Wednesday, June 12, 2013

What is a 'rim joist' and why should I care?

It’s the invisible part of your walls that nobody pays attention to, partly because it’s out of sight in some homes.  And maybe it’s because it has so many names that nobody knows what to call it the darned thing: band joist, rim board, rim joist, band board, ribbon, box band, box closure, and header joist.  But it’s all the same and the cavity looks like this:

If you have a basement with drywall on the ceiling, you won’t see it and you can’t access it.  If you have a drop ceiling basement, you can’t see it but you can access it by moving some panels around.  If you have a crawlspace, you have one of these, even if you can’t see it.  (And you can access it, even if the thought of going to see it isn’t attractive!)  Only houses built on a slab don’t have a rim joist.

Here’s why these things matter to your healthy home and energy efficiency: it’s a whole foot of wall that is usually uninsulated or underinsulated.  Worse, this is typically one of the leakiest parts of a home’s assembly.  And worse yet, if improperly insulated, it could cause serious health concerns.  This is a big triple whammy.  So what’s the fix?

The first thing to note is that fiberglass is not the answer.  It is the path of least resistance, as it is easy enough for a homeowner to cut squares of fiberglass and pop them into place in a rim joist opening.  We love DIY’ers, but this is a bad solution.  It’s a bad solution because fiberglass is very air permeable and therefore allows too much heat or cold to travel through the insulation.  And because it is so air and moisture permeable, it creates an even bigger threat: mold and, eventually, rot:

Think of your glass of tea in the summertime.  The contents of the glass are ice cold, and attract moisture to the surface of the glass, causing condensation.  The same thing happens in your home.  The surface of the rim joist gets so cold, or hot, that it attracts condensation on the inside (in winter) or outside (in summer) of the rim joist.  Because fiberglass is so air and moisture permeable, it does almost nothing to stop that surface from collecting moisture.  This happens day after day, the surface never gets a chance to dry, and bam: mold develops.

So the best solution is to create a thermal barrier in these cavities that stops the air flow and doesn’t create this big temperature differential leading to condensation and, eventually, mold.  This is accomplished with rigid foam board or two-part spray foam.  These products serve the dual purpose of insulating while removing the risk of condensation.  Rigid foam board can be cut into rectangles and held in place around the perimeter with a one-part spray foam (like Great Stuff in a can).  Two-part spray foam should always be installed by a professional with the proper training as improper installation can lead to serious air quality problems from off-gassing of the materials.  Whichever solution you choose, 2" of material is the standard application.

And as for the leaky part: concrete and wood are two rigid materials which never align perfectly.  And yet that’s exactly what exists in every home with a basement or crawlspace.  Responsible builders understand this and use what’s called a sill gasket to fill in any gaps between the wood structure and the foundation.  You can spot one in your home by looking for a thin piece of foam (usually green) sandwiched in between where wall structure sits on the concrete foundation.  If you don’t have one of these, your home is a perfect candidate for air sealing due to air infiltration that makes it harder to condition your home, year-round.

Final thought on rim joists.  Check out the heat loss from this uninsulated rim joist.  Blue = cold, orange = hot.  (Click for a larger image.)