Storage unit rental companies charge plenty of rent, especially for climate-controlled storage. But there’s an even more expensive storage unit that you may already pay for, perhaps without realizing it.

Consider what portion of your house you use for storage. If your belongings reside only in cupboards and closets, they may not take up an inordinate amount of space, unless your house has huge walk-in closets. But if you have a “junk room,” or if you store a dusty exercise bike in your bedroom, long-ignored boxes of books in your basement or have a spare closet filled with clothes you haven’t worn since the turn of the century, you definitely use a percentage of your home for storage.

The person with a 1500 square foot home and a 10’ X 10’ spare room full of rarely used belongings is dedicating 1/15 of their rent or mortgage payment to store stuff they don’t use anymore. So if they pay $1500/month in rent or mortgage payment, they’re paying at least $100 a month to store their stuff. (It would actually be a bit higher than that when you include house or renter’s insurance.)

Now, $100 a month isn’t a fortune, but there are certainly more gratifying ways to spend that money, and when you look at it as $1200 a year, it would look much better in a vacation fund than going towards storing unused stuff.

But let’s up the ante. Suppose the house also has a full basement that is one-half storage and one-half recreation room. That brings the house’s total square footage to 3000, and the total square footage used for storage to 750 square feet in the basement, plus 100 square feet in the spare room, for a total of 850 square feet (out of 3000) being used for storage. This means 28.3% of their $1500 monthly payment ($424.50) goes toward paying for climate-controlled storage of items they don’t use anymore.

We can all think of better ways to use $425 a month, which is over $5,000 each year.

That said, I’ll bet it never occurs to most people who want to move to a larger home that if they’ll just get rid of the stuff they don’t use that is stored in their current home, they’ll have much more room and they’ll no longer be paying hundreds of dollars a month for in-house, climate-controlled storage.