Posted by Ralph Doncaster on January 19, 2008, 9:25 pm
For stack effect you are right. However in a winter blizzard with
50mph winds, it makes little difference.
Posted by Ralph Doncaster on January 19, 2008, 9:22 pm
On Dec 22 2007, 3:11 pm, nicksans...@ece.villanova.edu wrote:
Maybe for a slab-on-grade bungalow with heavy local shielding and
average January temps that stay above freezing.
It's a big read, but well worth looking at Tim Weber's paper:
When I factor stack effect http://en.wikipedia.org/wiki/Stack_effect
and a 5m/s avg wind speed for my house (36' from the threshold of my
basement door to the 3rd floor ceiling) and the minimal shielding from
the treeless 2 acre lot, the ratio is in the range of 4-5x, not 20X.
For a well-insulated house (i.e. R25 walls & R50 ceilings) the German
Passive House standard of 0.2-0.6ACH@50 makes a lot more sense than
the 1.5ACH@50 required for R2000. Although I haven't seen any
construction details for a Passive House, I think getting to the
0.6ACH@50 should be achievable with a sealed interior vapor barrier.
I am <1.3ACH@50 and only sealed the ceiling vapor barrier. If I had
known how much heat I'd be loosing to air infiltration I would have
used a continuous, sealed vapor barrier as well as glue/nail the
exterior OSB sheathing. The extra (labor & materials) would have been
<$00. With electricity @10.7c/kWh as my heat source, 0.6ACH@50 would
save me over $000 in a heating season vs 1.3ACH.
Posted by daestrom on December 23, 2007, 8:22 pm
Uh... Yes, Yes, and maybe??
Air exchange is simply air flow and air flow is *always* caused by a
pressure gradient. The trick is, a temperature difference between inside
and outside can create a pressure gradient. And of course there is mother
nature and wind.
Well, it can't leak in and out at the same place *at the same time*. But a
place that lets air leak in some times of the year may be letting air leak
out at other times. Or on windy days, obviously it depends from which
direction the wind hits your house, but air is leaking in one side and out
the other sides.
On cold days, the warm air in your house weighs less than the same volume of
outside air. So gravity will make the air pressure in your basement
slightly lower than the air pressure outside around your foundation. So the
pressure difference tends to push cold air into your basement. It could be
cracks between the footers and the foundation sill, gaps around wiring
entrances, seams around the basement windows, lots of places.
And once some of that air gets into your basement, it tends to 'lift' the
warm air in your house up towards the attic. If there are any places it can
get out, it will. A couple years ago I followed the advice in some
government pamphlet and went into my attic and carefully pulled back the
insulation everywhere. When I found some places where the insulation had
dirt/dust on the underside of it, I knew I had found a spot where warm air
was leaking up into my attic. Just little things, like sealing around where
some wires went down into an interior wall for a wall switch, the extra
holes in electrical boxes, holes where plumbing vent pipes come up to the
attic. Lots of little things all over, but taken as a whole must have been
equal to a four or six inch diameter hole. Even just a crack the width of a
pencil lead between wall and ceiling can be a significant air leak.
Well, if it's 70 F inside and 20F outside on a calm day, a two story house
might have a pressure difference of about 0.155 psf. That kind of pressure
difference may seem pretty small, but it would force air through a hole
about 2.3 inches in diameter at your 83 cfm. A lot of little holes can add
up to that size in no time at all.
A modest breeze of just 7 mph can create a pressure difference of about
0.122 psf even with no temperature difference inside and out.
Hope this helps...
Posted by Jeff on December 24, 2007, 5:22 am
I notice that Nicks special case formula (it's not specifically for
house air exchanges):
CFM = 16.6Avsqrt(HdT)
implies a direct relationship to wind. Although it also implies no
leakage if no wind. I'm sure there must be some residual velocity effect.
I think I can see how the wind would induce a pressure difference,
just as wings have lift.
> So the pressure difference tends to push cold air into your
Hmmm, I've done a bit to seal my unheated basement but it is still
very leaky. I see that I need to revisit this as I was not thinking
pressure, only temperature. Insulation only takes you so far.
How did you get these pressure figures, and from there to the volume
flow through the "port"?
I'm thinking I need to look carefully at my very exposed west wall
(westerly winds) and the leaks in the basement. Since the last round of
weatherizing (filling in cocoon cellulose in the walls where I missed
before) I've noticed a substantial drop in what it takes to heat some of
Posted by nicksanspam on December 24, 2007, 1:13 pm
No. Just the vent size and the height and temperature differences.