Posts Tagged ‘tankless’

Radiant Slab Heating – Insulating Under the Slab

Thursday, February 23rd, 2012

Radiant Heating

In-Slab Systems

Radiant Heating has come to be known as perhaps the most comfortable type of heating available today (next to laying in the sun at least).  In-slab heating is a common option to consider for basements, slab floors, and garages.  While building codes do not necessarily call for it, there is no question that  insulation under the slab will positively impact the performance of any in-slab radiant heating system.  Here is what every contractor should know about insulation and in-slab heating.

Insulating Under the Slab

As was noted in our piece Radiant Heating:  In Slab Systems – What You Need to Know About In-Slab Systems, the first thing to keep in mind is “when heating a slab floor, the goal is to efficiently heat the slab and direct as much of that heat as possible into the living/working space above it – practically speaking  6-8 foot air space above the slab floor.”

The use of an insulation barrier under the  slab is a critical component that will result in much greater efficiencies for the heating system.

Without an insulating barrier, the slab will likely be resting on a bed of sand and/or gravel.  Even though sand is not considered to be a good thermal conductor, it is also not a good insulator.  It will allow heat to escape in a direction that is not in our target area – the 6-8 foot air space above the slab floor.

Insulation options: There are a few different options to provide an insulation barrier for a slab installation.  Three of them are discussed here.

Rigid foam sheets:  If the ground is leveled off well, a 1’ thick layer of blue foam is sometimes used as an insulation layer.  The cost of material is relatively inexpensive.  Because it is typically made of closed cell foam, this can also effectively serve as a vapor barrier, but would need to be taped or sealed somehow where pieces butt together in order not to compromise this functionality.  If working outdoors, care must be taken to hold the pieces in place prior to pouring the slab.  Pieces can be crushed or broken while being walked upon during the installation phase as well, sacrificing performance.  Lastly, while a 1” thick layer of foam will typically provide an R-5 insulation value, the foam itself will still allow some heat from radiant tubing in the slab to pass through it into the ground (i.e. – someplace other than our target area).

Spray foam:  This method has many risks with it.  The foam itself should be closed-cell foam.  If it is not closed cell, it will likely lose its insulation value over time.  Spray foam has no inherent vapor barrier capability.  This would need to be added perhaps through a plastic layer both below and above the foam.  Perhaps the most difficult aspect of using spray foam is the ability to maintain a consistent thickness and density prior to pouring.  For all of the above reasons, using spray foam as an insulation barrier under a slab is strongly discouraged.

Insulating Tarp or blanket:  An insulating tarp provides a built in vapor barrier above and below an insulating InsulTarp6layer of air (much like the bubble-pack used for shipping in packages).  The upper layer of the tarp also serves as a reflective layer to help to maximize the amount of heat being directed to the target area above the floor.  Insulating tarp is easy to work with.  It can simply be unrolled over the desired area.  It can be easily cut and taped down to adjacent pieces in only a few minutes.  It can be walked on by installers without worry to integrity or performance of the product.  It does not take up as much thickness as a rigid foam sheet and best of all, it provides superior insulation performance (typically in the R-6 to R-7 range).


While the per square foot cost of the insulating tarp may be more than that of rigid foam, it’s benefits during installation (quicker and easier) and its superior insulating performance and contribution to toward the increased efficiency of the radiant heating system provides a quick payback for both the installer and the homeowner.  Going back to the goal of our in-slab system – to efficiently heat the slab (and not the earth below it) and maximize the amount of heat being directed into the air space 6-8 feet directly above the slab – it is easy to see that the use of an insulating tarp makes sense in any in-slab radiant heating system.

How to select a heat source for your radiant heating system

Monday, March 29th, 2010
Jason Murphy

Jason Murphy

Selecting a heat source for your radiant heating system is easy for large projects since efficient boilers and hydronic geothermal heat pumps offer exceptional value and performance.

Problems arise when you try to select a heat source for small radiant systems of less than 1,000 square feet.

This is a problem for our customers since many radiant heating systems are indeed small. Many customers are adding radiant heating systems to new rooms, barns, and garages.

Radiant Heat Source Options

You have the following options for radiant heat sources:

  • Boiler (Electric/Gas/LP)
  • Tankless Hot Water Heater
  • Traditional Water Heater (Electric/Gas/LP)

On-demand hot water heaters, or instant hot water heaters, are typically the first choice since they are small and most users consider these devices to be cost effective. Water heaters are often considered as an option to reduce costs. Each option has important considerations before you decide on a heat source for a radiant system.

Instant Hot Water for Radiant Systems

If you are planning to use an instant hot water heater, you need to make sure the heat loss of the radiant zone exceeds the minimum output of the water heater. For example, if your water heater modulates down to 15,000 BTUH but the 300 SF radiant zone only needs 5,000 BTUH on the coldest day, your water heater will short cycle and overheat. The solution for a small radiant zone would be to use a buffer tank or to use a traditional tank style water heater.

Water Heaters for Radiant Systems

Tank style water heaters for radiant heating systems present a much different problem. You need to make sure the water heater recovery time is sufficient to satisfy the radiant zone’s heat loss on the coldest day. Water heaters are designed to make cold water hot, and radiant systems operate with a 20 degree temperature differential, which sends warm water back to the tank.  When incoming water is warm, a water heater cannot transfer heat in the needed amount of time or at the required flow rate.

Water heater recovery is measured in gallons per hour. Flow rates for a radiant system can be calculated at 1 gallon per minute for each 10,000 BTUH of heat loss. Due to recovery time, water heaters will work only with very small radiant systems. Larger radiant systems can use water heaters but it is essential to correctly calculate heat loss and select equipment based on recovery time for a 20 degree temperature differential.

Boilers for Radiant Systems

Boilers are an efficient way to make warm water hot. However, when used with a small radiant systems, the heat produced by a boiler may greatly exceeded the radiant zone requirements. Boilers such as the Argo Electric Boiler work great for small radiant systems as you can configure the 2nd heating element with a delay based on the heat loss, thus reducing risks of short cycling. Another solution to this problem is to use a deactivated water heater as a buffer tank.

There are many reasons to use a boiler for small radiant systems. Many boilers can simultaneously heat water for both radiant systems and domestic hot water. Additionally, a boiler provides the opportunity for future expansion of your radiant system.

Important Considerations

When selecting a heat source for a small radiant system, it’s important to consider the heat loss of the radiant zone, the flow rate required to satisfy the heat loss, and the correct equipment needed to make sure your system meets both current and future requirements.

If you require any assistance please contact Eagle Mountain at 1-800-572-7831 or contact us.