You finished the basement so you could use it. Then November shows up, the furnace starts running most of the day, and the room is somehow still the coldest one in the house. Socks-on-the-carpet cold. And if you sit near an outside wall, there’s a draft you can’t quite trace.
A lot of the time, the source isn’t the floor at all. It’s about seven feet above it, where missing or inadequate rim joist insulation can allow cold outside air into your basement.
What is a Rim Joist?
Your foundation wall ends at the top of the concrete or block. Sitting on it is the wood framing that holds up the main floor. The rim joist is the board that runs around the outside edge of that framing, capping the ends of the floor joists.
It’s a strip of wood roughly ten inches tall, and at that spot it’s often the only thing between your basement and the outdoors. An inch and a half of lumber, then siding. It’s also full of seams, since this is where wood meets concrete and where most pipes and dryer vents leave the house.
In a lot of older Rochester and Southeast Minnesota homes, that strip was never insulated. The walls upstairs got insulation and so did the attic, but the rim was skipped. When the basement was finished later, a ceiling went up and hid it.
If you’re not sure what’s behind your walls, understanding the different types of home insulation can help you get a better idea of what may have been used throughout your home.
Why Rim Joist Air Leaks Can Make Basement Floors Feel Cold
Cold air is heavier than warm air. When it leaks in at the top of the foundation wall, it slides down the wall and spreads across the floor. So the leak is overhead and the discomfort is at your feet, which is why people buy thicker carpet pad and space heaters and never find the real cause.
With the furnace running, warm air rises and escapes through the upper part of the house, and the house pulls replacement air in down low. The rim joist is one of the easiest ways in.
Air leaks aren’t limited to the basement. If your home feels drafty or your furnace seems to be working overtime, it may also be worth checking other areas where your home may be losing heat.
Signs Your Rim Joist May Need Attention
Some coolness is just how basements are. A concrete slab sits on ground that stays cool all year, so that floor will never feel like the one upstairs.
These are different:
- Moving air you can feel along the top of an outside wall
- A floor that is much colder near the outside walls than in the middle of the room
- Frost or dark staining on the wood
How to Check Your Rim Joist for Drafts
Pick a cold, windy day. Twenty degrees with a northwest wind is about perfect. Go to the utility or furnace room, where the ceiling is usually left open, and find the spot where the top of the foundation wall meets the wood. Hold the back of your hand a few inches from the rim board, at the corners and anywhere a pipe or vent goes through. The back of your hand picks up temperature better than your palm.
Have a drop ceiling? Lift a tile along an outside wall and do the same thing.
If old insulation is already stuffed in there, look at its color. Fiberglass that has gone gray or black in patches has been filtering dust out of moving air for years. That tells you the insulation was never stopping the air. And if anything looks damp or moldy, leave it alone and make a note of where it is.
How batts and air sealing close it up
There are two steps and the order matters. Air sealing comes first, which means closing the gaps and seams with caulk or sealant so outside air can’t move through. Then batt insulation goes in, cut to fit each joist bay so it sits snug on all four sides without being crammed. A compressed batt insulates less than it should. A loose one leaves a gap.
We like batts for this spot. In older homes no two bays measure quite the same, so cutting each piece to fit beats forcing a standard size. They also come out cleanly if a plumber or electrician needs to get in there later.
Skip the sealing and the batts only slow heat. They don’t stop air.
Want a Second Opinion?
If you check and aren’t sure what you’re feeling, we’re glad to come take a look and tell you what we see, even when the answer is that it’s fine.
Our team provides insulation services in Rochester and surrounding Southeast Minnesota communities and can help determine whether air sealing or additional insulation could make your basement more comfortable.