new construction - geothermal/radiant floors?
Last Post 07 May 2015 07:21 AM by Bob I. 28 Replies.
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susanmUser is Offline
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20 Mar 2015 03:40 PM
Hi, I am building a home along the Susquehanna River in Northumberland Cty, PA. The 1st floor will be 12.5' up, to stay out of potential flood waters. The first floor walls will be 10" round Eastern White Pine logs, 2nd floor cape cod style, traditional stick built, same with roof. A large cathedral ceiling, approx. 22' high. Anderson 400 windows lowE, south facing will have smartsun. We are thinking of pouring 1 1/2" of concrete over pex stapled on top subfloor, and tiling on top, using geothermal to heat water. We will have a Harman wood stove for extra, but not main heat source. I wanted the radiant for comfort, but am being told it is not worth the extra expense, or that geothermal may not even produce enough hot water to heat the house. Oh, we will spray the underside of the first floor with closed cell insulation. I just want to be comfortable, I'm afraid with an air geothermal system all my heat will be up in the cathedral ceiling. Distributing the air conditioning is another nightmare, but my main question is will a geothermal system work with radiant heat in this situation, and is the cost worth it? HELP!!!
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20 Mar 2015 05:46 PM
Designed properly a ground source, water-to-water heat pump is a perfect match for radiant floors.

The question is more of the type of construction, window-to-wall ratio.

As for value, the "geo" system is more in question since payback in residential applications can easily match expected life.

Where vaulted ceiling and big glass are found in the same room--nearly all of our lakeside properties--you will find radiant floors or empty rooms in cold weather.
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Dana1User is Offline
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20 Mar 2015 06:36 PM
To meet IRC 2012 code min for under-floor R in US climate zone 5 (that would be you) requires. R30, which would be 5" of closed cell foam, which is something of an affront to the planet, between the environmental hit of that much polymer and that much HFC245fa blowing agent,, which is a powerful greenhouse gas (about 1000x CO2). Worse yet, the ~R5 thermal bridge of the joists would rob it of a large double-digit percentage of it's performance. 

Unless there is some good reason for the crawlspace it's better to build the first floor slab-on-grade, with 4-5" of 1.5-2lb density EPS (Type-II or Type-IX, respectively), and run the radiant tubing in the concrete, which is better heat transfer than under-subfloor radiant systems, for lower water temp requirements.
susanmUser is Offline
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20 Mar 2015 07:21 PM
We need to be raised because we are in a flood zone. We plan on 2" of sprayed closed cell insulation on the bottom. We are exempt from code because the house is "recreational", but we want it to be right.
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21 Mar 2015 07:36 AM
Code immunity notwithstanding, if you're going to be using the floor as the radiator over the crawl space with only ~R12 foam you'll then have to insulate the foundation to at least R15 continous (an insulated concrete form, or 4" of EPS on the interior of the concrete) or the heat loss out the bottom of that floor will be excessive.

Alternatively you COULD put R21-R30 batts under the 2" foam to get it up to code-min, but the thermal bridging of the joists will continue to be an issue.

A third and better option would be to skip the closed cell foam except around the band joist/beam (for it's air-sealing properties, install full-depth blown insulation between the joists (or full depth batts, if you fit them perfectly), then add 2" of foil-faced rigid polyisocyanurate insulation strapped in place by 1x4 furring 24" o.c. through-screwed to the joists, and tape the seams with a temperature rated aluminum foil tape (eg. Nashua 324a), and use can-foam to seal the edges to the foundation. (I'd do this even without the radiant floor. Low density cellulose & 2" of polyiso as a thermal break. The cellulose willl sag an inch or so over the first 20 years lowering the overall R-value slightly, but that also reduces the thermal striping of the joists apparent in the floor.)

To keep water temp requirement sufficiently low to enable use a heat pump solution you'll have to use an above the subfloor approach such as WarmBoard (tm) or a lightweight non-structural concrete slab (gypcrete, etc.). Run the tubing perpendicular to the joists to avoid exaggerated temperature striping of cold joist edge & warm tubing run- it'll be more even that way.

Log homes are notoriously difficult to air seal. If the interior is going to be exposed logs there's not much you can do about that other than maintaining it with periodic leak checking & fixing, but if it's going to be a paneled or wallboard interior you can use housewrap detailed as an air barrier between the logs and the finish wall.
susanmUser is Offline
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21 Mar 2015 07:52 AM
Thank you for the responses. As an alternative, does it make sense to forget about the geothermal system, use minisplits to heat and cool, combined with the Harman wood stove. We would still run the pex and pour the 1 1/2" concrete on top of subfloor, (we can do this ourselves, so the cost is minimal), and maybe use a propane furnace to heat the floors in extreme cold if the minisplits can't do the job? Ideally we'll be in the south during the winter anyway!

