How much heat energy is lost through the floor of a house?

I am asking this as I do not recall anyone speaking of any study to this end.  Many comments about adding insulation under a house with a crawl space and the benefits of doing that.  I do not recall hearing of any study or before and after type of readings.

Heat travels towards cold and radiant heat travels in every direction, including straight down.  There is great amounts of discussion on the heat loss through walls and ceilings with lots of facts and figures.


I dont recall seeing any type of facts and figures like that from heat loss through the floor.  Did I miss that somewhere on here?


Has anyone ever applied / installed any type of insulation under a house and gathered before and after floor temperature readings from inside the house  to determine the temperature difference and energy savings??



The RCC classroom





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I just remembered seeing a report by a federal adency, many years ago, that basically said  "65% of heat is lost / gained through the roof, attic and ceiling.  35% through the walls, doors and windows'.

They made no mention of heat loss through the floor. 


Surely there must be other data besides that report.

Hi Hal,

Search "Basecalc basement heat loss".  From memory Canada (nrcan) has/had a software for calculating heat loss through basements and now I see ornl uses the same term.  But you are correct that there is a ton of data out there "somewhere".


Good morning Bud,

Took a quick look at the site and a couple others listed on the Yahoo search.  I was kind of hoping to find a general or maybe generic answer like that old report I referred to.  No such luck there.  Most seemed to be only concerned with homes that had basements.

If we can ever say that a certain percentage of heat is lost through the floors of a home, it would surely vary a great deal depending on the climate of that area.

I have never applied the RCC that I have worked with to the underside of the wood flooring of a house.  However, my counterpart in Texas had done that "A few hundred times" over the last 20 years or so.  I asked what the results were and if he had any data on the results at all.  He said all he had was the people saying, especially little old ladies, it made a great difference.  Statements like  "My tile floor isnt cold under my feet anymore", The linoleum in the bathroom and kitchen isnt cold anymore, Very comfortable now", and similar remarks.

I asked him if it made a noticeable difference in their heating bills.  He assumed so but didnt know for sure because he never asked.  I asked why not and he said he didnt need or want to because every residential he did led to 3 or 4 more residential calls and he didnt have time for those little jobs because of all the commercial jobs he had.  "Joe, you're not helpin me here buddy!"


Painters for the East Bay Regional Park District, in the SF Bay area, coated the underside of a mobile home in the Oakland Hills.  .  They did a before and after test in the winter, temps in the low 40's all that week.  The unit had been unoccupied for a few months and was as cold inside as it was outside.  Propane heater runs full blast until the thermostat is satisfied.  It took 4 hours 45 minutes to reach 72 degrees.

They sprayed the entire undercarriage and the 32 foot length of metal heating duct.  They gave it 3 days to cure and then repeated the same test and temperature readings.

It only took 45 minutes to reach 72 degrees this time.  That is a reduction in heater run-time of 84%  .

Based on that one single application and results, I am under the strong opinion that the heat loss through the floor of a mobile home is tremendous.  I am also, based on my guy in Texas statement to me, the heat loss through the wooden floor of houses is quite alot more than anyone realizes.


I would be very interested to see floor temperature readings before and after insulating the wooden floor.


With conduction that ends at the bottom of the joists and convection that is naturally suppressed due to the "warm over cold" winter condition, radiant heat loss is the only remaining path (assuming no air flow) and from articles I have read it IS substantial.  I know you know this but for our readers, radiant heat transfer occurs very rapidly.  The limiting factor is how fast the lost heat can be replaced via a path to the source.  When a basement or crawlspace is exposed to mother earth, both the surface above and the earth are radiating energy, the cooler earth radiating less of course.  It becomes the difference between the two sources that determines the basic exchange.

Add a radiant barrier and everything changes.  I just went looking but have evidently deleted a reference from ASHRAE where they provided some equivalent r-values for having a radiant barrier in the floor cavities.  I'll keep looking.

Also, here is a building science link that mentions the importance of radiant heat loss.  Maybe they provide a reference or related document.


I found the buildingscience article interesting.  Interesting that they noted the wet bottom of the fiberglass batts were caused by the humidity in the air in the crawlspace.  I also found it interesting there didnt seem to a relationship there to explain why that happened.


Wet insulation is no insulation.  The moisture build up in the fiberglass is due to the radiant heat energy from the floor passing right through the fiberglass and carrying the humidity along for the ride.  Alway humidity in air and always air in the fiberglass.


The radiant barrier needs to be against the the wood flooring.  The fiberglass is stopping some heat loss by convection, not radiation. 

In the case of a Radiant Control Coting being applied to the underside of the wood flooring, it maintains an intimate bond with the wood.  The floor stays warmer because the RCC stops the heat enrgy from radiating out to the ground.  At least that is how they are supposed to work.

