Archive for the ‘Slab Systems’ Category

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).

Summary

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.

What You Need to Know about In-Slab Radiant Heating Systems

Thursday, February 23rd, 2012

Radiant Heating

In-Slab Systems

Radiant Heating has come to be known as perhaps the most comfortable 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.  It is also commonly found in commercial buildings and other non-residential structures. Keeping in mind that once the slab is poured, any changes to the system are very difficult and very expensive to accomplish, so careful planning and knowing what is required for both a successful installation as well as a comfortable and efficient heating system is a must.  Here is what you and your customer need to know about in-slab radiant heating systems.

What you need to know about in-slab heating

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 the 6-8 foot air space above the slab floor.

Slab System in Finished Basement

Slab System in Finished Basement

Items that need to be taken into consideration to maximize the performance of an in-slab radiant heating system:

Insulation:  Insulating beneath the slab is strongly recommended.  Ideally, the use of an insulation tarp that provides both a vapor barrier and a reflective layer over the insulation will ensure that the majority of heat suppied to the slab will make its way to the intended area, i.e, the 6-8 ft area above the slab.

See our separate white paper Radiant Heating:  In-slab Systems – Insulating Under the Slab for a more detailed discussion on this aspect of in-slab radiant heating.

Desired heated space vs. slab size: Does the slab extend beyond the area where heat is required?

A slab represents a thermal mass.  It will absorb heat as well as transmit it.  If only a portion of a slab is heated, it can be expected that some of the supplied heat will be lost to areas of the slab that are not being heated and therefore will not reach it’s intended area, i.e., the 6-8 ft area above the slab.  Sometimes this may be acceptable, but it is best to make sure that expectations of the system performance are clear.  If this is not acceptable, then the placement of thermal breaks into the slab floor should be considered by the appropriate party (homeowner, builder, designer, etc.).

For this reason, it is usually recommended that each segment of slab be treated as a single zone.  If there are thermal breaks in the slab, then multiple heating zones can be considered accordingly.

Thickness of the slab: In an in-slab radiant heating application, you are effectively transferring heat from the hydronic system into the thermal mass.  The greater the amount of thermal mass, the longer it will take to transfer enough heat to allow the mass to pass heat to the desired area (the 6-8 ft area above the floor).  We say longer because it is important to know that concrete has a certain rate of heat absorption.  If there is more of it to heat up, it is going to take a longer time to accomplish this.  The good news is that once the slab is heated, a thicker slab should give off this heat into our desired area for a longer period of time before requiring another injection of heat from our hydronic radiant system.

Size of the tubing:  From time to time, we hear about people wanting to use tubing with a larger diameter than ½” PEX in their in-slab heating application.  This is not recommended.  As mentioned above, concrete has a specific absorption rate.  We design our systems to optimize the performance of the overall system taking into account all the equipment involved with the creation and transfer of the heat.  Changing the size of the tubing will have an effect of the overall system efficiency (not necessarily a positive one) and would need to be accounted for in the overall system design. In addition, larger diameter radiant tubing may likely require a more powerful circulator in order to achieve the turbulent flow required for efficient heat transfer.   Bigger is NOT better when specifying tubing for a radiant heating system.  A larger circulator will increase the size and cost of your radiant system, and may not produce quantifiable benefits.

Manifold placement: In general, manifolds are located in or on a wall in a spot that is (a) hidden or otherwise out of the way yet (b) still accessible for occasional maintenance and (c) where it can be easily reached by the rough-in Pex-Al-Pex tubing from the mechanical room/area.   Care should be taken during the construction phase to make sure that any electrical wires are located safely away from the same area as the manifolds.

Spacing and Position of the tubing within the slab:  Spacing 12” on center using ½” Pex tubing.  Depth of tubing should be in the bottom third of the slab with the top of the tubing at least 2” to 2-1/2” below the top surface of the slab (deep enough to be below any fasteners that might be driven into the slab during or anytime after construction is completed).  It is important that the tubing not be at the very bottom of the slab where it might not be fully encased by the concrete when it is poured.  This would greatly reduce the efficiency of the heat transfer into the slab.

Heat SourceHow much heat and expected performance

Geo vs boiler system:  While boilers are capable of generating much hotter water than geo heat pumps, both types of heat sources should work very well in a radiant slab application.  If all aspects of the slab construction and radiant system installation guidelines are followed, there will be an appropriate amount of heat provided to allow the slab heating to work effectively and efficiently.

Summary

An in-slab radiant heating system can be both efficient and comfortable.  Investment in the proper components and attention to detail on all aspects of the design and installation should result in a radiant heating system that will perform optimally on a daily basis.