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Uninsulated basements are
subject to massive heat loss
- Keep the Heat in your House -
At first glance, the basement
seems like the perfect spot for a woodstove. You don't have
to worry about cleaning up woodchips and dust in your living
room, the floor of the level above will be toasty warm, and
your basement installation might put your stove closer to
your woodpile - for less hauling! Unfortunately, many stove
owners are sorely disappointed when they find themselves
continually feeding the basement stove without getting much
heat upstairs in the living area.
| If you had
a 20' x 30' basement
with concrete or concrete block walls, your heat loss
could easily be over 1 million BTUs per day through the
walls. Concrete or concrete block walls
have the same R-value as a 3/4" thick particle board. |
If your basement is truly part of your living area and is insulated and finished,
it can be treated like any other area of your home concerning stove placement.
Many basements, however, are either partially finished or not finished at all.
If this is your situation and you plan to install a woodstove in the basement
in order to heat the upstairs, this article is for you. The focus of the article
is on how to make the basement an effective and efficient spot for a woodstove
installation.
The heat loss from an uninsulated basement is similar to heat loss from a shed built with 1/2" particleboard walls (R-1.31) and no insulation. You are heating the great outdoors.
A. Measuring Heat Loss
No matter what the
BTU rating of your stove may be, the other half of the heating
equation is the amount of heat loss out from your home over
a given period of time. There are two critical factors in
determining how much heat you'll keep in your home. The first
is the difference in temperature between the inside and the
outside of the building. As you'd expect, the greater the
difference, the greater the heat loss. The second factor
is the degree of heat retention in your walls, floors, and
ceilings. To keep things relatively simple, we will focus
on heat retention in the walls.
When looking at heat
retention in a basement wall, we have to consider three portions
of the wall: the part that is above ground, the part that
is below ground but above the frostline, and the part that
is below the frostline. Obviously, the portion of the basement
wall that is below the frost line will stay warmer than the
portions that are exposed or within the frost line. In cold
climates, the frost line is typically 2-3' below grade. If
you are heating an uninsulated basement with concrete walls,
the heat loss through the concrete that is above the frost
line is astronomical. Consider the following example.
Imagine a 20' x 30' x 8'high basement
with 8" thick
concrete walls and two feet exposed (above grade). If the
temperature inside is 70° and the temperature outside is
20°, the heat
loss through just the 2' exposed portion of the wall is 15,625
BTUs per hour (370,000 BTUs per day). Let's further imagine
that the house is located in a cold winter climate where
frost extends 2' below grade. This means that the 2' above
grade and the 2' feet in the frost zone will all essentially
be exposed to the 20° outdoor temperature. The 4' that is
below the frost line will be exposed to a relatively balmy
ground temperature of 50°. With the upper 4' of the basement
wall exposed to 20°,
and the bottom 4' exposed to 50°, the total heat loss through
the cement walls would be 43,750 BTUs per hour (1,050,000
BTUs per day!). This equates to over four cords of oak or
sugar maple firewood (at 20% moisture content) to warm only
the basement over three winter months.
B. Keeping the Heat In the House
The R-value of a material is a measure
of its thermal resistance. The higher the number, the greater
the resistance and the better the insulating value. Concrete
has a very low R-value. For example:
R-value of 8" concrete
block:………….... 1.11
R-value
of 12" concrete block…………... 1.28
R value of 8" poured concrete:………......
0.64
R-value of 4"brick………………………. 0.80
R-value of 1/2" sheetrock………………..
0.45
R-value of 1/2" sheathing..........................
1.31
For comparison:
R-value of single pane glass…………....
0.91
R-value of 2" of Expanded
Polystyrene (beadboard)……..................
8.00
R-value of 3 1/2" Fiberglass Batt……… 11.00
R-value
of 1/2" Polyisocyanurate
Foil-Faced Foam(Thermax™)…............
