[Greenbuilding] Attic heat loss mystery

nick pine nick at early.com
Sat Feb 5 13:30:19 CST 2011


Paul Ladendorf <paulladendorf at ...> wrote:

> I'm trying to figure out why my attic and heat storage lose way
more heat than expected. Today was sunny all day. The attic/storage
peaked at 85 degrees at 2:30 pm. By 4:30 it was down to 82. At 5:30
it was down to 79 and its now 7:00 and its down to 74.3. During
this time, the house lost only 3 degrees from 70 to 67 with no 
supplemental heat so obviously there was transfer to the house
(apparently just the right amount to keep the house temp steady).

So the attic floor is uninsulated? Warm air rises, but an uninsulated
floor can transmit heat down by radiation. Does attic air flow down
through the house to return to the sunspace? Is the top of
the sunspace glazing below the attic floor to make a heat trap?

> I assume This still can't account for such a rapid loss from
the attic/storage. 

It may not be an actual storage heat loss. Are you measuring
the attic air temp or the container temp? On a sunny day, with
no direct gain, it's normal to have water containers surrounded
by warmer air during the day and cooler air at night.

>If it does, then there really is no point in having a storage if
its got to give up its heat to maintain the house temperature.
In that case, I might as well just put vents in the wall that
separates the house from the sunspace because at this point
the attic is a liability (its not insulated from the house) and
not an asset. If I would have insulated the attic floor at least
I wouldn't be wasting heat keeping it up to room temperature.

Some vents in the sunspace wall could keep the house warm during
the day without fan power to bring down warm air from the attic...

> I have 150 sq ft of glass in my porch which is insulated from
the house and vents up to the attic. I have 750 gallons of water
in two liter pop bottles for heat storage up in the attic.

In full sun, 150 ft^2 of glass might transmit 150x200 = 30K Btu/h.
A 2-liter bottle has about 1.2 ft^2 of surface (which loses water
vapor over time--water-filled layflat polyethylene greenhouse air
ducts on shallow shelves would be easier to top up.) With a slow-
moving 1.5 Btu/h-F-ft^2 airfilm conductance, 1419 bottles would
have a total conductance of 2601 Btu/h-F, so the air around them
bottles would be about 30K/2601 = 11.5 F warmer than the water
during the day. In 6 hours, the bottle water temp might rise by
30K/1419/4.4 = 4.8 F.

> The attic is approx 400 cu ft. Rafters are insulated to R43 and
the 65 sq ft end wall to outside, R20. Other end wall separates
attic from loft and is r8.

If the loft is heated, the (8' tall x 50 ft^2?) attic conductance
might be 50ft^2/R42+65/20 = 4.4 Btu/h-F. If the attic air is 74.3
and it's 30 F outdoors, the attic air might be losing (74.3-30)4.4
= 195 Btu/h (not much) to the outdoors. If all that heat comes
from the bottles and none goes to the house, the water might be
74.3-195/2601 = 74.2. In full sun the next morning, the attic air
temp might be 74.2 + 11.5 = 85.7 F, close to what you measured?

> The electric co-op came out again and... did a blower door test
and my score was 760 with 940 being the benchmark (anything less

Perhaps natural air leakage of 940 cfm at 50 Pascals was the minimum
ventilation standard according to ASHRAE's old 62-1989  standard,
which required 15 cfm per occupant + 0.35 ACH for the house. BPI
says if you make the house tighter, you need to install a mechanical
ventilation system, eg a bathroom exhaust fan that turns on when
the house RH rises to 50% or CO2 rises to 1000 ppm. That's a good 
idea, since the house won't have any fresh air at all on a still day
with equal indoor and outdoor temps. ASHRAE's newer standard says
7.5 cfm/occupant + 0.01cfm/ft^2 for the house, but a house with
no smokers or volatile chemicals might have less.

Nick




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