Hello all,

This is my first post, thanks for letting me join the forum. 

I was a building contractor for two decades before I switched careers in 2000, so I have a bit of experience building and remodelling homes.  I plan to build myself a new single-story 1200 sf home under some Live Oak trees here in Biloxi, Mississippi and I am seeking advise for planning and installing an in-slab radiant floor heating system. 

The slab on grade will be 28 x 42 .  The house will have three bedrooms and two baths on one end, and the kitchen / living / dining area (basically one big room) on the other end.  Three people will be living in it full-time.

The ground temperature around here is 70-72 degrees based on what I've read (not measured) and we get a lot of sunshine during the winter.  Although the house will never see this sunshine since it is under the trees, the roof of the new garage / shop building will always be exposed to the sun, so I hope to take advantage of it by installing a solar drainback water heating system on the garage roof.  This solar system will supplement a gas-fired water heater which I'm planning to use to heat the water for personal use and also the in-slab hydronic floor system.

I have several questions so let me list a few of them to start: 

1- Does it make sense to install insulation beneath the slab in this area which has such high ground temperatures?

2- My understanding is that hydronic floor systems are slow to heat up and cool down when installed in concrete slabs.  This seems to suggest that using a programmable thermostat may not save money, or perhaps not enough to bother with.  Is this true, or does it still make sense to use a thermostat that raises and lowers the temperature each day?

3- Does it make sense to create two or more separate heating zones in a small house like this?  Or should I just "keep it simple" and save money by putting the whole house on a single thermostat?

welcome!

the trick in your area is that you flip back and forth from "need heat" to "no heat" fairly regularly. the startup on a slab takes awhile. I would consider outdoor temperature sensing controls a must and I'd consider strongly the use of floor sensing as well to help manage the possibility of shortfalls and overshoots. it can be done but it's not a dumb thermostat on a wall hooked up to a water heater with zero system intelligence.

absolutely you must insulate the slab.

absolutely skip the programmable thermostat

zoning is dictated by the building needs. it's possible you won't need multiple zones but impossible to say without at least looking at the floorplan.

I would consider a lightweight method first (panel radiators, radiant wall) in your case.

Hello NRT.Rob, thanks for your quick reply.

I have focused on hydronic floor heating because my primary goal is to have warm feet in the winter.  I decided on a slab-on-grade floor because it is impervious to termites while also being compatible with hydronic floor heating.  I do not think my feet will be warm, or at least not the "bare feet on the floor" warmth I'm imagining, if I go with panel radiators and radiant walls.

Is there another alternative that might give me the warm feet I desire, while also dealing with the "flipping back and forth" issue you mentioned?  Perhaps a different type of floor that is still heated hydronically?

Here's a rendering of the floor plan for heating zone suggestions:

With good insulation and a mild climate, you might want to consider just using the system used for A/C for heating. A radiant floor isn't going to be very warm most of the time.

 A radiant floor isn't going to be very warm most of the time.

Hi jonr,

Wow, this is a BIG surprise to me!  I figured that since the entire house would be heated by the floor, it would be warm most of the time.  Can you tell me why wouldn't it be warm most of the time?  I was actually looking forward to going barefoot 'comfortably' all winter after this house is done!  Are you saying the floor would usually feel cool rather than warm?

a few degrees above room temp would be the norm, depending on your heat load.

there is a difference between conditioned and unconditioned floors in all cases, but the difference can be small if your heat load is very low.

a few degrees above room temp would be the norm, depending on your heat load.

That's better than 15-20 degrees below room temp, which is what it has been for years in our current 80 year old house which is built two feet above grade on posts -- with flow-through air below and no insulation in the floors.  In other words, a few degrees above air temp sounds downright toasty to me ... :)

Were you to install radiant floors in the 80 year old house you describe, the design floor temperature would reach up in to a range that would feel neither cold or hot. we call it "comfortable". But when you build a tight home in a mild climate the heating system is going to operate at a lower output and perhaps cycle off more depending on design and controls.

