This is a great resource for owner/builder DIY numbnuts like me – wish I’d found it about 5 months ago! Here’s a situation you guys can have some fun with while maybe sending me in a good direction. I’ll apologize in advance for the long explanation...and mention that I’ve talked to some of your members over the phone and they have been very helpful.

I have a faux-adobe home under construction in central Colorado – elevation 8000 ft. This is a high desert / mountainous area known for lots of sun and relatively mild winters. Still, it gets plenty cold at night fall into spring and we do have occasional periods of downright nasty weather with much wind and temps near zero. The structure is 2 level; 1500 sq. ft. on the lower level, 700 sq. ft above. It’s a simple 44 X 34 rectangle footprint; the upper level sits on the back half of the rectangle. It has a great views and a solar orientation – along with lots of south facing glass, appropriate overhangs and glass:mass ratio. It is 2X6 construction and was just dried in last week. The front half of the lower level is slab on grade with ½” pex on 6”-12” spacing installed – tight spacing towards the perimeter. The slab sits on 2” blueboard; the whole foundation is ICF so there is also good perimeter insulation. The rest of the house is BCI joist with OSB subfloor. The upper level is open to the lower level, so there will be lots of air movement between the two. We purchased enough ½” pex for the rest of the house, along with 4 t-stats and an 8 zone manifold, but have not installed it or picked a fuel source as yet...for good reason as you will see if I haven’t lost you already! I should mention that we plan on spray foam insulation to R-38 in ceilings, R-21 in walls. I plan on batt insulation to R-30 in the subfloor joist bays with reflective bubble foil sealing it all off. The ceiling that is also the second level floor will get R-13 batts – as much for sound deadening as anything. For better or worse, the spray foam insulation lets us build on 24” centers, skip the housewrap and not have to install any venting or air exchange systems.

I’ve come to realize that the idea of radiant floor in this particular design has a couple inherent issues – mainly the water temp requirements for slab vs. wood subfloor. I’ve also learned that the radiant system design is (to put it nicely) flawed. The designer (a self-described expert, that I now know has questionable credentials) had called for a tankless water heater and simple staple-up system. Thankfully I never purchased the tankless, and with what I now know I certainly won’t. We can go propane or electric for fuel source; space limitations make electric attractive, although efficiency and operational cost lean me towards propane. From what I know, regardless of fuel source, we will want outdoor reset and a closed system.

Okay, the brass tacks and why I’m putting this out there – I’ve learned that adding aluminum heat transfer plates will help overcome some of the built-in problems with simple staple-up – but there are a number of choices and attendant cost differentials. I’ve only looked at omega-curve U-shaped style plates. The heavy gauge extruded plates drive the cost up to a point where I wonder if we wouldn’t be better off to abandon the system altogether and revert to baseboard. The lighter gauge stamped plates I’ve found for about half the cost and bring things back to the sensibly affordable range, but...

Here’s a couple more factoids before I get on with the actual question I have: Heat loss calcs have been done by a couple different supposed experts and range from 34K to 70K btu/hr. With all that glass I know we’re going to see significant loss at night, even though the windows are super efficient low-e/argon/high quality. I also know those windows are going to provide tremendous heating when the sun is out during the winter months, and the slab will store then release some of that heat. We’re installing a small woodstove to supplement all this – but code (and common sense) won’t allow us to proceed beyond framing inspection without some sort of mechanical heating system (hence the in floor).

Obviously, we have a lot of options at this point but will soon go past the point of no return with regard to the heating system. Frankly, I’ve been much more attentive to insuring that the place doesn’t overheat during the summer, and I think we’ve covered that pretty well with all the opening windows and thermal escapes on the upper level and slab on the lower level.

I should also mention that we had planned on hardwood or bamboo floors as we are not fans of wall to wall carpet and respiratory issues play a big role in that. However, knowing what I now know about what in-floor can do to wood floors...

As I see it, here are my options – bearing in mind that I’d really like to complete the in-floor system provided it can be efficient, quiet and not break the budget. I think the best way to frame the options are in a “what would you do if it were your house” hypothetical:

1) Forget about it. Put in baseboard, stoke the woodstove, be happy, hope the slab isn’t so cold you have to carpet it, and spend the money on a nice entry door or something.

2) Do in-floor only on the lower level; use the heavier plates. The upper level will get plug-in baseboards if it needs it.

3) Do in-floor only in the slab; baseboard the rest of it

4) Use lighter gauge plates and in-floor the whole thing

5) Go with the original in-floor design, forget the plates, hope for the best.

6) Bite the bullet, dorko – put in the heavy plates for both levels, pay up and enjoy it.

As far as propane vs. electric, cost of operation here says that propane would have to hit $3.50 gallon to be cost equal to the $0.13 kwH our rural co-op currently charges. It’s about $1.50 / gallon right now. I do plan to add solar components over time, which somehow makes me lean towards electric, but that’s only a plan...and if I’ve learned anything so far, plans are necessary but don’t always pan out – even good ones, which 90% of the project has.

