House addition on a pier, should I be getting cold feet on this project?
I was the recipient of a 10” x 40” x 40’ laminated beam when a local grocery store was demolished. I got it at a ridiculously low price so naturally I couldn’t resist it.
I was challenged to figure out just what to do with my prize. I love to repurpose things in a creative manner. I thought it would be no problem to use it on an addition to my own home. Building on piers is hardly a new construction technique , so naïvely I thought that it wouldn’t be that hard use this in zone 6B if I treated the floor like a wall since cold air would circulate under the addition floor. Ignorance is bliss, right? The more I have dug into the building science of my self imposed challenge, I now realize continuity of my control layers is only part of the penalty I get for building in this way. I recently listened to a Building Scientist (who is far more educated than myself, and shall remain unnamed) that did a similar project creating an R30 floor, wall and roof assembly only to find out from his wife that she was not impressed with his lack of ability to create comfortable living space. She said her feet were comfortable walking on the portion of their house overlaying the basement, but the the moment she stepped onto the floor of their addition on piers, she now had cold feet! Happy wife = happy life. Right? The opposite of that may also be true. I feel for him because every time his wife gets cold feet, he has a constant reminder of his building prowness! Any coments on what went wrong and how I can do better would be appreciated. I am sure he did a very good job of calculating recommended R values for his climate zone and keeping control layers in continuity within his ability to do so. This guy is actually quite brilliant and has built some excellent structures he be proud of. Like so many things in building science, the more we know, the more complicated we find out things are. To make my situation even more challenging, a portion of my addition with be extending an upstairs bathroom 8 feet so people may be barefoot when the space is used! What was I thinking? I’m in climate zone 6B! The footings, piers and beam are already in. I put them place last summer while I was in the confident but ignorant stage so backing out now has it’s penalties also. I am waiting until this next summer to do the actual remodel so perhaps some of you sage folks can give me some advice for the floor assembly. I initially was going to use wire mesh for pest control, ZipR with taped seams, Rockwool and then Advantech subfloor. (From bottom to top) I initially was thinking of insulating floor and walls to R40. It looks like enough to me on paper, but now I am getting “.cold feet”or at least am less confident in my calculations after listening to the comfort failure of someone I hold in high esteem.. Thank you in advance for any Input, advise, or any other light you can shed on this subject. This is the first time I have built anything on piers. By doing so I have violated my own KISS principle philosophy. By challenging myself in this way, I hope to grow by this experiment and if I am to be completely honest, preserve my ego for hauling home that laminated beam that was calling out to me to repurpose it. Hopefully, this exercise is not to the detriment of people’s feet and a constant reminder that I should have done something different.
Andy (Wannabebuilder)
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Replies
Generally no amount of insulation will get warm (or not cold) floors in an overhang. My bedroom is in an overhang with 16" of insulation bellow, the floors are cold if I turn off the floor heat.
The fix is easy. Add in some floor heat. You don't need to fully heat the place with the floor heat, you just need enough to get the floors a couple of degrees above room temperature. Resistance mat is probably the simplest. The energy use of this will be pretty minimal even if it is electric resistance. You can also do it with a small loop off your hot water tank if it is gas.
You could also try setting the registers to blow across the floor, just make sure to get good throw and coverage of the whole area. Not easy to get right, floor heat is the simpler option.
“You can also do it with a small loop off your hot water tank if it is gas.”
I have a heat pump hot water heater. Perhaps what you said is obvious to some but the “if it is gas” portion of your statement is a bit perplexing. Thank you for your explanation.
Andy
I think the implication is that if the water heater is resistance electric there is no efficiency gain compared to resistance electric floor heat, at a significant cost in complexity.
With a heat-pump water heater the calculation is more complicated, because you're heating air inside of the house, and then extracting some of that heat to heat the water, and then putting some of that heat back into the house. So the only net heat flow is the electricity used to run the heat pump -- same as resistance heating. But it's not just about net heat flow, it's about getting heat to where it makes you most comfortable.
Even the smallest natural gas or propane tank has enough burner capacity to heat a fairly large well insulated space, that is is what I meant by gas.
