Energy Audit-Electric Underfloor Heat

I recently conducted an energy assessment on a 25 year old home with electric underfloor heat.  Back when I was working as a residential electrician, we installed several of these systems in new homes, heated garages and shops.  The first systems used a cable that we plowed into a sand bed under the slab.  (I was the “horse” that had to pull the plow, not fun.)   A few years later, the company that supplied these heating cables was sued because of a high failure rate.  A change of product design had us installing cables imbedded in a drywall panel.  These panels were made by a Canadian company who originally designed them for use in ceilings.  As it turned out, they worked underground also.  The advantage was these could be wired in parallel, meaning if one panel failed, the rest of the system could continue to operate.  If a cable failed in the old system, that entire circuit would fail.  Later an improved system emerged, one where the cables where woven into a fiberglass mesh.  Lighter and easier to handle than the drywall panels, these are occasionally still installed today.

Back to the energy assessment, the home had the old cable system installed and is still operating as designed today, but the homeowners are a little discouraged by the cost.  A recent electric bill of $350 for the month to heat 2400 square feet.  And this is at a reduced rate of $.05 per kilowatt.  That’s  more than 6500 kW for the month.  In comparison, the typical household will consume around 500 kW of general service electricity per month, lighting, refrigeration, laundry, entertainment, etc…

To understand why this home is expensive to heat with the underfloor electric, we need to examine how the system was constructed.  The home has a 5 foot concrete block frost footing, typical for my area.  To create the slab on grade, clean sand is brought in to fill inside the footings.  This soil becomes a heat sink, storing heat that is produced by the heating cables.  The cables where typically installed a few inches under the slab, in the soil, then the slab was poured.  In the case of this home, 1 inch extruded polystyrene insulation (XPS) was used to insulate the outside of the concrete frost wall.  No insulation was used anywhere under the slab.  Next, because of the reduced electric rate the customers receive, the company supplying the electricity is in control of when the heat is on.  It only heats during the overnight hours.

Lets review the 2nd Law of Thermodynamics, heat wants to move from hot to cold until it becomes equal.  In the case of the ground, there’s a lot of soil that needs to be heated to reach equilibrium.  Combine that with the low R-value of the XPS insulation on the slab edge and footing wall, some of the heat that is produced continues moving in the ground and some out the concrete footing wall.  Couple that with the heat loss of the home, 1995 wasn’t known for well air sealing details, great windows and doors, or high insulation levels, the cost to heat the home is high.

Other building components can also affect heating.  This home has a large amount of carpeting.  A check with my thermal imaging camera showed the temperature of the slab where there was no carpeting at 75°F.  The carpeting was at 85°F.  A check of the temperature under a blanket that was laying on the floor, 90°F.  All this was telling me less heat was warming the air temperature in the areas with carpet.  I’m sure if I check under the carpet, the temperature of the concrete would be nearing 100°F.  We’ve known for a long time that the right carpet pad is needed when used with in-floor or underfloor heating systems.  There is still some insulating by the right carpet and pad.  The uncovered concrete is loosing the heat to the air at a much faster rate than the concrete that is covered, but the uncovered area is much smaller than the area with carpet.  Eliminate the carpet, there’s less insulation, more heat will end up in the living space instead of being trapped under the slab.

One other disadvantage of this electric underfloor heat, it’s very slow to heat and cool.  At the time I was in this home, the outdoor temperature had risen to 40°f.  The temperature inside was nearing 80°F.  Windows were open to cool the home down.  The homeowners will be fighting warm inside temperatures for several days before the slab cooled.  Once the slab is cooled, it will take a couple days to increase the temperature of the slab should outdoor temperatures drop.  The flip side can also be true, if there was an extended power outage during cold weather, the slab would maintain a comfortable temperature for several days.

What options do the homeowners have if they decide to change heating systems?  There is currently no ductwork in the home.  Because the electric cables were installed instead of hot water tubing, there’s no way to switch heating fuels to maintain the in-floor heat.  Air source heat pump mini-splits are an option.  The home is in climate zone 7, average insulation and probably poor air sealed, a second source of heat would be needed.  A thermostatically controlled gas fireplace might be a good choice as the second source.

Personally, I like the feel of in-floor heat, it’s extremely comfortable, but my experiences with energy audits and assessments tell me there are better choices.  I’ve been in several hundred homes over the years where the cost of the heat wasn’t aligned with a homeowner’s expectation.  I’m sure part of that is many of these homes had inadequate under slab and slab edge insulation.  In my opinion, code minimums are half what should be installed.  If your installing in-floor heat in your next home, make it a hot water system.  More options for the fuel choice.  And bite the bullet, double the insulation.  You’ll have a better chance of not needing someone like me to tell you after the fact you need more insulation in an area that is no longer accessible.

Leave a Reply

Your email address will not be published. Required fields are marked *