With warm floors, come warm hearts April 2010

Article image

Miles Dyson – Inspection Connection LC

Las Cruces Bulletin

Warm Floors, Warm Hearts

The Earth is big and the temperature of the earth under your house is, on average, a little cooler than your barefoot comfort zone.  Physicists tell us heat energy moves from warm to cool spaces.  So if your little bitty 70 degree home is nestled upon the 60 degree Earth you hear a great big sucking sound as heat and utility dollars are moved by laws of physics to the giant mass below.

Even in Las Cruces where we are primarily concerned with cooling homes, heat loss through slab floors in the winter months is one of the main consumers of utility energy dollars.

To break the heat loss cycle we need a barrier, a Thermal Barrier.  Our local residential code requires installation of rigid insulation at the edges of foundations beneath our homes.  Until I became involved in energy modeling of homes I was skeptical of the value of the dainty one inch sheet of Styro-foam installed just below the soil level around the base of my house.   I now know this insulation between the slab and the earth is effective but is really just a good first step in keeping my feet warm.

Some of the most energy efficient and comfortable homes incorporate a continuous layer of rigid insulation under the slab and foundation footings as well as the slab edge on the perimeter of the home.  Just like a camper who preps his sleeping bag spot with a roll of insulating foam, a continuous layer of insulation holds the warmth were we need it.

A LEED Gold home completed recently in Silver City incorporates ICF’s (Insulated Concrete Form) blocks as the wall support footing material at the perimeter of the home in addition to 2 inches of dense polystyrene rigid insulation board under the concrete slab.  These measures provide a complete thermal break between the floor slab and the ground.  This homeowner reports that through this colder than average winter her propane gauge hardly twitched.

Under slab insulation is inexpensive when installed at construction. Its use will typically pay for itself in reduced utility costs in climate zones like ours or cooler.  If a complete insulation layer under the slab of the home does present cost issues, a band of the same type of rigid board insulation installed at just the outer 4 foot perimeter under the slab can also benefit.  Homes designed with radiant hydronic (in floor liquid piping) or electric in-floor heating should always use under slab insulation to herd the heat into the home.

There are many types, thicknesses and grades of rigid foam board insulation.  Usually high density polystyrene (“e.g.: Styro-foam”) is used.  More exotic materials are available like poly- isocyanurate which is more expensive but provides twice the thermal resistance (R-value).  Foil faced foam board is sometimes installed under or adjacent to foundation walls.  This material is more expensive and the foil radiant barrier’s effectiveness is mitigated when there is no air space gap at one side of the foil (which is the circumstance with most slab installations).

Retro-fits to install rigid under slab insulation are not usually feasible on existing homes.  But, if you are thinking of building a home you should consider under slab installation to keep your feet warm and your heart happy!

For more information on home energy audits and green and efficient building practices you can contact Miles at amdyson@msn.com

Or you can visit these websites:

EPA – Energy Star for Homes






LEED for Homes



RESNET – Residential Energy Services Network



Inspection Connection LC

Home Inspections, Energy Rating and Residential Green Building Services