Water supply line routing
I’m building a house in Western Montana at 3500 ft, climate is not bad other than a couple week cold snap every year, though that may change. Had a couple of brain farts during the design in regards to water lines on exterior walls. It is a slab with radiant heat, so can’t go in the floor.
The specs:
Shed roof, unvented with 4″ ISO on the roof under rubber membrane.
Planning on R40 batts then drywall with very few holes other than some junction boxes for lights.
Walls are covered in 2″ of ISO with 3/4 airspace behind siding.
Planning on batts or something in the R21 range.
House will always be heated lightly with radiant, wood stove is the work horse. It will never be colder than 70 in the house.
That said, I only have about a 7′ run where I can’t get the lines in an interior wall. I then have two shower heads, a sink and a tub that are on exterior walls. There would be maybe 10ft of line at the most in the exterior walls for each of these fixtures.
Any suggestions? Personally I don’t think there is any way the pipes would freeze even in the walls. Bathrooms, including shower floors are heated with radiant.
Thanks guys, any input is appreciated.
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Replies
Stan,
Leaving 1 1/2" for the pex or copper fittings, you still have 4" of space in the stud cavities. Coupled with the 2" of ISO on the exterior it means the waterlines are protected by 6" of foam.
Line the entire back of the cavity with the foam, but don't surround the pipes or add batts. Air-seal the perimeter and where they penetrate the studs or bottom plates. Remember to provide mechanical protection (plates) on the face of the studs wherever the pipes are closer to the surface than 1 1/4".
Oh, and check with your BI first. Some jurisdictions don't allow waterlines in exterior walls.
Thanks Malcolm...I guess my only thought on that is sandwiching the OSB sheathing between closed cell foam. Do you think this is not an issue just because it is such a small space?
I'm waiting to hear from the inspector before I move forward. Do you usually see any issues with going through the ceiling?
Stan,
That's a pretty big concern for me to have completely missed. We commonly use the 4" of foam in stud bays with waterlines, but never use foam outside our sheathing. I don't know how big the risk is. Perhaps someone else can chime in.
A lot of builders here run their supply lines on the bottom chord of the truss, rather than stub them all up through the slab. The inspectors have never have any problem with it.
Stan,
If you are just talking about a few stud bays -- the stud bays with water lines -- then I wouldn't worry about sandwiching the sheathing between two layers of foam.
The key to preventing the water lines from freezing is strict attention to airtightness. When pipes freeze, you can be fairly sure that there is an air leak nearby.
Follow Malcolm's advice in Comment #1, paying close attention to the caulking and foaming.
Stan,
Be very sure you insulate correctly. I live in a home with a similar design. Slab on grade, radiant floor, water lines in the attic. My water lines have frozen every winter once the outside temperature drops below 28 F for any period of time. There is maybe 6" of cellulose blown in the attic. I also have one water line for my fridge that runs in an exterior wall, this freezes pretty fast. The only water line that does not freeze is my kitchen sink, it was ran under the slab.
Fortunately you have much less exposed line that I do, I have one run of nearly 30' that is perpendicular to the trusses. My suggestion would to be careful. I should also note, my lines are 100% PEX in the attic. There have been no leaks. I believe the lack of fittings in the attic really helps this.
Michael,
Sorry to hear about your frequent problems with pipes freezing. Was your house built with attention to airtightness? Did the builder confirm the air barrier work by performing a blower door test?
Martin,
The attention to air tightness was not even thought of. The house is only 10 years old. I will be posting pictures of the work I've done to add 4" of XPS with taped/foamed cracks and sheathing and XPS was also taped on both layers.
I have areas where I can nearly reach my fingers through cracks in the sheathing and touch the back of the drywall. There is no insulation in this crack originally. The one door I've removed, you could see daylight through the RO and the frame, they did blow some cellulose in this crack though ;). I will post the pictures, they are laughable and I live here...
The guy who sold it to me/GC'd the build still swears these things are all my imagination and the house was wonderful the first 7 years he lived here.
Michael,
I don't like the idea of routing water pipes through an attic. But your story is basically a story about sloppy building practices and the total lack of an air barrier -- not a story about where water pipes should be located.
If your builder had even an elementary understanding of the need for air barriers, your pipes wouldn't be freezing.
Michael,
It sounds like your design is pretty different than mine seeing that you have an attic. Also sounds like with an attic you'd need a heck of a lot more than 6" of insulation even with 4" of XPS on the roof. Good luck with all that, unfortunately your builder probably truly believes its just fine. Seems like a select group that actually cares about performance, but at least that number is growing. This is why I'm building my own house.
Martin and Malcom, I appreciate the info. I still have to hit all the exterior blocks between the rafters that separate soffit from interior with closed cell spray. Maybe I'll just use that on the 3 stud bays that will have pex in them, or even a combo of that and leftover XPS.
Thanks again guys. Now we'll see if the inspector even allows it.