Does it make sense to put extra insulation in the roof system when I have 10" round log walls? The logs are double tongue and groove with a layer of foam on each tongue, and will be held tight from top to bottom with all-thread.

Thanks!
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21 Mar 2015 12:27 PM
Minisplits + an electric boiler for the radiant floor (for comfort when you're there, and as the "Hail Mary" backstop to the mini-splits when you're not there) is a heluva lot cheaper than a GSHP system. Even though propane is cheaper energy than propane at this year's prices, as the shale production drops due to the drastic reduction in drilling rates prices will again rise. If you're only using the radiant floor during the shoulder seasons and as a backstop you would likely never make up the difference in up-front cost between a propane boiler (and it's maintenance costs) and an electric boiler.

And, with the crashing price trends of renewable electricity there is going to be a deflationary trend on electricity prices within 10 years.

It's always worth building to code-min where you can, more if your energy costs are higher than average. Even if you're not there during the winter and holding the line at 55F the heat loads are real. Taking it to R49+ on the attic insulation with blown cellulose is really cheap compared to many other things (like radiant floors or high-performance windows), as is 2x6 / R20 construction (or better) on the upper floor.

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27 Mar 2015 06:06 AM
Since the minisplits already give you everything you need, with the possible exception of ultracoldweather backup, the radiant and or geo system adds $10,000 to $40,000 in cost. Putting a portion of that into better insulation would give you more comfort. Is radiant the best use of your budget?
newbostonconstUser is Offline
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27 Mar 2015 07:49 AM
My house is has a geo water to water heater for the floor which are in 1 1/2 concrete with pex. 2x10's insulated with fiberglass. We love it. We are building another house this summer and doing it the same way but insulating better. (complete icf)

We are going to do geo for the write offs but cost wise month to month NG water heater is just as much.

We had our water furnace geo go out and I just turned on the NG hot water heater and the bills were the same all winter.

If you are on well you can do a pump and dump geo with out a field. And install costs might be half as much. Good luck
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
susanmUser is Offline
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28 Mar 2015 11:55 AM
This discussion has been very helpful. Thank you to all for your input. Here's what I'm thinking at this point...

I think we'll go with mini splits for our main heating and cooling. One big reason is not needing the ducting - the logs make it very difficult.

I still want to put the radiant tubing on top of the subfloor and pour the concrete over it. If we don't do it now, it will be almost impossible to do later. We will insulate the floor as Dana1 suggested in option 3 above. Since we can pour the floor ourselves, I think it will cost around $1700. Then we can decide how best to heat the water for it. The radiant will probably not be used regularly, just as a backup in extreme cold - we will have a Harman TL300, and burn wood when we're there, besides having the mini splits.

This house will be built this summer. It will be a weekend place for the next 7 years or so, then hopefully more full time. However, at that time we plan on going south for the winter months, I hate to go overboard on the heating when we may not need it 100% of the time, yet if for whatever reason we can not go south, I need to be warm!

I think the geo system is just way too expensive, even with the tax credit. We have 2 different quotes in the mid to high $30,000s, before the credit.

Any other input is greatly appreciated!
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28 Mar 2015 12:31 PM
This is helpful, and candid information. Thank you
MA<br>www.badgerboilerservice.com
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28 Mar 2015 12:34 PM
I think you are on the right track. My brain loves the mini-split, but the old bones leave floor on most of the cold months.

How big is this house? What is the R-value of your logs?

Open stairwell?
MA<br>www.badgerboilerservice.com
susanmUser is Offline
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30 Mar 2015 01:28 PM
The house is 30x50, 2300 sf, 2 story great room with lots of windows facing south. That's another reason I want to pour the floor - to absorb the sun in the winter. I believe the logs are 1.3 R value per inch, they are 10" round logs. I'm not sure how to factor in the thermal mass of the logs v/s strict R value, or if I should.

It is an open stairwell, but a large great room probably makes that a non - issue!
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30 Mar 2015 02:22 PM
I have been involved in many such designs. The first thing is a detailed heat load.

If you are thinking about passive solar, you have to model for it, detailing the size, quality and orientation of the windows.

Tile over concrete is efficient but will "feel" cold in all but the coldest months, with rooms not heated by the sun going cold in sunny weather.

A comprehensive design considering the options and matching expectations with reality.

