As summer has gone, and the furnace needs to run again, I find myself aware
of how often the furnace is kicking on and it's only mildly cold compared to
what will be coming to us here in NH. I know I have drafty windows that I
have yet to seal, and need to check the make-shift seal around my poorly
made attic hatch. And thoughts of how much heat gets wasted up the ceiling
vent in the bathroom even though we keep it turned off most all the time...
and then wonder how well the walls were actually insulated on the pre-fab
house, especially given the quality of the window installation...
This left me wondering, and then searching the internet to no avail,... how
long SHOULD my house hold heat? Searching that out leads right into some
very complicated mathematical calculations regarding BTU's ,sizes,
efficiencies, blah blah blah, which is all great stuff for calculating but I
am interested in something far simpler. Specifically, rate at which a home
should drop in temperature when the heat is off. The purpose of which would
be to determine if there is a problem with insulation or drafts, based on
ACTUAL heat loss instead of calculating, extrapolating,... and then just
wondering and waiting for the heat bills to add up!
I'm picturing a chart (or calculator) that assumes a house that has been
kept at say 68 degrees for a long enough time so that heat loss isn't just
from the air at the thermometer into other interior objects. From there, it
would be nice to know degree drop per unit time with heat off, for
determining the homes ability to hold the heat. With such information, one
could easily determine if they need to make improvements or not! The
variable would be outside temperature. Additional variable could include
wind-chill, otherwise you'd have to test on a calm day! Sunlight would be
very significant, so this would have to be a night testing only. But the
point here would be to eliminate all the house size and BTU's, which really
just muddies the situation when just trying to determine if draft and
insulation is actually sufficient. I.E. all the calculations in the world
don't really determine if the insulation in that wall is really sufficient,
or if those windows were really installed correctly (mine are energy
efficient but some were Z'd when installed so I have to seal around them
every year). The closest I find to what I'm looking for is when someone has
been timing their furnace cycles. But that leads to furnace efficiency
complications, on to BTU's, and well... just seems it would be so much
easier to just be able to watch the inside temperature with a timer, and log
the drop over time along with the outside conditions, and determine the
buildings ability to hold the heat.
I would think this information exists somewhere, but I have yet to find it.
If you made it this far, thanks for reading! Meanwhile, I'll just go do
what I can in the "draft" department and hope propane prices don't break me
There is no simple answer, you have to *DO* the calculations. The type of
construction, doors, windows, shade, which way does the house face, raised
foundation/basement/slab, degree days, mean low and mean high temps, etc.
are all needed. There is a reason its called "Building Science". No 2 homes
are built exactly the same, and there is no such thing as a "Rule of Thumb"
other than most folks have 2 thumbs.
Here's an example of *WHY* you have to do the math.... I have a customer who
has a a home with 1100 sqft of floor space that needed 2 tons of
heating/cooling, another with 3,000sqft of floor space whos home is also
heated and cooled with 2 tons.
If you don't *DO* the math, your just guessing.
Another thing, it does make a difference what side the unit is on.
On my sunny side the brick and condenser are much hotter than they
should for cooling. The air might be 10-20 degrees hotter, and the
don't necessarily add up. If it were on the other side, way better
I should have dug a big trench and gone heat pump for cooling and
Currently electric heat is cheaper than oil or gas.
Thanks. Still it "seems" like there should be some sort of guide to
determine how efficient your home is based on rate of temerature loss!
I'm assuming (dangerous word) that your cited examples of the 1100 and 3,000
sqft homes BOTH are sufficiently insulated and airtight, but the 1100 has a
lot more doors and windows? If not, I would think it needs some
If I did "*DO* the calculations" for my home, would I then be able to know
at what rate my home should drop from 70 degrees, say on a calm evening
where the temperature outside was a steady 32 degrees?
