Do It Yourself Energy Efficiency Projects: Your Home’s Thermal Envelope (Part 2): Mudsill and Rim Joists

By Vernon Trollinger, January 19, 2010, Energy Efficiency, Home Improvement

There are a few things you can do around your home to save money during the remaining winter months.  These will also save you money during the hot summer. Basically, you need to air seal your home's "thermal envelope"; the sum total of the home's insulation systems including walls, ceilings, foundation, floors, windows, and doors. These work more effectively with good, tight fits that seal-out the weather, moisture, and air.

By having a tight seal, the less energy you waste or lose by exchanging it too often with the air outside. 

You might believe that repairs like these are beyond your ability.  Never fear, these are easy tasks for any average person.  Plus, you might want to look at the Federal Energy Efficiency Tax Credit and save on your taxes, too.

One word of warning:  you will get a little dusty and dirty.  

So, let's look at your mullsill.

The mudsill is the board that is bolted flat on to the top of the foundation wall. An example of one is a 2 x 8 inch board bolted onto the final course of cement blocks. It provides a bed to attach the flooring joists and banding boards for the first floor of the house. Depending on how well it is installed, it can let in a lot of air and moisture. 

Places to look for gaps is where the mudsill is fastened to the foundation. A common building practice now is to put down a plastic foam gasket over the foundation before attaching the pressure-treated lumber that will be the mudsill. In older homes, a paper-backed cellulose material or tar paper was put down.  Sometimes, nothing was used. To find gaps, get as close as possible to the mudsill from the inside and look for daylight shining through between the mudsill and the foundation wall.  You can also feel for a draft of cool air.  Wetting your hand with water will make this easier because a draft will feel colder. 

If your foundation is made of cement blocks, examine the vertical joints between the blocks. When these blocks are put into place, the mortar between the blocks often slumps leaving thin mortar or none at all. Over time as the house settles, holes can appear. These might be tiny holes that let through tiny amounts of air, but if your home has 10 or 20 of them then you're letting in a lot of weather plus insects. Seal every hole you find with mortar or silicon caulk or expanding foam.  Also, be sure to seal any gaps where plumbing or wiring enters the house. 

Another place along the mudsill to look for is where the rim joists attach. The rim joist (sometimes called "banding joist") is the piece of wood that closes off the end of the flooring joist or is the last floor joist underneath the exterior wall. The bottom edge is not necessarily an air-tight seal. In fact, I lived in one older house where there was a half-inch gap between the rim joist and mudsill. Now, while this seems small, the gap ran for the entire length of the house: 25 feet. It was the equivalent of leaving a 24 inch by 24 inch window open all the time. Some expandable foam quickly sealed this gap and there was a noticeable improvement in comfort and energy cost right away. 

Visit the Bounce Energy Education Center for more tips on saving energy. 

Stay tuned for next time:  Windows!
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