Thanks for anything you guys can offer!

Craig in CO

Or, (7) simply use a primary/secondary pump setup to provide the higher temperature fluid to the non-slab radiant zone, and set up the mixing valves for the slab zone vs the wood structure floor on different fluid temperature reset curves and carry on.

One statement in your description troubles me: "skip the housewrap and not have to install any venting or air exchange systems."

You MUST still install a heat/energy recovery ventilation system to meet Code minimum air changes for the house, and this is a requirement for a well sealed envelope.

You can also add a hot water coil on the supply air from the HRV/ERV for supplemental heat if you so desire.

Thanks for the reply.

We do have some options for the wood vs. slab part of the conundrum, and your suggestion is a good one. I've also been advised that a mixing valve to knock down the temperature of the water going to the slab zones would work. And that adding heat transfer plates may allow us to operate the staple up system at lower temps than the 180 F I've been reading about, in addition to reducing expansion/contraction noise and providing more efficient transfer of conductive heat.

The real trouble we've run into is the cost of doing all this and the space limitations of a mechanical room that was not designed for a traditional gas boiler/sidearm set-up. If I had 20K to put into the heating system and didn't mind not being able to put a washer & dryer in the house - no worries! But I don't, and had originally planned for a woodstove backed up by electric baseboard. Our heating season is only 4-5 months, and with the passive solar aspect we're likely to not need a whole lot of mechanical heating. I switched to in-floor based on the guidance and design of someone that I should not have trusted so blindly; he had assured me the system could be in for under 5K. I know more now than i did when i bought into his plan. Well, his system would come in under 5K, but it would more than likely have lots of problems.

The venting matter is another question. We are not code required to install ERV/HRV, nor to install housewrap or provide any venting in the roof spaces. Spray foam insulation is new to me, and has me a bit nervous about condensation potential - although I am assured by the installer there simply is no potential. Still, I am a little concerned about condensation in the crawl space joist bays as they will get batts, not spray foam. Naturally we want to make sure our IAQ is good; we live in a very clean air region, have 14 operable windows and 5 exterior doors in a relatively small house on 5 rural acres, we do not smoke and will likely not have any gas appliances. Our building site is in an area that consistently has scouring winds, and the house is designed to create a thermal chimney through the second level loft and upstairs windows and door. We think we'll be able to air exchange just fine, but will pay close attention and be mindful of the potential for having to install mechanical venting assistance. There may be days when we don't really want to open the place up to change air naturally, but we believe those days will be few and far between. Still, any thoughts you'd care to pass on in this realm are more than welcome.

I see my original post has gotten lots of looks but only the one reply. Sorry if I was too long winded or was asking for guidance on something you'd rather not address. Basically, I screwed up at the design level and am just looking for the best solution. There just isn't anyone in the area without a financial interest in what we're doing (small towns!), so its proven very tough to get unbiased information.

Heat transfer plates for the wood structural floor - yes, absolutely use them to help lower the required water temperature. Question: has a proper design been done on the radiant to determine what the actual temperature it needs to run at to provide comfort heating? With a tight enveope and passive solar, you may not need to run the wood structural floor at a very high temperature anyway.

Mech room too small: Well, some piping design and little Grundfos style pumps won't take up very much room to create a primary/secondary piping setup, so trying to diagnose this over the internet won't work too well.

House Ventilation: whether your local Codes "require" it or not is irrelevant. You MUST have some form of ventilation system to make up the air from the exhaust sources in the house - this is basic building physics for a tight envelope. Visit Buildingscience.com for some fundamentals. Unless you are going to make sure you always have one of your high performance windws open at all times (then what was the point of the tight envelope?)

without load calculations and expected water temperature requirements, advice is just guessing. plates might be all you need. you might need plates AND mixing. you might not need either one, if your loads are small, though with passive solar that's unlikely unless you have significant thermal shades or the like to help with the 'night time load' or 'cloudy day load'.

you might be ok just adding some baseboard or radiators to the high load areas more cheaply than adding heavy plates.

i agree ventilation is a requirement for a modern tight envelope. If you do not make a tight envelope, then you are just making an energy inefficient, leaky house with unpredictable and likely poor wintertime IAQ. not to sound harsh, that's just the cold facts.

I have to agree with Geoff, and Rob on thier comments.

You need some room by room heat loss numbers to determine those water temperatures in those areas.

Bite the bullet use heavy plates keep the water temps lower. You were miss lead that the install would only cost 5K. You would be hard pressed to get a conventional F/A system for that. Radiant is a step above F/A.

Ventilate, Controled ventilation is more of a concern with the tighter homes of today. You need controled air changes in the winter months to keep your home efficient. You have a would stove right?

I don't know how much room you have for mechanicals but with some well thought out designing you can get alot in a small area. Is it possible to designate a portion of garage area. I can.t belive how many times I see house designs that forget about an area for mechanicals.

Gordy