Still no before and after temperature readings of the floor inside the house.  LOL  Oh well!  Maybe some day.


The RCC classroom



For self-heating homes I'm using an insulation board under the joists to be able to use that space as a duct space to move air to store heat/cold in water-filled pipes, it will reduce radiative losses by having two more layers of foil along with insulation and an air seal to separate basement air from the area between joists.

By doing remodels my experience in the Pacific NW is that it's typical for batt insulation to get soaked in many areas of the home for similar reasons, the dewpoint falls within the batting and moisture can migrate to that point, with the insulation board it eliminates that, true for exterior walls as well.

For my purposes the RCC will go on the insulation board not on the flooring because I want heat to transfer through the floor.

From your posts I'm still trying to figure out a thermal model, R-factor was measured but it's 0.083 or something like that and from examples it definitely reduced heat gain over white roofing paint so that's where most of the benefit was derived, reducing long-wave radiant energy transfer so what I'm working to include in the model.

The technique has changed radically and the crawl space now considered how to add thermal-mass to an existing home. The key is a layer of concrete blocks laid sideways and the holes are used as ducting so it can support the weight of the mass and is easy to install, using roof/attic heat to warm it is intended yet it's plugged into the existing heater to capture that air first and return to the rooms and back to the heater so most of the very hot air is used by the mass.

This is a lot better way since you can add more mass and engineer the amount of kJ's stored to match the heat-loss rate of the building. Other drawbacks to pipes were the costs & labor to install, difficulty sealing the space with plumbing & wiring and the weight preloads the floor so might need an extra girder.

The image is a brief summary of the idea, a bladder water tank is used as the main mass, these are available in any size & gallonage so what's adjusted to heat-loss rates. Some of the crawl space dirt is used as thermal mass to soak up what radiates down from the source by installing a layer of soil-contact foam insulation below about 18" of dirt, this increases the efficiency of the system a lot so worth the trouble and you find things from when the home was built:

Hello again Bud.

The ORNL test that determined the R rating of R-0.0083 was based on 'Thickness of the material applied'.  This was a generic formula for basing the thickness of a material to determine an R- rating.   For that test I applied a thickness of 12 - 15 mils if I remember correctly.


Installing insulation boards on the joists to create an air cavity, with an RCC applied to the side of the insulation board facing the wood floor?  As IO understand this.  Wouldnt the heat loss through the floor be a greater expense than the heat saved in those pipes?  OR, are you doing that to keep the water warmer, to prevent them from freezing?

An RCC sprayed directlyu om to the underside of the floor creates an intimate bond.  Now, speaking only of the coating I have worked with, an "Intimate bond" means that the RCC will penetrate into and become part of the wood.  An airtight seal that stops the heat from radiating out of the floor.

Attaching insulation board to the joists surely cant make an airtight seal, I wouldnt think.  How brittle those insulation boards are could make a differenced if one gets broken.

I would go with foam wrap around the pipes if your intention is to protect thm from freezing.  Rcc to ther wood as I mentioned. 

Dont know if this will help you or not.



Hal,  Unless you are looking at some type of cantilevered overhang or 'bump out' looking for floors will not get you much. Start looking at foundations and slabs.

Hanging any type of insulation from a floor will not help much. Insulation must be in contact with the air barrier and batts must be encapsulated on all 6 sides.  Gravity will pull the insulation away every time.

The software based on the DOE2 engine (almost all in use) will use this distinction, not one of floors; for the info you want.

Hi John,
Do those spec take into consideration spraying a Radiant Control Coating directly onto the wood before any other insulation? Do they consider an RCC at all? Or just some other type of radiant barrier material hung below or attached to the wood? The RCC should penetrate into the wood and become a part of the wood. It should NEVER giv up that bond, Our RCC can and has been applied to concrete slabs and works well there also.


I pointed you to possible data sources for heat loss information through the floor.  I merely pointed out that foundation walls might give you a better chance at data.

Now you are asking about effects of specifications for a specific material applied to the floor.  Different question.   Typically, manufacturers, such as Johns Mansville or Certainteed for fiberglass insulation follow an ASTM standard and have their own products tested by an independent lab to this standard.

Beginning your data search with a source other than the manufacturer is not the best.  Start there. Then perhaps you can find confirmation the testing is accurate or not accurate.

What Radiant Control Coating are you referring to?  A specific brand?  A generic?

Thanks for the link John.


I am the one that had to do most of the testing with the coating I have worked with for many years..  I have to refer answers I give as a generic to all RCCs but all my years of data is on one. 

I had originally hoped others would join my group below and tell of their experiences with RCCs.  That did not seem to be happening so I changed the name of the group to THE RCC CLASSROOM as that seemed to be what it turned into.. 

There are many statements there that refer to the product specifically.




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