The "good" thing about a "real" log home is the steady heat load presented by the modest R-value of the logs. The floor will be on a lot when it is cold or cloudy.
MA<br>www.badgerboilerservice.com
BadgerBoilerMNUser is Offline
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30 Mar 2015 02:22 PM
I have been involved in many such designs. The first thing is a detailed heat load.

If you are thinking about passive solar, you have to model for it, detailing the size, quality and orientation of the windows.

Tile over concrete is efficient but will "feel" cold in all but the coldest months, with rooms not heated by the sun going cold in sunny weather.

A comprehensive design considering the options and matching expectations with reality.

The "good" thing about a "real" log home is the steady heat load presented by the modest R-value of the logs. The floor will be on a lot when it is cold or cloudy.
MA<br>www.badgerboilerservice.com
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30 Mar 2015 05:46 PM
Hard to beat ductless minis for bang for buck and efficiency. A 30 SEER unit with 12,000 BTU will run you around $2,500 - $3,000 installed.

Many building science studies show that in a tight, high R-Value home, properly designed, that one or two minis can heat/cool the entire home. Spending $3K- $6K for the homes heating/cooling needs beats spending $30k on a ducted system that gets 12 SEER and creates hot and cold spots. I have never seen a ducted home system be properly zoned, there always ends up being a room that gets too cold or too hot when the system is running. Ductless Minis are simple and reliable.

With minis you can't hear them or feel them. The air distribution is subtle and a constant air temp is achieved. Instead of a typcial ducted system which blasts you with cold air, you then freeze to death and then the system turns off. It then gets hot in the room and then the system cycles over and over the same way.




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30 Mar 2015 06:01 PM
Mini-splits heating and cooling a log home...

Maybe cooling with a large open great room if the sleeping rooms are on the north side of the house and no computers or plasma screens in the bedrooms. An isolated loft bedroom with solar exposure would need a separate head.

It's not easy heating a multi-level, multi-room home with mini-splits. I am in one now...
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31 Mar 2015 01:47 PM
I love air source pumps but there are a lot of ways to skin that cat. The ductless wall units are nice and cheap but we're starting to see backlashes against them up here from aesthetic concerns and air movement... if you keep them small, the air movement is less but then you're not hitting as much of the load in a home/room such as this and you're doing a lot more backup heat. If you make them large they move more air and are more intrusive unless you have a great location where you aren't blowing on people.

I do find they are oversold in modern construction as well. Yeah, they can heat the place, technically, but if you try to do it without room by room distribution in the vast majority of homes built today, the non-directly-heated rooms will be significantly less comfortable. That's just physics. It's POSSIBLE to design a home where that is not true, but then you're looking at very diligent near passivehouse design which almost no one doing and even fewer are really doing well. Or if you're happy with 5+ degree differentials between main space and auxiliary rooms, go for it. In a log home though it would probably be worse than that. Personally I wouldn't ever do that in any house without someone showing me the numbers that prove that the bedrooms will be comfortable.... usually they aren't.

I'm then finding by the time you do enough wall mount units to handle a typical or higher load home with a broken floorplan you're in a similar price point as hydronics or ducted systems. Not the slam dunk it might be for small, efficient, open concept homes.

If you're looking for bang for the buck that includes room by room comfort, consider panel radiators with minisplits in some key areas for heating assist and summer cooling. If winter comfort is a lesser concern than the minisplits or other air source heat pump option might be good enough with the backup you describe.
Rockport Mechanical<br>RockportMechanical.com
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31 Mar 2015 08:07 PM
Yes. What Rob said.

And, you have to balance the heating load against the cooling load. Here in Minnesota we get the worst of both but our real time AC operation is roughly a tenth of the heating. Still, much is compromised to move air, since you can't cool anything without moving a minimum amount of air.

We have settled on Unico high pressure for most of our work and as we are doing more new home construction have a hard time looking back.

Mini-splits where it makes sense but objective thought it the first order of business.

We just installed a twin head Juditsu to cover the shop and office. Naturally nearly ever surface is radiated but radiant cooling is a challenge around here.
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01 Apr 2015 04:18 PM
The Fujitsu xxRLFCD mini-duct units are now hitting the performance levels that the wall-coil type mini-splits were only a few years ago, with decent capacity down to -4F (better low temp capacity & higher efficiency than the Mitsubishi SEZ/SUZ mini-duct units), which would be fine in an eastern PA location, which can solve the room-to-room distribution issues without going with high-R construction.

Until you run the heating & cooling load calculations it's a bit silly to be locking into any one solution though, and during the design phase you have considerable control over where those numbers eventually land.
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