I will be building a house here in NH and will be two floors and have a slab on grade foundation about 24x40. My first train of thought is that this slab needs to be heated in order for the floor to feel comfortable. I do plan on using some passive solar in the house and plan to insulate the slab well. I'm just wondering, if my house is super insulated will this be over kill? can I incorperate a mixing valve to get the water temp down... is this practical? I am building a super insulated house to lower my utlity bills.. or do I just do away with the slab floor, build a crawl space above, insulate the floor and go with a mini split heat pump on each floor?

How much insulation are you calling super insulated? Are you going to do any of the work yourself? You say "some passive solar" is this solar tempering or are you going full out passive solar? There is also some conflict with passive solar (lots of windows) with super insulated, since the windows are holes in the super insulated envelope.

My opinion is that if you are pouring a concrete slab you should put the tubing in the floor before you pour, even if you aren't currently putting in the rest of the radiant system. If you can do the radiant tubing yourself, and it is relatively easy to do, it is not an expensive proposition. Radiant tubing gives you lots of flexibility, since you can heat the water using a variety of methods.

My new house is well insulated but definitely not super insulated, with solar tempering, lots of south facing windows balanced with the thermal mass of the floor etc. I am in the Finger Lakes of NY, so it is cold. I have a frost protected shallow foundation, slab on grade, with radiant in the slab. I designed and installed the entire system myself, not something everyone might want to tackle, but there are plenty of places you can go to get a design, both online and off.

There are some folks that say passive solar and a radiant concrete floor with lots of thermal mass don't mix, but I am very happy with mine.

On the other hand, if you go with true super insulated, and have an open floor plan, a couple of mini splits will probably do the job. There are lots of folks on this site, well versed in this stuff, who will hopefully chime in.

-Rosalinda

Thanks Rosalinda

The house will be r 40 walls and r 60+ in the roof.I will be doing most of the work myself.This is my first home that I will be building and I am trying to get a good game plan going before I start construction. What temp does your infloor heat run at??

How does one insulate a slab? Doesn't the slab just sit on raw soil?

My slab is on top of 2" rigid foam. Pieces around the edges tops cut at a 45* angle. Foundation and raw dirt doesn't suck the heat out of the slab.

ChrisJ

I use a 50 gal 100,000 btu high efficiency Vertex condensing hot water heater to provide both domestic hot water and heat for the radiant floor isolated through a heat exchanger. The hot water heater is set at 125 degrees with a 20 degree differential, so it does not fire until its water temp reaches 105 degrees. The hot water on the radiant side runs around 100 degrees with a return temp around 80, though there is some variation depending where the hot water heater is in its cycle. I keep the house pretty cool, as much above 62 degree air temp is too warm for me.

My house is set up so that the ground floor is an at level basement with storage, utility room, office etc. The main floor is the second story. I only have heat in the slab on the ground floor. This is my first winter in the house, and it has been fairly mild, though with some very short bouts of very cold weather. I am not sure, but I might want to add some radiant to the bathroom and living room upstairs. I designed my system to allow me to add loops to the main floor if I decided I needed them.

Do you know what system you are using to build your house? Make sure you get some of that insulation on the outside of the framing to stop thermal bridging. That is the one thing I could not do on my house that I would have done differently if I could have. My insulation is R21 walls R70 (attic) and R50 (cathedral) roof, R5 under the slab, R30 slab perimeter and R10 down 4 ft vertical around the perimeter in the ground.

I haven't done anything yet about covering the windows at night, though I will eventually do something. I like the house very cool for sleeping, so set back the radiant thermostat 5 degrees from 8 pm to 6 am. The hot water heater rarely kicks on at night at all, unless is in the teens or single digits outside, or very very windy and cold. The HWH also rarely runs on sunny days.

I saw your post about running electric baseboard in the rooms upstairs and a mini split or something else in the open ground floor. I think a radiant floor heated by your domestic hot water heater might work well for you on the ground floor. On the other hand a mini split would be 200 to 300% efficient, which might be a better option for the ground floor as long as the floor is well insulated from the ground.

-Rosalinda

Can radiant floor heating involving water pipes eventually leak? What is a mini split?

Posted By ChrisJ on 20 Jan 2012 09:00 PM
My slab is on top of 2" rigid foam. Pieces around the edges tops cut at a 45* angle. Foundation and raw dirt doesn't suck the heat out of the slab.

ChrisJ

Is insulating the slab mostly done for heating climates or can it be done for cooling climates also?

Will not the foam get crushed with the weight of the home sitting on it?

Can radiant floor heating involving water pipes eventually leak?

Older systems had a reputation for leaking. New systems, done with PEX tubing, are supposed to have done away with the leakage problems.

What is a mini split?

Short for "Ductless Mini Split Air Conditioner or Heat Pump". It is like an air source heat pump that has an outdoor main unit and heating and cooling air handlers in several different rooms of the house so you don't have to have ducts.

Is insulating the slab mostly done for heating climates or can it be done for cooling climates also?

Mostly for heating, since the ground is almost always cooler than the inside of the home.

Will not the foam get crushed with the weight of the home sitting on it?

The weight of the home sits on the footings, not on the floor. Reinforced concrete weighs about 150 lbs per cu FOOT, so a 4" slab typically weighs about 50 lbs per sq FOOT. Underslab foam usually is rated 15 lbs per square INCH or higher, which calculates out to at least 2,160 lbs per sq FOOT.

However, there are some locations where homes (including the footers) sit entirely on foam. Foam used for those purposes is often a bit higher density than standard underslab foam.

