Wednesday, February 17, 2010

Radiant Floors: nice, but missing the point?

This is a variation on some points that this article effectively makes, except I'm going to be a little more academic about it:
We often think of radiant floors as inherently desirable for most projects.  For us temperate climate Northerners (and Southerners in Land of the Fuego), there’s an almost universal appeal of a very warm radiant energy source in the midst of a cold thermal environment.  It’s a neurological/evolutionarily programmed tendency.  When it’s cold, we are naturally drawn to warm things.  It keeps us from freezing to death, and therefore passing on our genes more successfully.

I’d like to suggest, here in this blog posting, that radiant floor systems are poor investments in most cases, compared to a well-insulated envelope.  Radiant floors in addition to well-insulated envelopes is good, perhaps expensive insurance. 

In most cases, radiant systems are placed into, and used most effectively (from a comfort standpoint) poorly insulated houses (e.g. Eichlers!).  The reverse is also true-- if you insist on poorly insulating the house (and that means cost-engineering out good windows) then radiant floors probably are a good way to make a comfortable environment.

Good insulation IS a good radiant floor. 
In fact, it’s a radiant house, not just a radiant floor. 
A prominent reason why we feel cold inside our homes in the winter is that the temperature of our surroundings (floor, wall, ceiling) is cold.  The same goes for feeling overheated in an air-conditioned house in the summer—the surrounding elements tend to be quite warm.

What defines comfort is that somehow the heat transfer and moisture transfer between our bodies and the local environment are in some comfortable equilibrium.  There’s LOTS of research and guidelines on this (go Bears!).  When we’re surrounded by coldness (cold walls/floor/ceiling), we need something pretty warm to balance out the energy transfer to/from our bodies.  This could be in the form of air temperature (convecting heat to our bodies) or radiant elements in our floors/ceiling.  The same goes for cooling as heating. 

For a house that has indoor wall/floor/ceiling temperatures that are fairly even around 65-75 degs, this should keep us comfortable all year long, regardless of outdoor temperature.

Radiant heating should be subordinate to insulation
The choice, in my mind, is how you provide such an even radiant environment in the floors/walls/ceiling.  One way is to not insulate well, but embed some sort of heating elements into your assemblies to counterbalance all the energy loss/gain to the outside.  You would consume a lot of energy, since all the energy you’re putting into keeping the environment the same radiant temperature leaks easily to the outside. 
The other way is to insulate really well, which should achieve the same goal 99% of the time.  In such a well-insulated system, you could go ahead and install a really expensive radiant element into your assemblies, but you’d really only be using 1% of the time.  And, who knows, that might be worth the expense—not my call. 

Fun/cheap backup heating sources
Standard practice: plug loads

In the Passive House certification program, the way they handle supplementary/backup heating is via things plugged into the walls—like refrigerators, lights, hair dryers, etc.  These all heat the house.  In a well insulated house, these are all you’d need to heat the house.  Passive House is very deliberate about quantifying these loads in their houses.  In a modern lifestyle where we leave everything on all the time, the problem is not that our house would ever get cold—it’s that it would be too hot all the time. 
Humans/Dance party?

For one of my projects considering Passive House, the backup heating was actually to just invite the neighbors over for a potluck.  Humans are roughly 200 Watt space heaters, so having a bunch of folk over for dinner would do wonders for heating the house on a cold day.  If it’s a really cold day, then maybe they would get a little dance party, and the heat from a 1 hour dance-fest should last the house for a couple of days of heating needs. 

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