Use good gaskets if you hope to get a decent air seal between logs. Even then, I'd like to see blower door results.

I agree with Dan; from an energy efficiency standpoint, you wouldn't want radiators on exterior surfaces (walls, crawlspaces, etc) at all. But from a comfort standpoint you DO and then the less insulation between the radiator and the interior the better. On the other hand, you can always add insulation to offset any increase in heat loss caused by having external surface radiators or a little insulation between the radiator and the interior.

Posted By tbomb on 07 Nov 2013 07:37 PM
Wow so many smarter people than me out there...

Anyways here comes the bomb drop.... Log home. Large pine logs running horizontal. Chinked with a traditional mortal mix. Many people have built with the identical wall style and say it is a very easy home to heat (thermal mass I suppose). Very few windows and doors compared to today's standards. He is not sure yet on roof structure. SIPS, Rigid foam built up system, fiberglass, blown in cellulose, has all been mentioned for the roof. 2 storey, VERY OPEN CONCEPT, only bathroom and closets will be completely closed off.

This updated pic solves a few problems I suppose. I am really hoping I can install the 1/2" ply from the bottom up then do everything else from topside down. Which means hydronic tubes would need to be secure before the 2X flooring is installed. Anyways, this should spawn some more thoughts/ideas for everyone. Thank you greatly.

CLICK HERE UPDATED FOR PIC

Use EPDM roofing membrane, not radiant barrier as the vapor barrier in your new stackup.

DON'T leave a gap between the blown insulation and the vapor barrier- pack it in there tight (2lbs density if fiberlgass, 3.5lbs if cellulose). If you leave the gap, air leakage into that gap would be a HUGE heat loss. (And log-home= air leakage, guaranteed.)

A foot thick log home with mortar chinking performs no better in a cold climate than a 2x4 studwall with R15 Roxul, and it's impossible to air seal (unlike the 2x4 wall).   Even if you air seal the hell out of it on a particular day, the dimensional changes with temperature & humidity over the course of a year always opens up new leaks- it's a fool's errand. (My brother lives in a log home- I know this movie.)  Yes, there's a thermal mass benefit which lowers the peak temp somewhat, but with the air leakage it's still an energy pig compared to a true high-R house, or even a tight code-min house.  If the logs are 2.5-3' thick, maybe it'll perform well enough to heat with radiant, but not without heat spreaders to get the heat out of the PEX and into the floor.

As drawn you would have serious heat-striping across the floor, with hot-stripes above the extra-hot PEX, and much cooler floor temps between them.  With heat spreaders the water (and PEX) temps come down, and the temperature distribution at the top surface of the flooring is more even.  A WarmBoard (tm) or Roth or similar system would be a superior (if expensive) way to go.

If the radiant is truly intended as the backup/auxilliary heat, it's simply not worth the money.  You'd get more/better service and at least 2x the efficiency out of ductless mini-split heat pumps.  They work somewhat better than pellet stoves and wood stoves in open floor plans, since they distribute the heat more evenly (though when it's very cold out having a hot-spot like a stove in the middle lets you pick your comfort level by your proximity to the stove.)

If the client starts sobbing over the loss of radiant, you can always use electric resistant matting in limited areas. On a good timer, it goes well with mini-split heat pumps giving a luxurious feel to tile floors in bathrooms, and even the kitchen. Please use proper insulation beneath the floors, however.

I know a contractor who did a lot of log homes in Idaho and Montana before moving out west and starting one in milder Puget Sound. Unfortunately, this client knew what was up and could point out the energy shortcomings of the log construction. That contractor actually cared and I think I once caught him sitting in his truck slapping himself in the face for past transgressions; "Bad!....Bad!....Bad!...". They sold that oinking energy pig and built another fancy one with concrete "log" siding. Looks just as good and performs properly.

Reading between the lines, I believe OP's client has always dreamed of a log house and nothing we say will change that. The same with the 2x floor, its part of the dream.

Some might be surprise at how comfortable a log house can be. Much has to do with the craftsmanship of the builder. But as Jon said, use gaskets. And don't use mortar, use Perma - Chink. It looks the same but is far superior. There is no question about the logs continuing to move and shrink, but that is allowed for by the craftsman. Synthetic chinking and gaskets remain flexible and can accommodate this.

