In zone 4 (Northern VA), what is the best way to keep pipes from freezing in a 2×4 gypsum sheathed wall with aluminum siding?
Picture attached but the vent pipe will be cut out and adjusted to run to the other stud cavity and a 2×6 will be added above the copper to brace a pedestal. There is 2 3/8 inches between copper and exterior gypsum and a little over 1 inch between the pvc and exterior wall. Rain does not hit this wall because it is the house side of a semi-enclosed porch so it has another wall roughly 4 feet in front of it and it has a living space above the porch so it is not open to rain from above.
Secondary to keeping the pipes from freezing my next concern is to protect from moisture build up within the wall. I understand in zone 4 it may be best not to have vapor retarder other than painted drywall yet I think having a vapor retarder behind the pipes may be the best way to keep them from freezing.
Here are the options I am thinking. For all the options I would use pipe insulation on the copper.
1. Put two inch R-10 rigid XPS foam behind the copper pipes and cut it to fit behind the PVC but only install it to the height of the copper pipes. Then install unfaced fiberglass or Roxul in the rest of the cavity including in front of the pipes. This would allow any possible moisture to escape through most of the exterior wall and allow for warm air in the house to pass through the drywall and hit the pipes.
2. Put 2 inch R-10 rigid XPS foam to the full height of the cavity and fill the rest with unfaced fiberglass or Roxul. This would give the highest R value and stop cool air coming in but may cause moisture problems.
3. Use great stuff foam around the studs and fill the cavity with unfaced R-15 fiberglass or Roxul This would allow warm air from the house to pass through the drywall to the pipes and would stop some of the cool air from coming in.
4. Same as number three except use faced fiberglass or in the case of Roxul use a level one plastic vapor retarder. The vapor retarder in both cases would be tucked behind the copper pipes to allow some warm air to pass through the drywall to the pipes.
This is my first post, sorry for the length. I lack knowledge on insulation and would appreciate any advice!
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Replies
Zack,
I think you forgot to upload the photo.
Here is my general advice to anyone who asks this question:
1. It's a bad idea to locate any plumbing pipes in an exterior wall if you live in a freezing climate.
2. If you insist on violating this fundamental rule, the best approach is to install oversized studs at the location where you want to install the pipes (or to strap the studs to thicken the wall). Bring the pipes as close to the interior as possible. Install insulation with a high R-value per inch between the pipes and the exterior sheathing. You want to meet the minimum R-value for walls required by your local code. Don't install any insulation between the pipes and the interior surface (usually drywall). Pay close attention to airtightness in this stud bay.
Concerning your questions: I don't know what you mean by, "I think having a vapor retarder behind the pipes may be the best way to keep them from freezing." Vapor retarders don't prevent freezing or stop the flow of heat. What you really want is (a) an air barrier, and (b) insulation with sufficient R-value. The best products for this are either spray foam or rigid foam. If you use rigid foam, the foam needs to be properly sealed at the perimeter with caulk or canned spray foam.
To make this work, your wall has to be deep enough to provide enough room for insulation behind the pipes. If you have a 2x4 wall, you don't have enough room to follow my advice. So you'll have to thicken the wall on the interior or move the pipes.
Whatever you do, don't put any insulation in front of the pipes.
Thanks for your feedback! With the pipes being in the floor for many years they were very close to the rim joist (that has the same insulation as the wall above) so I thought moving them 6 inches closer to the wall and increasing the R value would work. Also several surrounding homes have the pipes in the wall so that pushed me to move them. Although I already moved the pipes to the wall I have not finished the wall and have since decided to move them back to the floor. Although it is not ideal for a pedestal sink it will give me piece of mind not to have to worry about it.
In terms of the vapor retarder I was thinking it might be more effective to have a vapor retarder behind the pipes to minimize airflow and to keep the insulation from touching the pipes but I was conflicted as some advise against vapor retarders or air barriers in zone 4.
If I did put the pipes in the wall i now believe polyiso would seem to provide the highest R value although I am suspicious of applying rigid or spray foam directly to the wall to create an air barrier. I wonder what may happen if water some how made it's way past the gypsum sheathing to the foam. The water would seemingly have nowhere to go and no way to dry. It may create similar problems to those found on homes with improperly installed stucco and (more recently) improperly installed cultured stone.
Thanks again for your feedback, in the end the pipes will be moved back to the floor. Would have been better to make my mind up in advance but that would be too easy.
Zack,
You wrote, "I am suspicious of applying rigid or spray foam directly to the wall to create an air barrier. I wonder what may happen if water some how made its way past the gypsum sheathing to the foam. The water would seemingly have nowhere to go and no way to dry."
You have several fundamental misunderstandings about moisture flow through walls.
1. You don't want to design a wall that allows moisture to flow continuously from the interior to the exterior.
2. In winter, your interior will always be warmer and more humid than the exterior. That's normal. There is no need for the interior moisture to "go" anywhere. Ideally, the interior moisture will stay put.
3. Moisture won't condense on the interior surface of the polyiso in the scenario you are worried about. Moisture condenses on cold surfaces, not warm surfaces. Because polyiso and spray foam have a high R-value per inch, the interior surface of the foam will be warm.