Expert Tips on How to Insulate Existing Walls in an In-Law Suite

A long time Roseville client, Jeff, recently converted the back room of his house into an in-law suite for his father-in-law. The problem, though, is that the walls back there weren’t insulated well, and as the summer has started heating up, it’s becoming a problem. Jeff says:

“I need some help. My father-in-law recently came to live with my wife and me, and, surprisingly, that’s not the problem. It’s that I turned a spare room into an in-law suite for him and the temperature in there is always much warmer than any of our other rooms. I know the previous owner added that room on at some point and I’m guessing he didn’t invest in top notch insulation for the walls since nobody was living in there at the time. So, I’m wondering if you have any expert tips on how to insulate existing walls in an in-law suite? I’m not sure if you need to know this, but they also happen to be stud walls. Thanks!”

Jeff, first of all, I never for a million years thought your father-in-law could be the problem. I want to stress this—especially if my own in-laws happen to be reading. But, all joking aside, this is a common problem I see with home insulation: repurposed rooms being too hot or too cold. Nobody spent much time in them before, so it wasn’t a big deal. Then, someone moves in and quickly finds out that they’re not going to be comfortable. You’ve come to the right place, Jeff, because I do have some tips on how to insulate existing walls.

Tips for Choosing the Best Insulation for Existing Walls

There are two main categories of tips I’m going to give Jeff today. The first is what sort of insulation he should invest in. The second will be advice on how to get that insulation into his existing walls without a major disturbance to the rest of his household.

By my count, a homeowner in Jeff’s situation has five product choices, so let’s dive right his options for choosing the best insulation for existing walls:

• Loose-fill cellulose: Loose-fill cellulose is the most common and familiar choice for insulation. Loose-fill means it was blown in through holes at the top and bottom of the walls with low pressure, and that it falls short of the 3.5 lbs of cubic foot density it needs to be considered dense-packed cellulose (more on that shortly). This is the choice of many DIY handymen who want to spend as little as possible and do the job with a blower from a local hardware store. Do I recommend this for Jeff? Not really. This sort of job leads to spotty and uneven density because you can’t ensure it distributes evenly inside the wall. If you know how a radiant barrier works to keep out heat, you’ll know this won’t be good news.
• Dense-packed cellulose: The only difference between dense-packed and the loose-fill cellulose above is how it’s installed. This type of insulation is blown into the wall with more force, ensuring a denser layer and more even spread. It’s also tougher work best left to an insulation professional. With this type of job, Jeff can be sure his father-in-law will feel a noticeable difference in daily temperatures.
• Loose-fill fiberglass: This choice shares much in common with the first one on our list. It’s popular among DIY homeowners and can be installed with a blower. It’s a bit more expensive than the cellulose variety, but the difference in its constitution helps it to spread much easier. The downside is similar to loose-fill in that you can’t be sure that the entire surface of the wall has been sealed.
• Dense-packed fiberglass: This is a really tricky sort of insulation. By nature, fiberglass is not as dense as its cellulose counterpart, which means even though the dense-packed installation method spreads it evenly, it still might not do as good of a job keeping outside air from getting in.
• Injection foam: This type of insulation is exactly what it sounds like. You inject it into your wall and then it expands to insulate the areas around wires and outlets. It’s more versatile, which makes it a great choice for nooks and crannies. 

I’ve talked in the past about why foam is the perfect choice for the attic or other similar areas, but if Jeff’s room in Roseville is just four straight up and down walls shaped like a square or rectangle, I’d say he may not need to invest in injection foam, making dense-packed cellulose my top recommendation for Jeff, and anyone else asking about how to insulate existing walls in an in-law suite.

Getting Professional Help: How to Insulate Existing Walls

I wouldn’t take any chances with your father-in-law’s comfort, Jeff. I know I wouldn’t with mine. I would opt to go with dense-packed cellulose insulation. Now, to get this installed in your existing stud walls, you’re going to have to take down sections of the drywall to ensure even and thorough distribution. You’re also going to need a blower that’s more powerful than those available to rent at most local hardware stores; otherwise, the insulation won’t be as dense, i.e. as effective. Finally, you’re going to have to mix and apply drywall over all the open space and the studs.

Of all the DIY jobs I see homeowners try to tackle, insulation is a tricky one.

I’ve heard from homeowners that this type of DIY job can take as long as two weeks, with no guarantees that it was done properly. A professional insulation expert can do this same work in a day, two tops, ensuring that your father-in-law’s daily routine is minimally disrupted. Afterward, you’re more or less promised that he’ll have a room that holds in conditioned air year round, keeping him cool in the summer and snug in the winter.

Of all the DIY jobs I see homeowners try to tackle, insulation is a tricky one. It’s seemingly easy on the surface, but the margin for error is high, which means you might do a whole bunch of work and not be much better off than when you started. I wouldn’t chance it. I’d call an insulation expert and make sure my room is done right.

An existing wall insulation job completed by a professional won’t need to be revisited for the next 20 or 30 years. I know personally, I don’t like to leave much to chance with my in-laws, because the more comfortable he is, the more comfortable my life is, too.

Make sure your in-law suite is as comfortable as can be. Contact the insulation experts at Bell Brothers to make sure you pick the right of insulation for existing walls—and have it correctly installed.

Image courtesy Unsplash user Sam Wheeler