If the floor joist bays are filled right up with insulation, you will have a cold floor. You can use electric mat, but an alternative is to use a two floor system. The bottom one is filled and sealed and the upper one will allow for air circulation. The diagram I previously posted is one we use with cantilevers and forced air. We always allow at less a little hot air into this space under the walking floor to keep it warm.

Depending on OP's fuel availability, this system could work for the whole floor with gas forced air or if just tempering, possibly an electric force air to drive heat into this space at a much lower cost then radiant.

No matter what you do, a raise floor system on piling is a pita!

The owner has land and logs. He insists on building with as much local materials as he can within reason. He is stubborn and on a tight budget. But saying that, I respect him and quite frankly this job intrigues me, I'm growing sick of the plastics, drywall and plywoods myself. He would definitely choose immediate cost savings over future heating/cooling savings. Mainly because he is planning on using his own time and trees as his fuel source.

With all of the contradicting opinions out there perhaps I should have posted this first.

If you inherited the following how would you finish the job?

A 35x35, 2 story open concept log home on piers.
Traditional mortar chinking already complete.
Low amount of windows and doors.
2x pine flooring ready to install.
Steel roof.
Wood burning fireplace.
Attempting to avoid propane and cold floors.

No roof, floor, or backup heat system set in stone.

This is the shit storm I'm in....

1) Decide how much heat you want to get out of the floor and how many $ are available to do it. Plates will be more $ and deliver somewhat more btus.
2) Don't put the floor vapor barrier on the wrong side and air seal/insulate the floor well. Same for the roof.
3) Consider a wood boiler for radiant floor/separate room/fan coil heat and do heat storage in a big water tank (more comfort, less hassles with wood refueling).
4) Baseboard electric backup is fine - if you don't use it much. Otherwise consider mini-splits (which also provide cooling).

Posted By tbomb on 08 Nov 2013 08:34 PM
The owner has land and logs. He insists on building with as much local materials as he can within reason. He is stubborn and on a tight budget. But saying that, I respect him and quite frankly this job intrigues me, I'm growing sick of the plastics, drywall and plywoods myself. He would definitely choose immediate cost savings over future heating/cooling savings. Mainly because he is planning on using his own time and trees as his fuel source.

With all of the contradicting opinions out there perhaps I should have posted this first.

If you inherited the following how would you finish the job?

A 35x35, 2 story open concept log home on piers.
Traditional mortar chinking already complete.
Low amount of windows and doors.
2x pine flooring ready to install.
Steel roof.
Wood burning fireplace.
Attempting to avoid propane and cold floors.

No roof, floor, or backup heat system set in stone.

This is the shit storm I'm in....

Some thoughts, see attached. If radiant is just tempering, maybe an electric hot water heater might cut it. Is electricity cheaper then propane it that part of the country?

He is trying to avoid having a third heat source due to initial cost. Mainly because he feels he will rarely use a backup system.

He has considered an outdoor wood boiler but again ruled it out for cost reasons.

Thanks for the drawing FBBP, when I first mentioned a second insulation cavity he wasn't to excited about it. Again, the added cost of framing to hold the insul turned him off but he is starting to consider it....

Are we worried to much about the cold floors? Would the warm indoor air keep the pine flooring warm enough? With a proper cellulose system installed.

The floor will definitely be cold. With wood heat as the primary, most areas will feel like walking on ice. Heat rises. Even with ceiling fans the will be a definite heat gradient with wood heat. Even worse with an open two storey. It doesn't really matter how much insulation there is as far as the warm floor goes. Insulation doesn't provide any heat it just prevents heat loss. Even on conventional homes, unheated cantilevers are cold to stand on.

OP - you are going to put the sheathing on anyway. The extra cost of the 2x6 is pretty low. The savings of the cost of not having to crawl around under the unit nailing up the sheathing will probably pay for the lumber.

If you got lucky, you might find some used freezer panels (or any other metal clad sips) to drop in the bottom instead of the sheathing. This would at least give some protection from rodents.

