New construction - need some advice on in floor heating....
Last Post 02 Nov 2011 10:38 AM by ilgeo. 33 Replies.
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minkia38User is Offline
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22 Feb 2011 11:44 PM
Hello
I am building a new 3350sq foot house in Detorit Michgan area.
My wifes folks have electric in floor heating in their home in Minneapolis, and the wife loves it.
I want to add it to our kitchen, nook, and master bathroom.

Should I be considering electric or liquid heating?
I am on natural gas, and will be using 4" spray foam in the walls

I'm leaning towards a 95% effic furnace, BUT beothermal is also an option right now with the 30% tax credit


ANY help would be GREATLY appreciated
thanks in advance
Ray
Dana1User is Offline
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23 Feb 2011 02:25 PM
Per delivered BTU geothermal might be cheaper to operate, but much more expensive to install (even with the 30% tax credit) than a condensing gas furnace (for hot air), or a condensing gas boiler (for hydronic radiant floors.)

Resistance electric would only be a reasonable option if you were looking at a true high performance building envelope (R40+ walls, etc) or have the worlds lowest power rates. It's cheap to install, but expensive to operate in most markets. Before plunking down an extra $15-20K for geo, consider how much it would take to go much higher-performance on the building envelope (high-R on foundation, walls & ceilings + better windows + verified air sealing to a very tight level, etc.)

BTW: Closed or open cell on that 4" of foam? Any exterior rigid foam going under the siding (or behind the masonry)? If open cell it's not enough R, if closed cell in a studwall it's still lower performance than what COULD be done- there are cheaper combination stackups to consider that would deliver a higher performance wall for similar or less money. See:

http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

For new construction in your climate 2x6" 24" on center construction with flash & fill (1-2" of closed cell sprayed on to the sheathing from the interior as part of the air-sealing, followed by spray cellulose or sprayed/blown fiberglass), and 3-4" of rigid iso on the exterior would deliver R40+ whole-wall R values. Combined with R24+ insulated concrete form foundation and R16 EPS under the slab and R55+ of cellulose in the attic you may still be looking a less of a cost differential to your current plan than adding geo, with a bigger boost in performance. 4" of closed cell in a 2x6 stud wall is only delivering a ~ R20 whole wall R value due to all of the thermal shorts of the framing. (And with open cell it's substantially less.) Foam-clad 2x6 framing is a relatively easy build. Even if you split the difference in performance with only an inch or two of XPS on the exterior + flash & fill you're looking at substantially higher performance comparable cost. (A variation on 2b in the high-R walls document.)

No matter what your insulation package is, building for air-tightness and verifying it with blower door testing, and getting it down to < 1 air change per hour @ 50 pascals is a cost effective efficiency enhancement. Defining the air-barrier in the plan and implementing it in construction this isn't too tough a requirement, but retrofitting it to that level can be a bit much.

By going with a moderately high-performance envelope you can cut the heating/cooling requirements by 50-75%, at which point the relative efficiency of the heating systems starts to matter less. In Detroit even a smallest-of-the-line modulating condensing boiler might be overkill in such a house, but a condensing heating + hot-water combi might not be (or even a non-condensing combi.) If you can get it down to where design condition heat-load is under 25KBTU/hr (likely, with a high-peformance envelope), something like the Daikin Altherma air-source hydronic heat pump could supply both radiant floor heating and hot water at near-geo efficiencies for less money (more up front than a combi or gas-fired mod-con boiler, but with a lower operating cost.)

IMHO high-efficiency heating systems make more sense for very large loads, or low-performance building envelopes. For new construction it's an iterative process, but erring toward the higher-performance building when all else seems equal usually results in higher comfort, even when the operating costs are equal. Radiant floors can achieve the same comfort at a lower room temp though, but even that becomes moot at R60 walls and PassiveHouse heat load levels, at which point you don't really need a heating system, but putting in some electric radiant in just the spots where you might sit/lie can probably improve the comfort & efficiency of even one of those (compared to the usual resistance-elements in the ventilation air schemes.)
BadgerBoilerMNUser is Offline
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23 Feb 2011 07:14 PM
"My wifes folks have electric in floor heating in their home in Minneapolis, and the wife loves it."

Radiant floors are naturally comfortable, but if your in-laws are using electricity for space heating here in Minneapolis, they are paying 3 times too much to operate it! Further, they are burning coal to heat the floors...definitely not GREEN. Worse yet, finding the error of their ways, can't even change there minds and use clean burning domestic natural gas.

Geo-thermal, more properly ground source heat pumps, don't make sense in most cold climates where natural gas is available, as the gas is usually cheaper than COP3 electricity and much less expensive to install using condensing boiler and indirect for domestic hot water.
MA
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minkia38User is Offline
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23 Feb 2011 07:43 PM
Wow, that is awesome! I read those 2 studys, and they were very informative.

