How do you insulate a mansard roof outside bathroom wall?


I have a mansard roof home in the Chicago area. With recent weather, I had some frozen pipes and tub drain. They are, unfortunately, on the outside wall on the southern face of the house. I recently had the bathroom remodeled, with sub-zero temperatures arriving near the end of the process. The pipes and drain have been on the outside wall since I have lived here (15 years), and I believe since the mid-1980s when the house had been last remodeled.

My thought was to use insulation baffles in the rafter/stud spaces, with fiberglass insulation over it. That could be covered with caulked-in closed-cell rigid foam. Behind the new plumbing, my thought was to stack bats of fiberlass insulation in the space. This is pretty much as it was before the remodel.

It also occurred to me that I might build a compartment out of closed-cell rigid foam to fit around the pipes themselves. As for the drain, I think I am going to have to cut a hole in the ceiling below to access the floor joist space, and build up insulation there.

I guess I'm asking if this seems reasonable, or if anyone who has experience with the migtmares called mansard roofs has a suggestion.

  6 answers
  • Mogie Mogie on Feb 21, 2021

    Seems like spray insulation would work here. Spray foam insulation can adhere to anything making it great for under floors. It also solidifies as it dries to create a hardened wall of insulation, which keeps out insects and vermin. Spray foam insulation is also impervious to the moisture that can create mold.

    Drawback this that is is messy compared to fiberglass insulation.

  • Seth Seth on Feb 21, 2021


    What you do not want to do is insulate your pipes from the heat energy inside your house. If you completely encase your pipes in insulation, you will actually cause more problems because no heat energy is going to get to the pipes. Insulate between the outer wall and pipes for sure. Use closed cell spray foam insulation if you can afford it. And, absolutely insulate your attic space from allowing heat energy to escape into your attic, which could lead to ice dams. Also make sure the framing between the top of your basement wall and bathroom floor (If on the first floor) is also insulated. If you can , install removable panels in the wall near the pipes that can be removed to allow heat to get to the pipes when you are experiencing prolonged periods of cold. Never use heat tapes on pipes in walls that are closed up. Other options include using a 60 watt light bulb to heat the area near the pipes temporarily or to use a space heater aimed towards the wall where the pipes are.

  • Cynthia H Cynthia H on Feb 21, 2021

    Hi! I found this article and hope it helps:

  • William William on Feb 21, 2021

    You have the right idea. I would use rigid foam on any outside wall then fiberglass insulation over that. Instead of rigid foam around the pipes you can use foam pipe insulation. You don't really need to insulate the drain since it doesn't hold water except for the trap but if that was to freeze it would expand into the empty parts of the pipes and not burst them. If it's not too much it would be an extra measure to take. You can also use expandable spray foam insulation for any gaps. Like Great Stuff.

  • Ed Ed on Feb 23, 2021

    Thanks for the responses.

    I have done a couple of the things recommended. I have an access panel through which I pointed a space heater in order to thaw the pipes. The contractor who finished off the remodel floated the idea of heat tapes, but I don't know that they would be safe, so that idea was scrapped. As far as a bulb in the area, I think that may be worth consideration if I can run electric there. After all, there are attic lights, right?

    This is on the second floor, so the area I am referencing is behind the shingles. The space itself is about 2-1/2 feet at the bottom, angling down to inches near the top of the space. The pipes are against the tiled wall of the shower (obviously), with the drain below. Given your advice not to enclose the pipes, would it be advisable to use a closed-cell rigid foam panel, and leave the side facing the interior of the bathroon (still about 1-1/2 feet from the wall by the window where the side access panel is)? Alternately, I was considering expanding the box, and essentially walling off a larger area around the pipes to allow ambient warmth from the house into the area.

    For the drain, I will have to cut a hole through the drywall in the ceiling below in order to access the space. I'm not sure exactly what it looks like, or how close the drain is to the eaves below, so how and where to place insulation is uncertain.

    Spray foam seems impractical for the larger space, if I am understanding the advice correctly, but perhaps could be used surrounding the baffles between rafters/studs (I'm not sure what to call them since they are part wall, part roof in a mansard).

    I doubt I have adequately explained the space, so I hope you are able to understand the area I am talking about.

    • Seth Seth on Feb 23, 2021


      Your descriptions make your situation clearer and I now understand what you mean by boxing in the pipes. Use a double layer of 2" thick rigid foam panels so you get 4" total that will surround the pipes on 3 sides leaving the side facing the bathroom uninsulated. The four inches is important not just for the R value, but as a thermal break. You do not want warm air condensing on the inside of your roof sheathing. As you describe, you can't really fill the entire space between the pipes and roof slope with insulation, but I don't think you need to or want to as some air movement is necessary. The rigid panels should be a few inches from the pipes to capture the ambient warm air as you mentioned. If you go through the trouble of opening up the ceiling below the drain, that is the perfect place to use Great Stuff spray foam to fill weird shaped, hard to reach places around plumbing and framing, especially in that area where the soffit meets the roof and the interior wall. As to what you will find when you open it up is hard to say. Your drain line is probably connected to the main drain and vent where your toilet is, or the shower drain might have it's own vent depending on your local building codes. Either way you can insulate around it. Again, leave any parts facing the inside of the house where it is warmer uninsulated. You might want to consider installing a spring loaded hatch below the drain in case you need to get to it again. I also recommend using a spray foam gun that a large can of spray foam screws into. You can buy a can of cleaner as well and you never have to worry about clogged plastic straws and wasting a partially used can of foam. With the access panel operational on an as needed basis in addition to the foam insulation, you should be able to prevent the pipes from freezing. If you have an open access panel, I would feel a little better about using a heat tape that you plug in when needed and can monitor. Does anyone else in your neighborhood have a similar roof? I'm wondering what they may have done to solve that issue.

  • Ed Ed on Feb 23, 2021

    There are other mansards in the area, but I don't know that they have the same issue with having the plumbing on an outside wall. I think it is a bad idea in the Chicago area, since we have have extended sub-zero temperatures with enough frequency to make it a concern. The only other place in this house that has plumbing by an outside wall is the kitchen sink, but I have the furnace vented through the toekick below it, so no isues there.

    As for the tub drain, the ceiling below it is right next to the front door, so I doubt I would get approval from my wife to install a hatch. I would probably feel comfortable spray-foaming the space and sealing it back up, since, as someone mentioned above, the worst that generally happens is a frozen drain that won't burst because the water has room to expand. I am guessing that if I properly button up the space around the feeds and use the spray foam around the drain, I will probably be good.

    So, the plan for the wall area is baffles in the rafters/stud covered with insulation (probably fiberglass for this spot) and a 4" thick box covering the pipes on three sides with the area against the back of the tiled wall providng access to the ambient warmth from the room. I have the access panel on the side if need be.

    Does that sound...sound?