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Insulating cold floor, keeping plumbing serviceable?

I have a question about a situation that a relative has encountered. Zone 5 (SE Michigan), an addition has been added to an old house. The addition has an exposed floor on piers, built with trusses. There is some pink batt insulation near the top of truss cavity, and two inches of foam (XPS) covered by plywood at the bottom. The problem is that the (pex) plumbing which runs from main house and then along bottom of floor has frozen up repeatedly. I suspect that airsealing is part of the problem, plus it's hard to avoid voids with the batt + truss combination…and now that I’ve actually opened it up to look, I see that there is a roughly 4” void above the fiberglass batts. Since the perimeter walls below have not been insulated, this void is likely the primary culprit.

So, I would like to remove the batt insulation, pack in cellulose, and reinstall the 2” foam and plywood.
Two questions:
1) Remove everything, install netting/screen, blow in cellulose, then cover with rigid and plywood, OR, reinstall rigid and plywood first and then blow cellulose into a confined space? I’m kinda leaning toward using netting and blowing in insulation so that I can see what is going on, especially since the trusses are more likely to create voids than dimensional lumber.
2) How to make plumbing serviceable? One of the spaces between the trusses contains a bunch of plumbing, both pex supply lines plus PVC drains. The pipes are in the upper third of the cavity, roughly, but I don’t think they can be moved up against the floor. If we choose to blow in insulation, how can we make the “plumbing chase” between the trusses serviceable? I’m thinking of stuffing that cavity with batt insulation (Roxul ComfortBatt maybe, fiberglass otherwise) prior to netting and blowing cellulose.

Oh, just to make it more fun, there is only about 18” between ground and bottom of plywood. Floor area is 8x12 feet.

Asked by andrew c
Posted Aug 15, 2014 10:02 AM ET


8 Answers

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Answered by andrew c
Posted Aug 15, 2014 12:49 PM ET


If you tent the plumbing with the netting before putting up your other netting so that the plumbing is always on the warm side of the insulation it will be fairly well freeze protected.

On open-webbed trusses like that it's useful to put netting over the truss webs so that you can fill a single truss bay a time, which ensure a more complete fill. With a dense-packing tube you could could then re-install the foam + sheathing prior to blowing, and fill from the side from a single hole drilled at the end of each bay rather than working from below.

Answered by Dana Dorsett
Posted Aug 15, 2014 1:43 PM ET


Thank you, Dana. Putting the netting vertically, covering the truss webs, makes a lot of sense to make it easier to compact each area.
Let me ask a dumb question about serviceability - if I tent the plumbing, then blow in cellulose, would future plumbing repairs (potentially) be done from the top side?

I think I like your idea of tenting the plumbing, plus putting netting over the truss webs. I'll blow all the other cavities, and use Roxul batts in the one cavity that has plumbing. Sound reasonable?

Thanks again for the ideas.

Answered by andrew c
Posted Aug 15, 2014 9:10 PM ET


Dana's idea is great. Much easier to execute than anything I would have thought up. Let me just say that installing Roxul flat on your back under a house is a real pain. If it is just one truss bay for the addition, it might not be terrible, but still it wouldn't be pleasant.

If you are going to re-install the foam and sheathing, just cut out the section of the truss bay where the plumbing runs. I would overlap the edges of the plywood (so you're not leaving an air gap) and screw in the foam and plywood under that area. That way you could do repairs from underneath and let it dry out should there be a leak. You could even tape the plywood along the seams and just cut the tape and unscrew it if you needed to in the future.

Answered by Lucy Foxworth
Posted Aug 16, 2014 10:15 PM ET
Edited Aug 16, 2014 10:25 PM ET.


i'm curious as to any you anticipate having to service your pipes? Plumbing is typically installed in walls and ceilings covered with non-removable materials with the understanding that it is a permanent part of the building, and only parts like clean outs and valves necessary for maintenance are made accessible.

Answered by Malcolm Taylor
Posted Aug 16, 2014 10:58 PM ET


Lucy, thanks for the feedback. I read that you spent a lot of time working on getting your house sealed up and insulated to your personal standards. I like the idea of cutting so that we could drop just the plywood and foam under the truss space that contains plumbing if the need ever arose.

Thanks again to folks for taking the time to read and making thoughtful suggestions.

Answered by andrew c
Posted Aug 16, 2014 11:06 PM ET


I have a very similar situation with pex and drains in exterior bays. The one anxiety of a super-insulated truss or wood I-joist floor with no basement is weather protecting the in-floor and in-wall mechanicals (particularly water pipes) and then access for repair and clean out. On the drainage side, soil and water traps and clean outs need to be available for routine maintenance, but on the HW heating and water supply side, pex or copper, the installer is not doing the job without pressure testing the entire system for leaks. If the piping and joints are all visible as in a new build, pressure testing can be with water. In a buried and hidden tubing situation (after installing the insulation for example) this is done by air pressurizing each zone and run of tubing and monitoring with a pressure gauge for a couple of days.
Like Malcolm says, its normal practice to bury water pipes. Just conduct the pressure tests and make sure the insulation is on their outboard side.

Answered by flitch plate
Posted Aug 17, 2014 8:33 AM ET


Malcom and Flitch, that's a fair point. The plumbing has been in service for a while (at least a couple of years) with no evidence of leaks, so there's no reason to expect it to be different than any other plumbing. It's probably the fact that it has frozen up and had to be worked on a few times already that is making us think about servicing. Perhaps doing it right would be a better solution. ;)

Answered by andrew c
Posted Aug 17, 2014 8:54 AM ET

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