My builder said to upgrade from 2x4 construction to 2x6 construction, and upgrading also from blown in cellulose to 5.5" of spray foam, its an extra $7500

Since I am going to be living there for a long time, I want to make the most economical choice....

Dana1User is Offline
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24 Feb 2011 12:16 PM
Posted By minkia38 on 23 Feb 2011 07:43 PM
Wow, that is awesome! I read those 2 studys, and they were very informative.

My builder said to upgrade from 2x4 construction to 2x6 construction, and upgrading also from blown in cellulose to 5.5" of spray foam, its an extra $7500

Since I am going to be living there for a long time, I want to make the most economical choice....


And most of that cost uptick is 5.5" of foam vs. 3.5" of cellulose.  Every inch of depth of closed cell foam adds the better fraction of a buck per square foot over what an inch of spray cellulose costs. It's more R (R6/inch vs. R3.6/inch), but from a cost-effectiveness point of view...   Once you have an air-tight assembly you're primarily comparing R/$, and closed cell spray foam is more than a 2x multiplier on that basis (more than 3x sometimes.)

Closed cell foam can be used to adjust the vapor permeance==drying capacity of the assembly, and limits the wicking of wintertime condensation into the studs & sheathing, but the right amount of exterior foam can raise the average temp of all the structural wood above the dew point of interior air, rendering that aspect somewhat moot. (And cellulose can safely buffer a significant amount of moisture even if you skimp a bit on the exterior foam.)

At 3-4" closed cell foam can be structural- it adds rigidity to the assembly and "glues" the studs/plates/sheathing together, which can be third level of benefit in hurricane zones (but not so much in MI.)

The number of board-feet of lumber for 2x6, 24" on center framing is nearly identical to 2x4 16" on inch, with similar structural capacity, but fewer boards to cut==lower labor cost.  The cost the builder is usually a net savings, not a cost.   Going to 24" on center results in a lower framing-factor on thermal shorts through the insulation. It usually results in comparable or cheaper construction costs, with a measurable boost in thermal performance.

The installed cost of rigid foam per square foof is typically less per unit R than closed cell spray foam for larger flat surfaces, but can be comparable if there are lots of angles, joints, window-openings, etc.  But putting it on the exterior of the sheathing A: protects the structural wood far better than when applied to the interior and B: Boosts the whole-wall R more than an cavity-spray job by providing a thermal break over the framing. If you detail each cavity and stud plate with acoustic sealant or simliar caulking as an air barrier, doing a cavity fill with cellulose and adding 2" of XPS or iso sheathing (2-layers, joints overlapping by a foot or so, joints taped/sealed both layers)  to the exterior puts ~30-35% of the R on the exterior of the sheathing, which would be enough to allow you can use only standard latex paint as the interior vapor retarder in your climate (which isn't nearly as severe as Minneapolis' averages, which is where the thermo-hygric analyses for High-R wall studies are simulated.)   That would put you over R30 center cavity, and over R27 for whole-wall R.  That's quite a bit more R for less money than 4" of closed cell foam, and the sheathing & studs are better protected from moisture issues. 

If you design the foundation with ICF such that the plane of the exterior foam of the studwall aligns with the exterior EPS (even if you have to cantilever it an inch or two off the concrete, which may take an engineering signoff, but it's doable) you also get a huge thermal break on rim joist & foundation.  In termite zones you'd want to extend metal flashing out and over that seam (foam seal it) but that's nothing compared to the thermal shorts that floor & rim joists/foundation sills represent- a very cost-effective detail for enhancing the thermal performance of the structure.  (Some buildes use an exterior inset or step to extend the exterior foam sheathing a foot or so below the foundation sill on poured concrete foundations when the foundation sill are set back from the exterior edge of the concrete, but ICF + aligning the planes is usually simpler/cheaper to build.)

This is so NOT a radiant-heating discussion, eh?
minkia38User is Offline
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24 Feb 2011 02:23 PM
Not a rad heat discussion, but very important stuff I need answered none the less. VERY GOOD STUFF.
Maybe I can just hire you to come out!
Blueridgecompany.comUser is Offline
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24 Feb 2011 05:33 PM
Excellent analysis on the building envelope Dana,
Thanks,
Dan
Dan
BlueRidgeCompany.com
minkia38User is Offline
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24 Feb 2011 08:13 PM
So, I might be retarded, but according to Dana, where does the 3-4" rigid ISO go? between the studs and the bricks? so you would need an extra 3-4" of foundation/footings as well?
Dana1User is Offline
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25 Feb 2011 12:05 PM
The rigid foam is applied to the studs, and leave at LEAST 3/4" of cavity space between the iso & brick.

Be sure to vent the top of the cavity well to the exterior under the eaves (or in some instances, to a well-ventilated attic), and allow ample weep-hole area to the bottom to course to aid condenstion/penetration drainage, and some amount of convective purging of the cavity air.

