Insulate an overhang properly and safely

Last Updated on March 8, 2021 by Dave Farquhar

When the polar vortex brought single-digit temperatures to St. Louis, our kitchen pipes froze. They didn’t burst, but we had to go without a kitchen sink until I could thaw the pipes. Our kitchen is partially on an overhang, which makes it colder than the rest of the house in winter and warmer than the rest of the house in summer. I finally did something about it. Here’s how to insulate an overhang properly.

Old standards don’t cut it anymore

My overhang was insulated with a single batt of fiberglass insulation, about 2 inches thick. This didn’t fully fill the void between the foundation and the kitchen floor. The openings between the joists were about 14 inches wide, 28 inches long, and six inches deep, and there were nine of them. This meant I had about 12 cubic feet of uninsulated space underneath my kitchen, including the area where the sink was. In theory I had about R-14 going on, but there was more than enough room in those bays to get well above R-30 and there seemed to be little or no insulation on the outside wall.

Temperature change and room for airflow causes drafts and further cold air. One of the ways insulation works is by restricting that airflow, but in my case, my insulation was only fixing 1/4 of the problem. The result was a noticeably colder kitchen floor, and a noticeably colder area in the basement.

You can add additional batts of fiberglass insulation on top of what you have, if you wish. But getting those batts up into those gaps is hard. It’s much faster and easier to insulate an overhang with rigid foam insulation instead, and in my area at least, foam costs less. The only drawback with foam is you will have to install a fire barrier, but even factoring in the cost of some drywall, it’s still cheaper.

Using an infrared laser thermometer

You can use an infrared laser thermometer to find drafts and figure out where to start first. This especially helps if you insulate in non-extreme weather. When it’s single digits outside, the drafts are several degrees cooler and easy to notice. When the temperature outside is reasonable, the drafts are much harder to notice. But the project is much easier when the weather isn’t as harsh. It’s no fun lugging around 4×8 sheets of material when it’s single digits and snowing.

But if you don’t have an infrared thermometer, you can just insulate everything. Chances are you’re not going to stop at insulating a handful of bays, unless you run out of material.

Insulating kitchen pipes in an overhang

Insulating an overhang
When insulating an overhang, it makes sense to start by thawing out and then insulating the kitchen pipes to keep them from freezing again.

Since this problems is most noticeable when it knocks out your kitchen sink, it makes sense to start there, with the kitchen pipes, if they run through your overhang.

Thawing frozen up kitchen pipes

Be sure to thaw the pipes before you insulate them, as it’s going to be harder to warm them up if they’re insulated. You can use a hair dryer, a heat gun, an incandescent light bulb, or any other mild heat source you can get up there. Do not use a blowtorch or another heating method that involves an open flame or temperatures high enough to melt solder. If you melt the solder joints on your pipes, you’re going to have even bigger problems.

Insulating your kitchen pipes

Once you get the pipes thawed, insulate them. You can get foam pipe insulation at any home improvement store. and a six-foot length will cost less than $2. Half-inch insulation fits most pipes feeding kitchens, but if you want to be safe, you can buy 3/4-inch insulation and cinch it to fit with zip ties, although sizing it properly will give a neater appearance. You can get pieces specially formed for elbows and tees, but the most important thing is just getting the long runs insulated. Buying the elbows and tees to insulate every square inch can double the cost of the project. You can leave those joints uninsulated, or cut openings in straight pieces so they fit over them.

Just measure the length of your straight runs, cut the insulation to fit, and snap it on.

Insulate both the cold and hot water pipes. This saves energy and keeps pipes from freezing up. You’ll also get hot water much faster when you turn on the kitchen faucet. This is a minor quality of life improvement, but a quality of life improvement nevertheless. You only have to insulate the cold water pipe back to the point where it veers off to the overhang. But there’s benefit in insulating the hot water pipes all the way back to the hot water heater, and up into the bathrooms too.

