Insulating cantilevered floor with plumbing lines


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Old 02-01-19, 11:21 AM
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Insulating cantilevered floor with plumbing lines

My GF's cold water line for her kitchen sink freezes on a regular basis when outside temperatures drop into the teens. The pipes run through the cantilevered floor underneath. Apparently the floor is packed with fiberglass insulation and the pipes are wrapped. Evidently, this isn't adequate. I was thinking of using a thinner batt and/or rigid foam panels and lining the inside of the joist cavity containing the water lines. My thought is that the radiant heat from inside the home would warm the cavity enough to prevent freezing. What do you guys think?
 
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Old 02-01-19, 11:54 AM
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The insulation needs to be on the exterior side of any pipes... and air sealed. Fiberglass alone is not enough because air can blow through it. It's best to put foam insulation on the rim joist and just above the soffit and air seal that space by sealing all the edges. Then leave the pipes exposed so they can gain heat from the house. The pipes need to be close to the subfloor... not down the center of the joists. And if the pipes are in the wall, they should be brought forward to come up inside the cabinets. Often there will be a toekick duct that helps warm the area under the sink... perhaps there isn't enough heat in the area. She could also keep a cabinet door open under the sink in cold weather.
 
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Old 02-01-19, 11:56 AM
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You could try it. What do you have to lose? You would think hot and cold would freeze when it hasn't been in use for a while. Can you tell if they are insulated differently? Last resort could be heat tape.
 
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Old 02-01-19, 12:23 PM
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Sounds like a plan. Hadn't thought about leaving the cabinet door open. Good tip! No concerns with condensation after unwrapping the pipes and leaving them exposed? I'm not sure if they are PVC or copper. I want to say PVC. They come up through the bottom of the cabinet. Not sure how close they are to the subfloor. I have yet to check things out. Relaying info based on what her brother observed last time the pipe froze.

I've never worked with CPVC supply lines. I imagine it is very simple. If I need to route the lines closer to the subfloor, will a pair of Pex cutters work, or do I need to use a cutting wheel or hacksaw? Clean the cut edges with a chamfering tool? Secure joints with one step cement or use a 2-step solvent/cement like DWV connections?
 

Last edited by mossman; 02-01-19 at 12:45 PM.
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Old 02-01-19, 05:54 PM
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Wonder if this is why the pipes are frozen and the basement is cold as crap. So ridiculous!

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Last edited by PJmax; 09-08-19 at 11:50 PM. Reason: resized picture
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Old 02-02-19, 03:54 AM
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if that is the overhang that the pipe is in then you have probably the worst case of air infiltration we have ever seen!
 
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Old 02-02-19, 06:21 AM
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I'm assuming(and hoping) this was done by the previous owner and not the builder. Anyway, I'll be removing all the insulation and doing it properly.

Plan is to secure a 2" layer of foam to the band board using screws/foam board anchors, then glue the second layer of 1" foam to the first using PL300 and seal around the perimeter with spray foam. Then I could install 2" panels along the bottom chord of the floor truss and seal them as I go. I could them slide some R-13 batts on top. Would I remove the vapor retarder if done this way?

In addition, I could also attach some 1.5" panels to the plywood prior to nailing it back in place. If I did that, I could probably do away with the batts.


It would be ideal to insulate from the inside, but the basement is finished and cutting out drywall isn't preferable.
 

Last edited by mossman; 02-02-19 at 06:56 AM.
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Old 02-02-19, 07:09 AM
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Great Stuff can says application temperature should be between 60 and 90 degrees. It will be in the 30s tomorrow when I plan to do this :/. Do I need to hold off?
 
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Old 02-02-19, 10:36 AM
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It will be in the 50s tomorrow, so it will be fine. And I realize attaching foam board to the plywood is a dumb idea. I'll just fill in between the trusses by gluing 1-1/2" foam to the 2" panels then nail the plywood back in place. That will give me and R-17.5 along the band and bottom if cantiliver.
 
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Old 02-03-19, 04:22 PM
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Just finished up. Everything went as planned. Ended up adding some R-13 on top of the rigid foam, so ended up with an R-17.5 on the band and an R-30.5 along the bottom. It didn't seem logical to have a vapor retarder on the batts since theres an air gap between the subflooring and insulation, so I removed it. All gaps and crevices spray foamed, caulked/air sealed. Should be a huge improvement, and no more freezing pipes!
 

Last edited by mossman; 02-03-19 at 06:55 PM.
 

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