With brick veneer construction there is an inherent thermal bridge at the top of the foundation wall bypassing the exterior foam of an ICF foundation, unless you beef up the insulation on the interior side of the foundation. Insulated concrete forms are still the way to go, but assuming you went with R20-ish symmetric ICFs, spray foam the foundation sill & band joist to at least R15, and add 1" of XPS (R5) sheathing to the interior side of the foundation at least at the top half.

Review that high-R foundations document for other options, but also pay special attention the locations of capillary breaks- the breaks between footing & foundation wall, and between the slab & foundation wall, as well as under the foundation sill are as important with ICF & brick veneer as with any other type of construction.

To save on foundation wall thickness you might hang the foundation sill off the foundation a couple inches to the interior with only 3-3.5" of sill bearing on the foundation, as well as using a narrower brick or stone for the facing . Hanging the sill off the foundation may require engineering review the extra cost of the design detailing to save the yardage of concrete won't necessarily pay off in reduced foundation cost. (And just like having more R value than spelled out by code, nobody ever complains after the fact about having too sturdy a foundation.)
minkia38User is Offline
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24 Mar 2011 02:01 PM
just an update, I decided to go with 3.5" of spray foam, and fill the rets fof the 2x6 cavity with blown in cellulose.
thanks for all your help!
BadgerBoilerMNUser is Offline
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25 Mar 2011 08:24 AM
Excellent choice and the one I made. Though often dismissed for its high cost, 2# high density foam is impossible to beat in some applications such as my 1912 balloon frame which is now air-tight, structurally sound and nearly water proof from the hot roof to the previously porous rim joist.

After foaming my own old home (and soon the steel building next door) and doing the heat loads on my Wrightsoft heat load program I couldn't find a better application and since have specified 2 pound foam in nearly every retrofit heating job I design or install. I just can't get enough of it!

One of my subs does a blower door on his foamed jobs and the proof is in the door.
MA
www.badgerboilerservice.com
RosalindaUser is Offline
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25 Mar 2011 05:01 PM
I think the 2# cc in the band joist on my house was a key component to our great blower door result. Also the infrared camera could barely see the roof rafters and other roof framing components through the 7 to 10 inch 2#cc foam on the cathedral ceiling, a testament to how well it seals and insulates. But it IS expensive. Hopefully as it becomes more ubiquitous the price will decrease.

-Rosalinda

Sum total of my experience - Designed, GCed and built my own home, hybrid - stick built & modular on FPSF. 2798 ft2 2 story, propane fired condensing HWH DIY designed and installed radiant heat in GF. $71.20/ft2 completely furnished and finished, 5Star plus eStar rated and NAHB Gold certified
AshleySegerUser is Offline
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11 Apr 2011 07:57 PM
Posted By minkia38 on 23 Feb 2011 07:43 PM
Wow, that is awesome! I read those 2 studys, and they were very informative.

My builder said to upgrade from 2x4 construction to 2x6 construction, and upgrading also from blown in cellulose to 5.5" of spray foam, its an extra $7500

Since I am going to be living there for a long time, I want to make the most economical choice....

You might be able to cut your costs if you blow in your own spray foam. That's my current project and it's working out pretty well so far. Better read the instructions if you decide to use a diy kit though.

Ash@spray foam insulation
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11 Apr 2011 08:07 PM
If you do consider geothermal, then take a look at DTE's offpeak geothermal electric rate which supposedly cuts the cost about in half. Even better if you have some thermal mass (concrete or water tank) to avoid use during the peak period - at least in the common case of non peak load weather (where the geo unit has excess nighttime capacity to be stored).
minkia38User is Offline
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11 Apr 2011 10:54 PM
By thermal mass, do you mean just a large tank of water or slab of cement?
I guess that I have not ever heard of this option.
I do have a 40x80 insulated barn, with a full 4" cement slab about to be poured next week, about 75 feet from the house.....

jonrUser is Offline
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11 Apr 2011 11:23 PM
Either one could work.
minkia38User is Offline
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12 Apr 2011 12:51 AM
I guess I do not know what you mean at all by thermal mass. Care to expalin what you mean? or how I would go about doing/implementing it? thanks
jonrUser is Offline
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12 Apr 2011 08:42 AM
I would hire a pro to go through all the options with you. But thermal mass can take the form of concrete floors or walls. Or it can be a buffer tank that stores water (possibly a lot of it). In either case, the goal is to use as little of the on-peak electricity as possible. The thermal mass allows you to coast through that period.

More info at:

http://en.wikipedia.org/wiki/Thermal_mass

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12 Apr 2011 09:25 AM
2x4 with foam and an 1 1/2" thermax skin for the dreaded "thermal bridging" R31 exceedes the US standards for sidewall insulation and infiltration is next to zero if windows are properly installed. I still prefer this in most of my retrofit jobs and stick-built homes as well.
MA
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NRT.RobUser is Offline
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12 Apr 2011 10:21 AM
I would typically prefer cellulose if you're doing a well sealed external foam "skin" already. way more cost effective, typically. Greener too.
-=Northeast Radiant Technology=-
NRTradiant.com
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