3.30
R-values are cumulative. For example, if you were to
insulate a wall with R-11 fiberglass batts and sheath it
with 1/2" sheets
of Thermax™ and 1/2" sheetrock, the total R value would
be 14.75. The minimum insulation (R-value) recommended by
the Department of Energy for horizontal below grade surfaces
in cold climates is R-10 to R-15. In addition, the DOE recommends
R-10 to R-20 insulation for under a slab, which we have not
taken into consideration for this article.
If you are building
your house, you have the advantage of being able to insulate
properly right from the start. There are many excellent methods
for creating a well insulated basement. One method is to
install rigid-board Styrofoam® on the outside of the walls,
which will include the concrete or block in the “thermal
envelope". Insulated
concrete forms provide another option, one which incorporates
the rigid foam insulation into the basement wall structure
when the foundation is poured. But even if you are working
with an existing basement, you can do wonders by adding insulation
inside or out, wherever and however you can.
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| Uninsulated basements make for overworked stoves that consume mass quantities of cordwood and provide little useful heat in the living area. |
Insulating your basement walls to R-12 will reduce wood consumption by as much as 16 times in addition to allowing more of your hard won heat to move up into the living areas. |
| Heat Loss from 20' x 30' Basement at 20° F Outdoor Temperature |
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An uninsulated basement in a cold climate
can lose over 1 million BTUs/day. Insulation can reduce
this heat loss by 95 %
The calculations below assume
a 20' x 30' basement
with 2' above grade and uniform 8' concrete walls. Doors
or windows in poorly insulated homes would generally
make the heat loss calculations worse. The calculations
are averaged to take into consideration the fact that
outside temperatures are warmer below grade.
Concrete with no insulation (R-1.11):
20' x 30' basement
has an average heat loss of 43,750 btu/hr or 1,050,000
btu/day.
Insulate basement with two inches
beadboard (R-8.0): 20' x 30' basement has an average
heat loss of 3,240 btu/hr or 77,760 btu/day (93% decrease
in heat loss from concrete wall)
Insulate basement with 3 1/2" fiberglass batting (R-11.0):
20' x 30' basement has an average heat loss of 2,334
btu/hr or 56,016 btu/day (95% decrease in heat loss from
concrete wall) |
| Adding even a modest layer
of insulation to your basement walls will result in an
incredible reduction in heat lost through the concrete
walls. The results will be felt immediately - both in
less fuel used and in more heat in the home. |
C. An Insulated House Makes a Happy Stove
Let's return to our 20' x 30' x
8'high
basement with 8" thick concrete walls and two feet exposed
(above grade). If you were to insulate this basement with
2" of
expanded polystyrene “bead board" (R-8), the heat loss at
20°F outdoors would be decreased from 43,750 BTUsto 3,240 BTUs
per hour. At 0° outdoors, the loss would be reduced from
56,250 BTUs to about 4,200 BTUs per hour. If you were to
build 2" x
4" stud walls against the concrete walls, insulate them with
31/2" fiberglass batts, and finish them with 1/2" sheetrock,
you would increase the R value to 12. Going back to our 20° outdoor
temperature, you now reduce the heat loss even further, from
43,750 BTUs to 2,334 BTUs per hour. At 0° outdoors, the loss
would be reduced from 56,250 BTUs to 3,000 BTUs per hour.
Your wood usage during periods of 20° weather would be reduced
from over 4 cords for a cold three month period to about
1/4 cord, that's a decrease of 16 times!
Adding insulation
is one of the most cost-effective improvements you can make
to your home. The benefits are immediate, both in terms of
economics and comfort, no matter what fuel you use to heat.
With a wood stove, these benefits are even more noticeable
because you aren't
depending on a central heating system that uses energy to
move energy. No circulators, blowers, ductwork, or plumbing
are required to enjoy the radiant warmth from a soapstone
stove. Just be sure to make the most of it by keeping the
heat inside, especially in a basement.
Source: Woodstock Soapstone Company
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