When I have insomnia, I hear every car and conversation that passes outside my house. Or perhaps it would be more accurate to say that when I hear every car and conversation outside my house, I get insomnia. Either way, I’ve been shopping for sound-damping windows since July. In five months of looking, I’ve discovered that sound suppression is not really on the radar of the average window salesman. It’s not their fault; sound ratings for the windows they represent are buried in PDF documents on major manufacturers’ websites, if they are published at all.

There are two systems for rating how effectively something such as a wall or a window stops sound. STC stands for sound transmission class and is used more for measuring higher frequency noises such as voices and barking dogs. OITC stands for outdoor-indoor transmission class and was developed to better measure low-frequency noises such as airplanes and traffic. The higher the STC or OITC rating of a barrier, the better it is at preventing noise from coming into your home.

For example, the walls of a typical midcentury brick colonial might have an STC rating in the mid-50s and an OITC in the low 50s, according to Michael Kerr, an acoustical consultant and owner of Bay Acoustics in Baltimore. By contrast, the walls of a newer house, clad just in siding, might have an STC in the mid-30s and an OITC in the mid-20s. The key is to purchase windows with STC and OITC ratings as close to that of your walls as possible.

It’s easy to find windows with sound ratings in the 20s or 30s, to match walls made of siding. In fact, if you own that type of home, you should be careful not to waste money on windows that are better at blocking sound than your walls. “I see a lot of clients overdoing it with the windows, and they haven’t thought about the walls at all,” Kerr said.

By contrast, for those who own brick or stone houses, it’s difficult — and expensive — to find windows with sound ratings in the 50s, or higher, to match their walls. In that case, Kerr suggests choosing a window with a sound rating no more than 10 points lower than that of your walls. If your home has lots of glass, you’ll want to go higher than that. If it has little, you can go lower.

For my home, I found myself trying to choose among three window brands with sound ratings within one point of each other. Kerr told me that the human ear can’t distinguish a one-point difference in sound rating and that the margin of error in acoustical tests is three points, anyway. His advice: “Choose the window you like the look of, because . . . these one-point differences are not meaningful.” Other factors to consider: whether the window unit is well built — if it’s airtight, it will block more sound — and whether the manufacturer backs that up with a good warranty.

You should verify a window’s sound rating by reviewing the acoustical test report performed by a certified lab. “Don’t take the salesperson’s word for it,” said Casey Mahon, president and CEO of St. Cloud Window in Minnesota, which makes high-end noise-reducing windows. “Ask for a copy of the test data. If you’re a window manufacturer and you don’t have a test, you’re a wannabe.”

Kerr and Mahon both said not to bother spending money on new windows unless you can achieve at least a six-point improvement in sound rating. Of course, that means you would need to know the sound rating of your current windows, which can be elusive information because they weren’t published in the past. To overcome that, you can hire an acoustical consultant to take elaborate measurements in your house. Consultants, whose prices begin at about $5,000, often discover noise is coming through a home’s walls, vents or electrical outlets, rather than its windows. “People usually recoup our costs by not making bad decisions,” Kerr said.

If your windows are the problem, there are several ways to achieve better sound ratings with new windows. Here’s a rundown from least to most complex and costly.

Storm windows

If you do not need new windows, adding storm windows is an excellent option for reducing outside noise. Many now open just like any other double-hung window, so they do not have to be removed in the spring, and can be made to precisely match your existing windows. Alternatively, some manufacturers have started producing inexpensive and effective interior storm windows made of plexiglass that attach using magnets.

Double-pane windows

If you live in an older home, it could still have single-pane windows. Switching to double-pane windows — two pieces of glass with some air space between them — will almost certainly block more noise.

Thicker glass

Mass is one factor in blocking sound, so choosing dual-pane windows made of thicker glass is the next step up.

Dissimilar glass

You also could choose a dual-pane window in which the two panes of glass are of different thicknesses. One might be ­one-eighth of an inch thick and the other a quarter of an inch thick. These dissimilar glass panes block different sound frequencies, giving you more noise protection. This option can be affordable — and remarkably effective. For example, both Marvin and Pella make dissimilar glass windows with sound ratings just one point lower than they achieve with laminated glass, a far pricier option.

Laminated glass

Laminated glass has a layer of plastic sandwiched between the two panes of glass, which limits sound energy traveling through it. Laminated glass was originally developed to resist shattering in hurricanes, and ended up doing a good job of blocking sound as well. The downside is that laminated glass can add 15 to 80 percent to the cost of your new windows, according to window salespeople I spoke with about it.

Triple-pane glass

Triple-pane glass can have ­either a modest or a major impact on noise coming into your home. If the three panes are close together, they add a bit more mass to block sound. But if the panes are of different thicknesses and there is a large air space between them (more on air space below) they can be effective. Heike Lingertat of Northwest Washington bought deep triple-pane windows to make her house less drafty. “The side effect was silencing totally what is outside,” she said. “I hardly hear any cars whatsoever.”

Wider air space

A wide air space between panes of glass kills noise by disrupting the sound waves. If you’re struggling with major noise from something such as a freeway or a flight path, look for windows with an air space of at least two inches. More is better. “You can get improved noise control with lamination and different thickness panes, but you will never solve airplane noise unless you have a big air space,” Kerr said. Specialty window companies make windows with large air spaces, or you can achieve a wide air space by adding storm windows.

None of these techniques will solve your noise problem unless your new windows are installed correctly. One contractor told me that he would seal my windows with spray foam, which Kerr said is a terrible idea, because it hardens and conducts sound. Kerr said the correct material is acoustical caulk, which doesn’t harden. Finally, the window must fit tightly in the opening in your wall and close tightly, too.

So there you have it. That’s what I’ve learned. Now, if you’ll excuse me, I have some windows to select.