Insulating radiant panels
Radiant energy travels omni-directionally, through the path of least resistance, including in a downward fashion.
For many years — primarily during the cheap energy eras — the placement of insulation as it pertained to the myriad radiant panels was not given much thought.
If the panel did not perform to required abilities, the simple solution was to turn up the panel’s operating temperature. In many cases involving radiant floor heating systems on upper floors, this resulted in an increased back loss from the panel (downward flow of energy), which typically resulted in an underperforming floor on the area above the radiant floor and a significant back loss (actually a gain) to the area below the radiant floor.
When I was still in the installation and service business, I could count on getting at least three distressed consumer calls per year, and the conversation always went the same. “Our radiant floors don’t seem to be working well when it gets colder outside, and do you guys do air-conditioning? Our basement is exceptionally warm.”
Most consumers — and building contractors — do not equate these two problems as being one and the same. I would then ask who pulled the subfloor insulation package out and the typical response was, “Well, near the end of the project, our budget started getting too tight, and in an effort to pinch pennies, our general contractor said we could pull the insulation package out of the floor areas serviced by the radiant floors because heat rises.”
At this point, I had to go into my explanation that hot fluidsrise, like hot air and hot water, but radiant energy travels omni-directionally, through the path of least resistance, including in a downward fashion.
Put it in writing
To give you an idea about how important insulation is, I once did a job for a family that had built its home on top of a decomposing granite mountain. The basement was literally drilled and blasted to get the depth necessary to maintain adequate head height in the basement area. When we recommended the use of 1-in. extruded polystyrene insulation below the concrete basement floor, the homeowner said it was going to cost him more money in drilling and blasting than he could possibly expect to save from the placement of the XPS insulation.
We cautioned him that the system may have to run for weeks in order to charge the connected mountaintop with enough energy to stabilize the basement temperatures, and he scoffed at this suggestion. It took about 30 days of nearly continuous run time to get the basement to a stable temperature condition. He was shocked when he got his utility bill, and we reminded him that he didn’t think the conservation efforts we had proposed were necessary.
It is impossible to insulate a slab after the fact. Another disadvantage in this situation is if a control zone valve should fail, it may be weeks before the owner realizes he has no heat; and once repaired, it may once again take weeks of run time to get the zone back to a stable condition. Such is the nature of thermal energy. Mother Nature despises any differences in temperature, pressure, humidity, etc., and will do whatever she can to make them equal. Before the basement would heat up, you have to charge the area below the floor so energy starts flowing upward.
It is our job to counter Mother Nature’s efforts in order to deliver good, efficient comfort.
Many years ago, I was contacted by a local building inspector who asked me to help recommend some minimum insulation values he could then carry forward to the International Code Council for inclusion in the new energy conservation code that was under development. This was a huge step forward in requiring a minimum insulation R-value on the back or unheated side of a radiant panel. This standard is still in play today in the International Energy Conservation Code and has been added to since it was first conceived.
The unfortunate part of insulating most structural elements for the application of radiant heating and cooling is the hydronic contractor is not the same entity that will be installing the insulation (with the exception of installation in a basement concrete floor). In these situations, my recommendation is the radiant/hydronic contractor have a caveat in the contract that spells out exactly what the R-value requirements are, the method and type of insulation to be installed, and any special instructions (required air gaps, for example).
The general contractor or owner should initial the contract after that caveat indicating he understands the critical need for the insulation and accepts the responsibility to ensure it is installed to the radiant contractor’s specifications. This way, if the insulation package is yanked at the last minute, the radiant/hydronic contractor is alleviated of liability due to an underperforming radiant package that was not properly insulated.
As far as the required R-value is concerned, bear in mind the code addresses the minimum required R-values. A high-performance home will require more insulation than the minimum code requirements. A contractor must insulate to the code minimum.
The typical standard, where the energy code is not being enforced, is to require twice the R-value behind a radiant panel as there is going to be on the front of the radiant panel. I use the term “behind” as opposed to below to make a distinction for radiant walls and ceilings. Essentially, we are minimizing the unwanted back losses from the radiant panel to either a conditioned or unconditioned space.
A problem I have run into in the past is the woman of the house usually has a floor finish in mind that is not reflected on the building blueprints. All along, she had the intentions of placing large, thick, woven-wool Indian throw rugs all over your nice, new heat emitter. But if you don’t take these unscheduled R-values into consideration, it may result in an underperforming radiant floor.
Bring the consumer into the design discussions early and remind her that, other than for controlling the echoing of sounds, there isn’t much need for rugs and carpeting. If she insists on the rugs and padding, then you should specify the type of padding (hard, dense, thin, low R-value natural rubber pad) with as thin and dense a carpet as possible.
If she insists on a thick pad and carpet, such as in a bedroom, try and talk her into an alternative method of radiant comfort delivery such as radiant ceilings and/or radiant walls. In a bedroom setting, radiant ceilings make more sense. Besides, why heat the floor, cover it with thick padding and carpet, place a thick box spring and mattress above it and expect good comfort with minimal energy expenditures?
Radiant ceilings allow homeowners to cover the floor with bearskin rugs if they so desire, yet still allow them to maintain excellent radiant comfort within that space.
It is very important to be a part of the conversation early in the owner’s discovery process. He knows what he wants, but doesn’t know what to expect. It is our job as comfort delivery experts to fill those gaping questions with answers and avoid an unhappy customer in the long run.
We as an industry need to be careful about what we promise. If it is a super-insulated, high-performance home and the consumer decides to go with radiant floors based on the idea he will have “warm and toasty floors,” he will be disappointed. The floors will barely be perceptibly warm for the majority of the time. Such is the nature of the beast.
Even in cases where hydronic radiant floors are in the bathrooms, I recommend the installation of an electric radiant floor system in addition to the hydronic system in order to guarantee the consumer can and will experience warm floors in the bathing and dressing areas.
Again, it all boils down to expectations. If you promise warm floors, and the floors are only lukewarm at best when it is extremely cold outside, the consumer’s expectations may not be met. If you promise “excellent radiant comfort” regardless of the energy-emitting surface, then you can expect a happy, satisfied consumer who will tell 10 other people that you delivered excellent comfort. The key is in the details, and it is our job to control the consumer’s expectations.
Regarding “reflective” insulation, don’t waste your time or the consumer’s money. In order for an insulation package to be able to “reflect” radiant energy, the reflective foil surface must be 99.9% pure aluminum (hard to find in these days of recycling), must have 1 in. of dead air space on both sides of the reflective barrier, and the barrier should be smooth as glass and cannot have any dust on the reflective surface. If any one of these requirements is not met, the insulation will not have significant reflective qualities.