AND, any recommendations for covering a concrete basement floor? Wondering
how effective some sort of tiles would be at insulating against heat loss
into the floor. Be it vinyl, industrial,... ? I'm pretty sure anything
like carpeting will be a mold / mild due problem, plus my basement is my
The 1100sqft home is a 60+year old basic wood frame house
The 3,000sqft home is a 6 year old geodesic dome made from 8 inch thick
Load calc programs are designed to tell you how many BTUs will be required
for your house to maintain XX indoor temperature and humidity, when you have
XX outside tempetature and humidity as well as the airflow requirements for
Floor tiles and linolium do not have insulating properties.
None at all? I would have thought even vinyl tiles would conduct heat
significantly less than bare concrete.
Now I might just have to stick one tile down (leftovers from kitchen floor)
on the concrete and compare it with my bare feet... : )
You have at least two variables.
heat loss thru the shell.
Amount of heat available to loose.
That's why typical calculations result in a BTU/hour rate of heat loss
rather than temperature rate of change.
Do this thought experiment:
Heat your house to 70 degrees. Turn off the heat and graph
the temperature over time.
Now fill your house with water at 70 degrees. Turn off the
heat and graph the temperature over time. You'll get very
different rates of temperature change.
You can't get meaningful numbers from rate of temperature
change unless you know the thermal mass.
And even if you did, you could make the math look better
by bringing a bunch of bricks inside. That's not what you want.
There are also heat sources in the house like lights, people,
So, Why do you care?
I expect that you care because of money.
You PAY for BTU's, not temperature.
Somebody has the right numbers, but as I recall,
to first order,
you lose one btu/hour for every square foot of surface
times the temperature difference across that surface
all divided by the R-value of the material.
Remembering that the temperatures at various points
differ, sometimes significantly.
Then add in the losses due to infiltration.
Again, from memory 1.08 times the cubic feet/minute
of air in/out times the difference in temperature
gives you BTU/hour.
And there are latent heat issues if you have any
evaporation/condensation going on.
If you want to take into account things like wind,
you have to be aware that the R-value can make a big
If you have a window made out of copper, the surface
temperature on both sides is likely to be the same
independent of any air flow on the surfaces.
It won't be the same temperature as EITHER the inside or outside
temperatures, but somewhere in between.
The amount of heat loss will be VERY
dependent on air flow.
If you have a window with an infinite R-value,
the temperatures on the surfaces will approach
the temperature of the air on either side.
There will much less increased loss due to
air flow as long as the air temperature at
the surface stays the same. It's a continuous
function, so for typical
R-values you'll find the wind does make a difference,
but higher R-vlaue does reduce the effect of the wind.
This is NOT the same as the increased infiltration
thru the cracks due to wind pressure. You have to add that in too.
Never calculate when it's easier to measure.
If you're concerned about cost, measure what you pay for.
The methods can be simple or complicated depending
on your ability to separate out the fuel uses.
I have a gas furnace, but everything else is electric.
Simple for me to read the meter and know how much gas
I'm using per unit time.
Turn on all the exhaust fans in the place and wander
around with a smoke generator. I use an incense stick.
If air is moving, do what it takes to stop it from moving.
Wander around with a thermometer. A non-contact infrared
thermometer is quicker, but not essential.
Find the coldest place and do what it takes to stop
what's making it cold. Move on to the next coldest area.
It's not rocket science, but it's not simple either.
Google "resfen" and see if that helps any.
A few great ideas in there, a couple of which I was already planning on...
I burn propane, so there is no meter to watch... that is why I thought
following temperature drop would be so much easier. But I guess the bottom
line is this... I'm gonna burn what I'm gonna burn so they really the
numbers don't matter. I have a couple windows in particular that I "know"
are venting heat like crazy. I used to put the shrink type plastic over
them and what a difference that would make. But since declawing the cat's
has become an inhumane thing to do,... now I use white vinyl tape around all
the cracks instead. Hey, how many BTU's per cat per unit of cat food??? I
think if I calculated that... propane just might be cheap after all ; )
When I was heating the whole house with wood via stove in basement, I had
removed much of the insulation from between the floor joists beneath the
kitchen and living rooms. Now that I'm only heating the basement
occasionally with a propane stove, I have cold floors. I don't think it's
all that much loss there though. I know I have to get the drafts under
control. I also have thought in the past that some of my exterior walls are
colder than they should be. It's a prefab house that is supposed to be
fully insulated. I wonder if it was done poorly with gabs or even cavities
Any suggestions for buying a "quick thermometer"??? Are there any elcheapo
infrared video cameras yet? ; ) It would be nice to see what the exterior
walls look like. Yet, can't imagine I'd ever open them up for
Infrared imager is the way to go. Problem is the expense.
