Retrofitting with In Floor Radiant Heat and UnderFloor PEX Radiant Heating System

1. What do I need for an Existing Structure?

To properly size most components related to your underfloor heating system we highly recommend a heat loss calculation for your project if this is your primary heat source. This is even more important with an existing house install. Why? Heat loss is a critical step, as we can estimate the average output of a radiant floor at 25 BTU’s per square foot but windows, doors, insulation, and degree days all make a major impact on getting you just what you need.

The most common sizing mistake with in floor heat is in oversizing. This not only makes the new radiant heating system cost more to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Oversized heating equipment also often creates uncomfortable and large temperature swings in the house plus it will short cycle the hot water boiler and run outside the design parameters costing you more money.

We are not in the business of selling equipment that you don’t need and a little work up front can save you thousands of dollars in costs over the life of your system.


2. How do I calculate my heat loss?

Heat losses can vary in houses of different ages and locations. For example, here in Vermont - a new house may have a heat loss of 25 to 30 Btu per Square Foot, a house next door built in the 1970's may be 35 to 50 Btu per square foot and a house next to this one built prior to World War II - could be a high as 100 Btu per Square Foot. Get the Math? It's hard to tell what older structures Btu heat loss is without a heat loss of something else to that tells us what we need to know.

Have your architect or builder supply it to you as in many states like NH or CA it is required.

Calculate it yourself using software - go back to the Heat Lost Calucator under Pex Tubing Radiant Installs.

Or use one of the two different rough guides below.

Insulation Type and Climate Zone

(Please Note: We highly recommend that you do a heat loss calculation and provide the information below as a starting place)

 1)  No insulation in walls, ceilings, or floors; no storm windows; windows and doors fit loosely .... 60 to 100 BTU's per Sq. Ft.

2)  R-11 insulation in walls and ceilings; no insulation in floors over crawl spaces; no storm windows; doors and windows fit fairly tight .... 50 to 60 BTU's per Sq. Ft.

3)  R-19 insulation in walls, R-30 in ceilings, and R-11 in floors; tight-fitting storm windows or double pane windows .... 29 to 35 BTU's per Sq. Ft.

4)  "Energy Star Rated" house with R-24+ wall insulation, R-40 in ceilings, and R-19 in floor; tight-fitting storm windows or double pane windows; vapor barrier sealed carefully during construction .... 20 to 25 BTU's per Sq. Ft.

5)  SIP or Earth-sheltered house with little exposure; argon filled windows, and R40+ insulated .... 10 to 15 BTU's per Sq. Feet.

Climate Zone

Heating Sq. Footage by Climate Zone for a pre-1970's House

Houston, TX ZONE 1 --> 15 - 25 Btu's per square foot

Los Angles, CA ZONE 2 --> 25 - 30 Btu's per square foot

St. Louis, MO ZONE 3 --> 30 - 40 Btu's per square foot

New York, NY ZONE 4 --> 40 - 50 Btu's per square foot

Minneapolis, MN ZONE 4 --> 50 - 60 Btu's per square foot



Outdoor Design Temperature

The Outdoor Design Temperature (ODT), also referred to as the 2.5% design day temperature, is not the coldest day ever, but rather a temperature that is achieved 97.5% of the time.


ODT Chicago = - 8 Degree F

ODT Denver = 1 Degree F

ODT Minnesota = -12 Degree F

ODT Washington = 17Degree F


Simply multiply the appropriate factor above by your home's total heated square footage to arrive at your approximate required heating capacity. For example, if you live in Zone 3, your home is well insulated, and you have 2000 heated square feet, the equation will look like this:

 2000 square feet of "Energy Star" grade new construction but with lots of windows =

 35 BTU's per sq ft. is 70,000 Btu Load

Then, to calculate the output on a hot water boiler, multiply its efficiency rating by its listed input rating for the actual Btu output of heat. An example of a medium efficiency boiler. Of course, this is a very simple way to look at efficiency - but actually, it is more complicated. Factors such as, how long does it take to get to efficiency, condensing, direct vent or not, using pex and amound of water in the boiler all effect true efficiency.

87,000 Btu input X .86 efficiency = 73,000 Btu actual output



3. Existing Heating System

All hot water boilers sold in the U.S. must have a rating plate. Check the rating plate and get the:

1) For Example --> 92,000 Btu Input of your Hot Water Boiler X .80 efficiency of your Boiler = 73,000 Btu actual output

2) Count the total linear footage of baseboard in the home. Multiply this number by 600 BTU’s. This will give you the BTU output at 180 Degrees F. This number should be close to the Boilers actual output.