Insulating around the pipes in an overhang

Insulating the remaining void around the pipes will be tricky but it also helps. The problem is the gap behind the pipes between the pipes and the wall will be bigger than the opening to get through them. You’ll have to measure the space you have to work with for openings, cut pieces to fit, then fit them behind the pipes, closing the gap as best you can.

You won’t get perfection, but you want to just close as much of the space as you can. If the air can’t flow, then your pipes stay warmer. Don’t let your inability to do a perfect job keep you from doing a good job. Spray foam would make quick work of this, but if a plumber ever needs to get at those pipes, it will increase the expense.

The area in front of the pipes is the same story. Just cut pieces to fit, fit them in, and do the best you can. You’ll find that after you insulate the two bays on either side of your kitchen sink, the bay holding the pipes will stay much warmer anyway, so you don’t have to fill every cubic inch to make a difference.

Insulating the rest of the overhang

insulating an overhang safely
The easiest way to insulate an overhang is to fill the gap with pieces of rigid foam from below. Use 1-inch and 2-inch thicknesses to fill it as much as possible, then put a piece of drywall in front to protect it from fire. In cold weather, you’ll notice a big difference almost immediately.

To insulate the rest of the void, I purchased a 4×8 sheet of 2-inch foam. By cutting them into strips 14 inches wide and 28 inches long, I was able to stack them on top of the insulation already present and close up the gaps.

Foam has the advantage of being easy to work with, easy to install, and easy to remove. If a plumber ever needs to get to the pipes, it will be easy to do that. If an inspector doesn’t like foam, it takes 15 minutes to yank it back out.

To cut foam sheet, cut a straight line with a utility knife. You won’t go all the way through it. But if you cut from both sides, you can scribe a deep enough groove into the board that it will snap pretty easily, especially when you’re snapping off strips that are 14 inches wide.

Measure each space between the joists, because they can and will vary slightly. I generally cut my sheets about a quarter inch short of the width of the opening. Sure, a tight fit is best, but giving yourself a fraction of an inch to work with makes installation much easier.

I did this when it was single digits outside, so I noticed a difference in the temperature in the basement near the overhang after insulating just two of the bays. When I went upstairs to the kitchen, the floor under the insulated bays was noticeably warmer than the floor under the bays I hadn’t insulated yet.

Installing your fire barrier

Because rigid foam is polystyrene, a form of plastic, it burns very readily and the fumes are deadly. This means you have to protect it. Use a minimum of 1/2-inch drywall, but fire-rated 5/8-inch drywall is better. You’ll only need one sheet, so using fire-rated drywall will cost a dollar more.

If you weren’t able to completely fill the cavity that’s OK, but you’ll need to fill any opening behind the drywall. This keeps the drywall from absorbing moisture from the remaining air in the cavity.

Seal the gap tightly and cut a piece of drywall to fit over the foam completely. Here’s a shortcut for working with drywall. Draw a line across the drywall, then cut along the line with a utility knife. Just like foam, you don’t have to cut all the way through. Just cut as deep as you can, given the length of the knife. Bend the drywall along the line and it will snap. Measure each bay, and scribe and snap pieces of drywall to fit. It’s much faster and less messy than cutting drywall with a saw.

Attach the drywall to the foam with a plastic-compatible construction adhesive. Your local building code may require you to apply fireproof caulk along the edge where the drywall meets the joists. That’s a good practice anyway. It’s your house and your family’s life, and life is precious.

Installing a fire barrier in front of the pipes

You’ll probably want to do the pipes a bit differently. Cut two pieces of drywall, one that fits over your pipes, and another, smaller piece that fits under the pipes. It doesn’t matter which of the pieces has the opening for the pipes. Glue a length of 2×4 to the foam, instead of gluing the drywall directly to the foam, then attach the drywall to the 2x4s with screws. This facilitates its removal if you ever need to get back at the pipes. Apply fireproof caulk around the openings for the pipe and along the seam between the two pieces, and then around the edge. It won’t look great, but it’s easy and it’s safe. You can paint it if you want to make it look a little better.

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