Call up the local energy efficiency people. Utilities, govt agencies,
non-profits and try to find out who does energy audits. Some of those
guys do free audits. Some may have imaging devices.
I bought one at a swapmeet. Was designed for fire fighters to find bodies
in smoke. Doesn't work all that well. Can see blobs of heat difference,
but it has automatic gain control optimized for finding bodies.
They don't care the absolute temperature.
I ran around the house with it in winter. Can surely see the
cold areas around the doors and aluminum window frames. Can even
see wall-warts cause they're slightly warmer. After about 15 minutes
the novelty wears off. I got tired of hauling around this BIG thing and
went back to using a simple infrared thermometer.
Non-contact thermometers are getting pretty cheap. You can buy one
at lowes of home depot for under $30.
Harbor Freight had one about the size of a cigarette lighter for $6
at one of their sale events several years ago. The D:S factor is 1:1,
so you have to get right up close and personal to keep the field of view
small. I just pulled it out and compared it to my Kane May. Reads the
If you buy one, get the biggest D:S ratio you can afford.
I had single pane windows with aluminum frames. Stuffing felt weather
stripping between the window and the track mad a HUGE difference
in the draftiness of the place.
Next thing I tried was covering all of the windows with R-5 rigid insulation
Made another huge difference in heat loss. Painted the outside the
same as the house to appease the neighbors. They call my place the bat
Another advantage is that solicitors and religious zealots
think the place is unoccupied and don't stop. Although I
did worry some about burglars making the same observation.
Had a chance to get double pane windows installed last spring.
They're only R-3, so I expect they won't be as good as what I had,
but I can see out now.
I did put the R-5 stuff back on one window. As soon as it gets
cold enough to make meaningful measurements, I may do the other
windows again too.
We got to do an experiment a couple years ago when there was a heavy ice
storm. Power went off about midnight, I had been waiting for it while
listening to the steady snap-snap-THUD of tree branches breaking.
I dragged the generator to a convenient place in the garage, dragged
extension cords through the house and rewired the furnace to a plug and
went to bed. About 6 AM it started to get cold, so I fired up the
generator and ran the furnace, the fridge and freezer and charged the
cell phone. I let the generator run for one tank of gas, about 90
minutes. About 3 PM it started getting cool again, but before I was
ready to fire the genny up again, the power company got the lights back
on. So, at night, it took about 6 hours for the place to cool off, with
the sun shining, it held the temp up about 9 hours. It did a LOT better
than I would have guessed. When I say "cold" it was likely between 55
-60 F indoors.
It's a computer program to model such stuff.
Think it was originally designed for window heat loss calculations,
but think it also included walls, ceiling etc.
Been years since I played with it.
I built a flapper on a microswitch that sits on a furnace register.
Connected to an old Palm Pilot that measures the on-time of the fan,
subtracts out the extra time that the fan runs after the gas stops
then graphs the result. Calculates some percentages.
In my case, gas costs a penny a minute of run time.
The results are interesting and not intuitive, but relatively small
percentages in insulation/infiltration get washed out in the
I also put remote thermometers outside, in the attic, and under the house.
Don't have any way to easily collect the data and I'm too lazy
to manually graph it.
Haven't been thru a winter since I built it, so may learn something.
Thats all factored in for your particular climate when a proper heat
load/loss calculation is run, as well as required air flow for each room for
both heating and cooling. Yes, there are going to be different air flow
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