There are a few ways to calculate heat loss. Use the above information to get a rough idea. We strongly recommend that you download a heat loss calculator. Why? Because windows and doors make a huge difference to the heat load of your home. Once you have an idea of your requirements, we will be able to get you a quote.



4. Ways to install Radiant PEX with an existing floor

PEX Tubing Under the Floor - Typically under hardwood or tile flooring

PEX In Floor - Typlically in poured cement

PEX Over Floor - Typlically using ThermalBoard, VersaTherm or Creatherm Radiant Heat Mass



5. In Floor Radiant Heat System Slab on Grade

For residential slabs we recommend 1/2 Inch PEX tubing to be 12 Inch on center. Along walls with lots of glass or high heat loss the PEX should be 6 Inch to 9 Inch on center on the outside walls for the first 2 feet, and 12 Inch on center everywhere else. In Floor Radiant Heat System will give you the most output Btu, but also the slowest response time.

When figuring the over all length of tubing you will need you divide any 6 Inch spacing area by .5, divide any 9 Inch spacing area by .75 and any 12" spacing area by 1. This will give you the over all length of the PEX needed in the slab. You will need to add the length of tubing needed to get up to the pex manifold.

Typically pex manifolds are mounted 18 Inch to 24 Inch off the slab.



6. Installing PEX Tubing

Following good piping practices the maximum length of each 1/2 Inch PEX tubing runs should be no longer than 300 feet (300-foot maximum is code in many places). When the pipe loops exceed 300 feet you need to use larger circulators (pumps) to maintain this temperature drop. With larger circulators initial cost is higher and they usually require twice as much electricity to run. Most good radiant installers try to limit piping loops to below 300 feet. 

There are many correct ways of installing in floor heat PEX within a slab. The best way is tying the PEX to the reinforcing mesh or rebar. When attaching the PEX Tubing to wire reinforcing mesh or rebar it is recommend that a zip tie be used every 2 feet of PEX Tubing. 

Another way to install PEX in a slab is attaching the PEX Tubing to ridged insulation. The use of insulation screw clips or large plastic staples is common. 

We recommend an insulation screw clip or staple every 2 feet if installing the tubing over insulation only (no wire mesh). If you use 2 Inch polystyrene insulation it is recommended that you use a 6 mil. polyethylene moisture barrier. 

Installing the manifolds and keeping the lines under pressure (air or water pressure) for the concrete pour is highly recommended and required by code in many locations



7. Insulation

Insulation is always needed with any radiant heating system and especially needed under slabs. Why, if the soil has any moisture in it the moisture will wick away the heat at a tremendous rate making your system inefficient.


Today many radiant slabs being installed with insulation only around the perimeter. Their belief is that you should store the heat in the ground for use later. One problem with this notion is that a large portion of the heat is absorbed into the ground and never warms your home. Why do you want to pay to heat the ground? Slab Insulation is important for the entire slab.

 We recommend Slab Shield Insulation which was designed specifically for under slab applications. Manufactured using two separate layers of 1/4 Inch polyethylene foam with a pure aluminum center. This product is available in 4 Feet x 63 Feet rolls for easy application. It is simply unrolled and taped together (this is necessary for a complete vapor barrier to be achieved). With Slab-Shield there's no time wasted installing 4 Feet x 8 Feet foam boards. With a puncture resistance of 92.9psi you can work and walk on top of it without it crumbling apart.



8. Here's a rough idea of what it will cost

Below are some pricing guidelines. These numbers are higher than most proposals, but can act as a "stand-in" as you're creating your construction budget.


Mid Efficiency (87%+) hot water boiler: $1,500 to $3,000

High Efficiency (95%+) hot water boiler: $2,200 to $5,500

Tankless Water Heater as a heat source: $1,200 to $1,700

Per zone controls: $250.00 ea. zone

Slab on grade Radiant: $1.20 per square

Wood Underfloor Radiant: $1.70 Per square

Myson radiators: $260 per 5000 BTU

People consider radiant heating for its superior economic and comfort advantages. But with energy prices rising 35%+ this year, whichever efficient system you choose, you’ll appreciate the cost savings!