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One of the most common problems I hear about is leaky windows. It seems as if everybody has the problem. (Why do you suppose those replacement-window companies are always phoning you during dinner wanting your business?)

Once the temperature starts to drop, many people find their windows are fogged up or covered in condensation; sometimes it's so bad that there's water pooling on the windowsill or floor. So is a bit of water on the windowsill in the winter such a big deal? It can be.

Mould is one reason. It can grow anywhere that it has enough heat, a food source, and moisture. Moisture also can cause structural damage, such as rot and rust, as well as damage to surface finishes.

Moisture can get in through your roof -- if you have a leak -- or through your basement's concrete walls and floor. Remember, concrete is porous and water can move through it from the soil outside. But, most of the moisture in your house comes from you: your shower, your cooking, your washing machine, your dishwasher, your breath.

When people find they've got sweaty windows, the first thing they do is call in a replacement-window company, and spend a lot of money putting in new ones. That might make sense, but I've seen these problems with new windows, too.

There is a situation, however, in which you should replace a window.

Condensation forms when the warm air inside your house hits a cold surface -- like the glass in a single-pane window. Newer windows are double-glazed, and usually have an inert gas such as argon or krypton between their panes, which provides some insulation and reduces the chance of condensation.

There should be an airtight seal between the two panes of glass. If the seal is cracked, you'll get condensation in between the two layers -- usually on the inside surface of the outside pane. That's because the outside pane is the coldest surface. If this is the case, you probably need a replacement window.

But if your windows seem fine and you still have condensation, it's probably because air is moving between the warm area inside your home and the cold world outside. That's not what you want; ideally, there should be an envelope around your home, keeping warm air in and cold air out.

If there is air leakage around your windows, it's because they weren't properly installed and insulated. So cold air comes in around the window and creates condensation. It's easy to check by carefully removing the interior trim and looking to see how much insulation there is -- or isn't. Then spray in some low-expansion foam. But make sure you read the directions carefully: If you use the wrong type of foam, or use too much, your windows won't work properly.

There are other possible causes for condensation on the inside of your windows:

If you have a home built within the past two years, it's possible your wood framing is full of moisture. Have you ever seen a new housing development where they cover the piles of framing with tarps to keep the rain and snow off? No, I didn't think so! Without the cover, that wood will soak up moisture, and continue to let the excess moisture out until it is dry -- go figure -- and that will happen within your walls.

It might be that your home is very well built, and as a result, is too airtight. Think about it: Try to blow into a bottle -- you can't.. With the weak exhaust fans that are installed in many bathrooms (minimum code again), you will be lucky if even the slightest amount of air gets expelled. Poor air movement in your home will cause condensation. So, open a window just a crack downstairs and install stronger, better-quality exhaust fans. They get rid of moist inside air and replace it with dry, fresh air from outside. This will help you create some air movement. You also may want to look into a heat-recovery system (HRV).

It is possible that you have a problem with heating, venting and air conditioning (HVAC). For that you should contact a qualified heating contractor.

If you control the moisture in your house, and make sure your windows are properly installed and insulated, you can get rid of your wet-window problems.

Mike Holmes is the host of Holmes on Homes on HGTV.