If you keep the pipe near the wallboard, and protect it with plates, you should be fine. But if you are worried about whether that will work, and don't want to sandwich the OSB in foam, here's a design that will keep the pipe a little closer to room temperature:
Put a 1 to 2" thick foam board right behind the pipe, but only about one foot or less high. Behind that foam board, there's still batt insulation before the sheathing. If it ever needs to, the OSB can still dry to the inside, around that foam board. Put strips of foam across the top and bottom of the foam board, so you have a little 1.5" deep, 1' high service cavity for the pipe. Because that cavity has only wallboard between it and the room, it will be very close to room temperature, and that air surrounds the pipe, so it will be at the cavity air temperature.
Thanks Charlie. Honestly, my original thought was to just put the pex as close to the interior as possible and run the batts behind it. Also spray foam the perimeter of the cavity with an inch or two of closed cell to air seal. I think it would have to be very cold for a very long time for it to freeze the pipes even with that simple setup. Of course, I only have one shot to get this right so better to bounce ideas around people that have done it.
Also, PEX is a little more forgiving of freezing than copper, so even if it did freeze you'd probably only stop the flow, not create a leak.
In zone 6B you don't have sufficient exterior foam R for R40 batts in the attic without moisture issues.
Code-min is R49, and based on that IRC chapter 8 requires R25 on the exterior of the roof deck to skate with only paint on-wallboard:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
R25 is more than half R49, which means at least half the total center-cavity R needs to be above the roof deck for dew point control. At 4" polyiso performance will average AT BEST about R22 wintertime perofrmance in a zone 6 climate, when the polyiso is on the exterior, derated for temperature. Reality might even be R20, but for now assume R22.
So with R22-ish foam on the exterior, the most you can put between the rafters without a class-II vapor retarder would be R21 fiberglass, not R40.
On the walls, you're marginal on exterior R per chapter 7 since the real wintertime performance is probably a bit less than R11, but it's not nearly as bad off as the roof stackup.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
To deal with the roof, an inch of closed cell spray foam on the underside of the roof deck would put a ~1 perm vapor retarder on the exterior, and create a non-wicking condensing surface that's more than R25 from the exterior. If you filled the remainder with cellulose rather than high density fiberglass you'd probably have about R30 in cellulose, but it would wick and redistribute the adsorbed wintertime moisture. If you also added a smart vapor retarder under the ceiling gypsum it would behave as a class-II vapor retarder during the cool dry winter, but would become more vapor open when the cellulose warms up, releasing it's adsorbed moisture.
Thanks for taking the time Dana. You think that closed cell spray foam sandwiching the roof deck is ok?
I think that the points you bring up could be debated for countless hours, though not by me. Some of the information on this site contradicts some of the things you said. I'm not saying you're right or wrong, I think only time can say that. I simply went off local tested advice as well as my gut feeling, past experience and of course a lot of research online. Also the climate here is not as bad as people think. In general there is about 3 weeks of actual cold then the rest of the winter is in the 30's. During the coldest part of the winter its very probably that I'll have a foot of snow on my low slope roof which in theory would make up those missing 3 R's by a decent margin.
I'm not arguing one way or another, I'm simply giving you the thought process. As you know this stuff gets pricy so that is definitely in play. I appreciate your advice and am always open to learning/hearing from others.
You got me thinking of course Dana, since now is my last chance to do it!
Joes take on cellulose in my type of roof.
http://buildingscience.com/documents/insights/bsi-043-dont-be-dense
I'll share a little about where I live now while I'm building my house. Its basically a metal out building framed in with 2x6's to create interior walls. Same for the ceiling, obviously no building codes here 10 years ago when it was built. So at best R21 fiberglass top to bottom. It doesn't have any sheathing so the insulation is directly against the metal roof and siding. Windows are cheap off the Depot shelve vinyl. Now to make it even less efficient, it doesn't have any drywall. All the walls and ceiling are T&G pine. Only thing it has going for it is no recessed lights and its only maybe 700sf. Only heat source is a wood stove which rated at 1500sf which will drive you out on the coldest montana day even with as poor of a building envelope as this is. Most winter days we don't even burn anything, the residual heat from a small overnight load makes it comfortable all day. In short, as horrible as this all sounds in theory, it is a very comfortable place to live even in -20 weather.
As far as condensation or whatever you want to call it goes, there is almost no sign of it. Ice creeping at the eaves is there. Stand back outside with the stove cranking and you'll know exactly what spacing was used for the roof beams because you can see the snow melt. Yet, still no major condensation, mold smells, discolored insulation, none of it. It has no sheathing so nothing really to rot other than maybe the T&G, but if there was enough moisture to rot the T&G you would think a good amount of it would drip into the house. Roof pitch is probably 2:1 max.
I know people take this building science stuff super serious but how do we explain where I live now in this zone 6 climate?
Just food for thought.