You might find the "lessons learned" about building cabins in Canada on piers discussed in this GreenBuildingAdviser.com post (and embedded/linked articles) to be very informative.

Lessons Learned from Building on Piers in Canada

We often build in wildfire prone areas and we always encourage our clients to use sensible siding and roofing. These days there are beautiful concrete log siding products that provide fire protection along with practically zero maintenance and endless life. We have become fans of this company because of the durability, quality, and variety of their concrete log siding products and the ease of installation:

BetterThanLogs

The photo shows their HR12 product being installed on ICF.

Posted By tbomb on 09 Nov 2013 08:20 AM
He is trying to avoid having a third heat source due to initial cost. Mainly because he feels he will rarely use a backup system.

He has considered an outdoor wood boiler but again ruled it out for cost reasons.

Thanks for the drawing FBBP, when I first mentioned a second insulation cavity he wasn't to excited about it. Again, the added cost of framing to hold the insul turned him off but he is starting to consider it....

Are we worried to much about the cold floors? Would the warm indoor air keep the pine flooring warm enough? With a proper cellulose system installed.

The complexity & cost of radiant floors isn't going to cut it then. 

From an initial cost point of view electric radiant cove heaters mounted on the exterior walls at the ceiling level, directed down & across the room are only modestly more expensive than electric baseboards, and are far more comfortable, and won't accidentally get covered with combustibles.

With a R30 insulated floor, no fans and 20F outside, I come up with the floor being 1F colder than the air temp near the floor. Not bad, but a little warmer than the air would be nice. So I'd add the PEX, even if you can't have plates for cost reasons. PEX is cheap and it would be hard to add later.

Posted By jonr on 11 Nov 2013 02:22 PM
With a R30 insulated floor, no fans and 20F outside, I come up with the floor being 1F colder than the air temp near the floor. Not bad, but a little warmer than the air would be nice. So I'd add the PEX, even if you can't have plates for cost reasons. PEX is cheap and it would be hard to add later.

Pex without plates means you have to leave a gap to keep the insulation from actively interfering with getting the heat out of the tubing.  An empty cavity is a thermal bypass channel, which means you then have to get religion about  air sealing.

The floor stackup is so NOT radiant-floor friendly it's silly to spend the money on radiant floor as a backup/auxiliary heat unless you do it kind-of right.  If you're not going to spring for the plates, you're better off spending the PEX and electric boiler money on radiant cove heaters, which WON'T have heat-striping issues on the floor, have a better shot at keeping up with the load, and provide more comfort even when not keeping up.

With a high supply temperature and low expectations for the btu output, PEX without plates works. Only the owner can say what slightly warmer floors floors are worth in $ terms.

Posted By jonr on 11 Nov 2013 06:17 PM
With a high supply temperature and low expectations for the btu output, PEX without plates works. Only the owner can say what slightly warmer floors are worth in $ terms.

That is pretty much what I gathered from OP's comments. I think we all agree that 2x lumber is not a good part of a radiant floor system but still if all that is wanted is warm to the feet without slippers, a double floor system with a cheap heat source for the pex will work.

Anytime you stuff insulation tight to the floor, it results in a cold floor so leaving the insulation down c/w an air/vapour barrier allows the pex to heat the joist bays. If OP chooses to go the cellulose way, he probably should put some sheathing over the vapour barrier and below the final floor system. This space then also allows for plumbing and wiring if necessary.

The better contractors make their best effort to discourage clients from making bad mistakes and wasting their money. The best contractors (i.e., ethical and not desperate for business) walk away from clients that are too clueless to be saved from themselves knowing that a bad outcome will be a lose/lose for all involved.

"Anytime you stuff insulation tight to the floor, it results in a cold floor"

That was worth repeating.

If you want warm floors you have to follow accepted practice and present a load to the floor. Proper heat loads and design, including weather sensitive controls are a must if the floors are to be warmer than the surrounding "ambient" air.

It is true that everyone wants radiant heating but not all can afford it. The shame comes when they want it done right, can afford it and are misinformed.

Go Dana!