If you often suffer from cold feet, as I do, there is certainly nothing wrong with radiant floors and when it gets cold the only heat would be driven through the floors. Toasty is not a technical term. My slab-on-grade floor varies in temperature with the weather. The basement walk-out slab wants to stay a 66°F all winter unless the thermostat is turned to 68° and above, at which point the floors never field cold until spring when the heat is off.

I like your idea and don't think you will be disappointed with the outcome.

I hear this "no toasty toes" argument a lot, even from people who know radiant heat, but have never gotten over an old colleague who made a 20 year career of designing and installing radiant floors driven by condensing boilers... in San Diego, CA.

Tell me again why this fella doesn't want radiant floor heating? I also have clients that run the radiant floors in their basements year round as the floors can get cool from AC falling from upper floors and settling in the basement level. I am tempted but haven't gone quite that far for perfect comfort.

there is a difference between conditioned and unconditioned floors in all cases, but the difference can be small if your heat load is very low.

I'm not sure how much heat load the new home will have.  It will be 50% larger than the current house in floor area, with AAC block exterior walls and possibly SIP roof panels, so it will certainly be better insulated.  I just want the floors to be warm enough to walk on and sit on without feeling that they are sucking the heat out of my body.

A relative in this area has a slab-on-grade house with no insulation beneath the slab.  This is typically how they are built around here.  When I visit him in winter I can feel the difference immediately between his floors and mine.  Mine are 'cold' and his are 'cool' or 'neutral' but not 'warm'. 

My goal is to have floors in the new house that are 'warm' or at least 'slightly warm'.  Hopefully I can achieve this goal with a poured slab-on-grade.  If not, are there other hydronic floor systems that might feel warmer than heating a slab?

absolutely you must insulate the slab.

I definitely plan to insulate the sides of the slab since that's where it would otherwise be exposed to the much colder air temperatures.  But I read somewhere that the need for under-slab insulation depends upon the difference between the operating slab temperature and the ground temperature.  So ...

I was thinking that since the slab temperature may be only a few degrees higher than the 70-72 degree ground temperature, and since much of the energy required to heat the slab will (hopefully) be sourced from the sun for free, it might take me forever to recover the additional material cost of under-slab insulation.  Does this make sense, or am I missing something important here?

My goal is to have floors in the new house that are 'warm' or at least 'slightly warm'. 

You can certainly leave out the wall insulation and then your radiant floors will be toasty warm on cold days. But if you want to save money, put your $ into good insulation.

Once you have your building heating load numbers, you can run the free DIY software on our website to determine the required slab insulation values and hydronic radiant floor heating design parameters:

Borst Hydronic Radiant Floor Heating Design Software

If you don't have your building heating load numbers, you can use this software too:

Borst Heat Loss Analysis Software

It takes a little time to run the numbers, but the process is relatively cookbook if you can handle this type of assignment. BTW, very nice floor plan and we always like seeing reasonably sized homes.

any radiant floor will feel as "warm", the difference between slab and other floor methods is mass, and you just happen to be in a situation where you probably have a lot of short term demands. so you'll need to commit to keeping the slab warm if you want good results, which means floor sensing in addition to air sensing. not a big deal but it is not the absolute most efficient way to go and it does require system intelligence in that regard.

in your case though, as you note, you can already get "neutral" or "cool" with no heating in a slab in this climate. the difference will be noticeable, if you are looking for it, but it won't be large. I stand barefoot on low temp radiant all day and I can tell the difference between 69 and 72 very easily. If that's what you're looking for then use a system with a few brains and floor sensing and put a little insulation underneath so you don't have to heat up a ton (or two) of dirt in addition to your slab every time you want to ensure you have a comfortable floor temp.

The ground temperature around here is 70-72 degrees

This is deep ground temperature, such as you would have with a basement slab. Ground temperature close to the surface will be somewhat closer to the air temperature (ie, colder than 70-72 in Winter).