I am really leaning towards the mini splits over the radiant at this point. and will be building using the double wall method giving me a 11-12 in cavity to fill with dense pack cellulose. In my town the inspector told me straight up that he would not allow a FPSF and it was only to be used for garages and accesory buildings. I am now thinking I will just have a 4 ft crawl space and build on top of that. I like the mini splits because of their high efficiency. I am very interested in using some sort of sun space attached to the house with french doors to close it off at night.

First a proper heat load analysis to determine the space and domestic hot water loads and design temperatures in and outside. Crawl spaces are for sissy's. A slab is a terrible thing to waste and will feel cold (regardless of insulation) unless you cover them with carpet and pad or radiate them.

We design, install and service HVAC systems for super-insulated smaller homes such as you describe and nearly always use a sealed combustion direct-vent condensing water heater for combi space and domestic hot water duty in. In my recent personal remodel I used a Mitsubishi mini-split air-to-air heat pump for cooling, backup heat and dehumidification while relying on radiant floors on all levels for unmatched comfort. i excavated the existing basement floor for new insulation, full bath and PEX for radiant floor.

PEX will last indefinitely in a radiant slab application full protected from outside threats by the concrete in which it is encased, and inert to most common chemicals, most especially to potable water at the temperature ranges common to radiant floor heating. I included snow melting in my project (one of our specialties here in Minneapolis) demonstrating how really tough modern PEX tubing really is.

Don't be sold short on comfort while designing your dream home. Always put PEX in a basement or walk-out slab and then you will be Radiant Floor Ready®.

Posted By BadgerBoilerMN on 21 Jan 2012 01:51 PM
First a proper heat load analysis to determine the space and domestic hot water loads and design temperatures in and outside. Crawl spaces are for sissy's. A slab is a terrible thing to waste and will feel cold (regardless of insulation) unless you cover them with carpet and pad or radiate them.

PEX will last indefinitely in a radiant slab application full protected from outside threats by the concrete in which it is encased, and inert to most common chemicals, most especially to potable water at the temperature ranges common to radiant floor heating. I included snow melting in my project (one of our specialties here in Minneapolis) demonstrating how really tough modern PEX tubing really is.

Don't be sold short on comfort while designing your dream home. Always put PEX in a basement or walk-out slab and then you will be Radiant Floor Ready®.

When they lay the slab down and the re-bar, does PEX snake through or above the re-bar?

Have they ever done PEX in a post-tension slab?

How much warmth can a radiant floor produce? Is it just the floor that feels warm or does it actually permeate through the floor and warm the entire room also?

It's tremendously convenient to tie the PEX to the top of the rebar grid using rebar ties. That gets the tubing in the top half of the slab and makes sure it is the last to go in, after the rebar, so it doesn't get tromped on so much.

Rule of thumb for 1/2 inch PEX in slab is 25 Btu/ft2

The radiant heat does permeate through the floor and warm the entire room. In my case the radiant in the ground floor heats the entire 2 story house, but probably not to the taste of folks who like it warmer than I do.

I used plastic zip ties to hold the PEX to the rebar/wire grid, I was afraid the metal rebar ties would cut into or kink the PEX.

-Rosalinda

On commercial radiant floor panels and snow melt (exclusively) will will tie rebar to chairs and tube to rebar to suspend it in the tube (PEX is tough and rarely needs "protection"). Rebar should never be laid
directly on rigid insulation and tube tied on top as this would be a total waste of rebar.

There is no "rule of thumb" at 25btuh for slab or any other hydronic radiant panel. Furthermore, PEX rebar ties will not cut or kink PEX, we do prefer PVC clad ties for cleanliness and job quality. Some manufacturers recommend the labor intensive "wire" ties,
obviously catering to the DIYer.) In Rosaliinda's project, the tube should have been stapled to the extruded polystyrene (common practice by most professionals) and the rebar- if any - should have been limited to the FPSF and tube tied to it as necessary.

Careful of free advice.

Sigh....

Stapling the PEX to the polystyrene would have put the PEX at the bottom of the slab instead of in the middle, which is where the rebar and wire mesh was (chaired). Is there a problem with using plastic zip ties instead of vinyl coated rebar ties? They worked just fine, were easy, clean, and adjustable. My contractor used rebar and wire 'cause that is how he pours slabs, and the slab was an FPSF.

"we can estimate the average output of a radiant floor at 18-25 BTU's per square foot"
http://www.houseneeds.com/shop/HeatingProducts/RadiantHeating/radiantheatretrofit.asp
It is a guideline - a rule of thumb to get you started and give you a starting point in your planning.

-Rosalinda

People should know the depth of your experience, before they follow your advice, with their money.

A proper heat load determines the insulation below the slab, the tube size, length, position in the slab, design water temperature and ultimately the potential output of the radiant slab, provided the floor coverings are taken in to account.

Much of our work here in MinnMinneapolis involves serious design mistakes made by folks just following their thumbs.

My experience with radiant heat is very limited - based only on what I learned over the last few years and then did in my own house. Do I need to add a disclaimer to all my posts?

-Rosalinda

PS - ok I added a sig block spelling out my experience so no one is misled

Badger's a bit of pill about this sometimes, but the fact is that "max output" for 1/2" pipe in a slab could vary from nearly zero to over 50 BTUs/sq ft depending on room temps, water temps, flow rates, floor coverings, etc.

the 25/sq ft rule of thumb is worth just about nothing, really. it's grossly misleading in many circumstances. people have rule of thumbed themselves into a LOT of non-functional and certainly under-performing systems over the years.

Just saw a study the other day that says that 60% of radiant systems out there are either not held in favor by their owners, or are outright nightmares. rule of thumbs are a good part of the reason why.

the fact is, badger has a good reason to be a pill about it. but he is a bit grumpier than he needs to be sometimes