I'm in the process of finishing my basement and I'm looking to start sheetrocking in a few months. The frigid temps here in New York have been a really good test of the insulation I installed. Where the waste pipe exits the house, I have a cast iron trap with a vent that exits the house at ground level. I installed 1" rigid foam insulation right against the sides of the trap and spray foamed the gaps. I also installed Roxul in the framed 2x4 wall a foot away from the foundation to neatly cover up the pipes. I will be installing access panels to the clean outs.

When the temps were hovering around 10*F this morning, I noticed the area in the basement around the trap was very cold. I took out my infrared temp gun and realized the portion of the pipe that exits the house was 32* and the lower part of the pipe, where the water sits, was about 40*.

I've lived in the house for 10 years and I've never had a problem. But once the wall in front of the trap is sheetrocked, less warm air will be coming in contact with the trap.

This might seem like a silly question, but do I need to worry about water freezing inside the waste pipe?

Since the main vent stack is about 4 feet to the left of the trap and I have two inline vents located in other areas, is the vent connected to the trap necessary?

Thanks,
Logan

is the vent connected to the trap necessary?

IMHO, no

do I need to worry about water freezing inside the waste pipe? yes

Hi Logan

A lot of towns/cities require these whole house traps so my guess is that your town/city does require it, but it can't hurt to call the local plumbing inspector and see if the whole house trap is still needed. If it is not needed, hiring a plumber to cut it out and plug off the vent is not a bad thing. If the whole house trap is required, I would not worry about it too much, but I might install a louvered access panel to allow some heat to escape into the trap area. Here, I would insulate around the trap (but not insulate the trap itself) above, below and to the sides, and that should keep the cold from the foundation area from entering the home. After that, I see no way that the whole house trap could ever freeze up.

Back to you...

Mark

PS: I don't see a VAPOR BARRIER associated with the insulation? Check local codes as they may require vapor barrier to be installed here.

I definitely want to keep the whole house trap. I'm just trying to find out if it needs to be vented the way it is. Other newer homes that I've seen with a PVC trap don't have the venting like mine. It goes up through the house and out the roof. I have that, too. It's just 4 feet behind the trap.

During my basement insulation research, I read that the 1" XPS foam board would serve as a vapor retarder, allowing moisture in the block to dry to the interior if needed. My town's building inspector confirmed that the foam board in combination with the Roxul satisfies code requirements.

The louvered doors is a good idea. I may have to go that route to keep the trap warm, especially since I now see frost at the top of the trap where the vent exits the foundation. I noticed some moisture in that area yesterday due to the warm, moist inside air coming in contact with the cold cast iron.

Would it be possible to temporarily cover up the outside vent to warm up the trap a bit?

This trap in addition to a vent out the roof? Some codes require it? I don't understand.

Hi Logan

Ask the plumbing inspector if you can do away with the vent from the house trap. Since you have a separate vent going out the roof he may allow the trap vent to be removed and capped off at the trap... certainly can't hurt to ask.

The building inspector gave me the OK to cap the vent. Before making any permanent modifications, I’m going to temporarily plug the opening to make sure everything still works properly. Thanks for the advice everyone.