 I stand barefoot on low temp radiant all day and I can tell the difference between 69 and 72 very easily. If that's what you're looking for then use a system with a few brains and floor sensing and put a little insulation underneath so you don't have to heat up a ton (or two) of dirt in addition to your slab every time you want to ensure you have a comfortable floor temp.

3/4 inch of pink board XPS (I've seen it locally at Home Depot and/or Lowe's) would be plenty here since the main goal would be to provide an effective thermal break rather than an excess of insulation, correct?

Biloxi MS is in US climate zone 2, and ~R5 at the slab edge down to the footing depth would have a long term economic rationale even WITHOUT radiant floor, but under the rest of the slab wouldn't be called for

With a radiant floor you'd want at least R8 at the slab edge and ~R4-R5 under the rest of it.

3/4" XPS (pink board) is ~R4, but is usually more expensive than EPS (the stuff of cheap coolers and disposable coffee cups.) In the US all XPS is also blown with HFC134a, with a global warming potential more than 1000x CO2, pound per pound, whereas EPS is blown with pentane (GWP ~ 7x CO2). In applications where thickness and water vapor permeance doesn't matter a whole lot (like under slabs), using 1" of EPS is preferable and slightly higher-R than 3/4" XPS. What's more over 50 year the blowing agent of the XPS leaks out, resulting in a ~15% reduction in R-value, whereas the EPS loses it's blowing agent before it makes it to the distributor, and has a stable R over time.

Just be sure to avoid low density Type-I (1lbs per cubic foot nominal), which is easily damaged in handling, and has lower compressive strength. Type-II EPS (1.5lbs nominal density) is pretty cheap & common, but if for some reason you need 25psi compressive strength (not likely- you could park a loaded dump-truck on a 4" slab without compromising the foam) the 2lb density Type-IX goods will get you there without breaking the bank.

Type-II EPS in 4x8 sheets rated R4.2 runs about 40 cents per square foot($12-13/sheet) f.o.b. a real distributor's yard, sometimes even a bit less. That's about the same money as the crummier Type-I goods with facers rated R3.85 at box stores, eg:

http://www.lowes.com/ProductDisplay?partNumber=15358-46086-451156&langId=-1&storeId=10151&productId=3365576&catalogId=10051&cmRelshp=req&rel=nofollow&cId=PDIO1 (Not the right stuff!)

...and a few bucks cheaper than 3/4" XPS (pink, blue, green, whatever) rated R3.75.

Be sure to avoid polyisocyanurate for under-slab and slab-edge foam, since it can become waterlogged over time if in direct contact with soil. EPS and XPS are both pretty water-proof- either will take on some amount of water when submerged, but EPS gives it up more quickly when the tide goes out. (EPS is used all the time in marine applications as dock-floats, boat & surfboard cores, crab-pot buoys, etc. since it's both cheap and very water-tolerant.)

R5 at the slab edge down to the footing depth would have a long term economic rationale even WITHOUT radiant floor, but under the rest of the slab wouldn't be called for

Although if your feet are sensitive and the alternative is heating the floor, then you may want to add more. With enough insulation and perhaps some air circulation, you can get the floor temp very close to the room temp.

Although if your feet are sensitive and the alternative is heating the floor, then you may want to add more. With enough insulation and perhaps some air circulation, you can get the floor temp very close to the room temp.

For awhile anyway :-).

I have been thinking about the slow response time for in-slab hydronic floor heating, so I did some research into alternatives to a poured slab floor. This one seems particularly interesting to me:

http://www.warmboard.com/

Basically it's a plywood subflooring system bonded to aluminum heat distribution plates into which is pressed the pex-al-pex tubing. It may cost more than a poured concrete hydronic floor system (or not?) but it seems to be more appropriate for quick response times and room-by-room heat zones.

What do you folks think of it? Is this a good system? Are there better ones, or similar ones that might cost less? Or different systems that might be even better?