair and Seal Bottom Board”.  For new homes, make sure you inspect the bottom board upon delivery, and report any holes, tears or road damage to the manufacturer. If you need to make any repairs, follow the information in the manufacturer’s installation instructions. Replace missing, wet, damaged or dirty insulation. Allowing wet insulation to dry out does not make it as good as new, it must be replaced. Just be sure to maintain the same R value as was originally installed in the floor.

Tape over lag bolts with an approved bottom board tape.

Make certain that the material you use to make any repairs or patches are appropriate for the job. I know a lot of installers use “Flex-Mend” with great success, and it is approved for this application.  If the area to repair is too large, use staples and a backer board of ¼” plywood or similar materials.  On multi-section homes, you should tape over the lags used to secure the floors together.

Be sure to clean the surfaces where the patch is to be applied. The HUD Code clearly states that any patches must be just as durable as the original bottom board material, see 24 CFR 3280.305(g)(6). Duct tape is not approved for bottom board repairs!

Duct Tape is not an approved material for repairing bottom board.

 

If you are a professional installer that is using the Complete Installation Checklist for every home you install, you already know that there is a line item for bottom board holes and tears.

I hope that installers, retailers,  community owners, and consumers are on the lookout for cable television, telephone, or satellite dish service technicians that have no respect for the importance of the bottom board under our homes. Their all too common practice of slicing open the bottom board to fish their cables and wires, must stop!

Bottom Board cut in two places for fishing cables.

Finally, many savvy industry professionals are now performing annual service inspections on their customer’s homes. For those of you that provide this service, make sure bottom board repair/patching is on your inspection checklist. If you don’t offer annual service inspections, maybe you should. It can be a money maker for you, and a money saver for your customers.

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