Heat pump water heaters have only around a 4000 BTU heat pump. You can get away with heating a small area like a mudroom or bathroom with one, but any large area will cause the heat pump to run continuously which might void any warranty.
Andy,
Read this discussion if you haven't already: https://www.greenbuildingadvisor.com/question/new-code-option-for-cold-floors
On the bath, I would install in-floor electric heat on a programmable thermostat. Or put in another type of supplemental heat.
According to simple theory, if you achieve a whole-wall R-value in the floor (actually whole floor R-value) of R-30, the temperature drop from the room to the floor will be 1/30 of the temperature drop from inside to outside. So, for example, if it's 70 F in the room and 10 F outside, the floor will be at 68 F. That's actually warmer than an unisulated floor over a 60 F basement would be.
But in practice, several things can make it worse: 1) cold air on the inside surface of the windows falling to the floor, and 2) Any air leak around the perimeter of the floor--stack effect will suck cold air in there, and spread it across the floor.
You can aim to mitigate those two with meticulous air sealing, high-performance windows, and radiators under the windows. You won't have a luxuriously warm floor, but if all goes well, you won't have a problem.
Or you can use radiant floor heating. I used to agree with the consensus here that radiant floor heating a mistake in high-performance buildings--the argument is that if you get the floor warm enough to feel nice, you end up overheating the room, and if you don't run the floor that warm, it's just an expensive way to get the same result you can get inexpensively with panel radiators. But with the increasing availability of high-performance hydronic-output heat pumps, whether air source or ground source, I think floor heat has advantages in high performance buildings: the ability to get astronomical COP by running low water temperatures, and the quiet of a panel-radiator hydronic system with better flexibility on furniture placement. You can even play games with a programmable thermostat to have the floor warmer for a few hours in the morning or whenever you plan to be barefoot. But do get some expert help laying out the tubing to avoid needing high pumping power or other problems.
Joe Lstiburek claims that if you put the insulation at the bottom of the cavity and have air on both sides of the floor you'll have warmer floors. Not sure how true this is in practice. He has a good article on unconditioned crawl spaces here:
https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces
He recommends all the standard wall layers, in particular an air barrier and vapor barrier. He recommends polyiso attached to the bottom of the joists with seams taped, and fluffy insulation above.
One thing he doesn't specify but which I think is crucial is a "critter control layer." Everywhere I've ever lived rodents would invade exposed insulation. You need to have all insulation covered by plywood or equivalent, and it has to be airtight. If there are gaps where air leaks rodents will detect the air flow and chew through the wood to see what is behind it.
Here's the article where he makes that argument. https://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance
Specifically, his argument is, " Think 2-D heat flow and have a beer and it will make sense to you. Or play with THERM. Trust me the beer approach is better."
That's not very convincing. A better discussion is in this GBA thread:
https://www.greenbuildingadvisor.com/question/new-code-option-for-cold-floors
Jon reports results of the THERM simulation: average temperature is higher without the air space--on average it keeps the floor warmer. But the variation is smaller with the air space: 0.3 degrees vs. 1 degree F. That's consistent with Joe's clarified claim: that the comfort issue he's talking about is the variation in temperature, not the average temperature. I'm not sure that's sound reasoning--I'm not sure that having a 1 F variation in temperature is a comfort problem, if the average temperature is warm enough.
For what it's worth... I built my cabin on piers (climate zone 6) and insulated the joists with r40 -joists are sealed at the exterior with polyiso and seams are taped. When cold temps arrived our first winter here I discovered that it felt as though there was little to no insulation in the floor, and the 22'×16' cabin was an ice box. I installed an insulated and sealed "skirt" around the house and put down a vapor barrier on the ground beneath the house. The difference was remarkable. There's a lot of places in my neck of the woods on piers, and I thought that building on piers would offer me a substantial savings. It cost me much more in backache, neckache, and labor for sure and I'm still not as far ahead as I would have been if I had built a proper foundation.
I agree with your observation on what building on piers Really costs us. Are you aware the efficacy of Polyiso decreases as temperature decreases?