Email from Conrad Nobert to Bob Heath, Peter Amerongen and Rob Dumont:
Hi guys,
Well, we've pushed just about every boundary that there is to push in this
house. We have squeezed every damn piece of energy out of each component,
but there is one potential gain left that I can think of.
We're gearing up to purchase window coverings for the main floor south
windows and the second floor library. We want them for privacy and to block
out light when the space is getting drenched with it. Hunter Douglas has a
product called "duette architella". It is a honeycombed horizontal blind
that apparently has three air spaces in it. They claim that it adds R4.
Even if it's R2, that's a huge gain for the winter nights. My question is,
how can moisture possibly not build up behind these things? I would
appreciate any insights that you might have.
Thanks,
Conrad
Answer from Bob Heath
Hi Conrad,
Thanks for bringing this up as it is something that I have already been
thinking about and would have eventually asked you about.
If you were to get a true R4 from the window covering, that would be close
to the R value of a south window (R5) so the temperature of the inner window
surface would be about halfway between the room temperature and the ambient
temperature. At -30C ambient temperature, the inner window surface would be
at about -5C which is going to condense and freeze any moisture in the air.
The only way I can see that this would not occur is if there is not a tight
seal around the window covering which would allow warm room air to circulate
at the inner window surface and warm it to above the dewpoint. This, of
course, would mean that the effective R value of the window covering was not
R4. If you could get an airtight seal with your window covering, then only
the moisture trapped between the window and the window covering would be
available to condense and freeze and this might only be enough to lightly fog
the window, but getting that airtight seal with a moveable window covering
would be difficult. R2 would leave your inner window surface above freezing,
but you would likely still get condensation depending on the RH, and hence
dewpoint, of your room air.
I did an experiment in my old house last winter. I cut a piece of 2"
styrofoam to fit tightly in my kitchen window which was roughly 2 feet
square. In the morning after a very cold night I had a very thin film of
frost on the window. I suspect it was thin both because of the tight seal
which only allowed the small amount of moisture, trapped against the window
when I installed the styrofoam, to freeze and also the fact that I did not
have a humidifier on my furnace so the RH of the house air was very low
(probably 10% or less). In my case the R10 of the 2" of styrofoam was much
higher than the double-paned window, so the inner window surface temperature
was probably around -20C.
Unfortunately, it seems that the only solution to this problem is to have
the insulation on the outer window surface. Exactly the same reason why the
rule of thumb is that at least 2/3 of your wall insulation should be outside
your vapour barrier (think of your inner window surface as your vapour
barrier).
B.
Answer from Peter
These are good insights, Bob.
The COG R value of the south windows is 5.7. With the 1/3 - 2/3 rule
of thumb ( I wonder where that came from. +20C minus -40C = 60C ;
20C- 0C = 20C which 1/3 of the total difference, but we can get
condensation on the glass with no interior insulation if the RH is
high enough.) we could go up as high as R 2.75 without concern.
I think we should just try it. R2.75 would make a decent
difference. The total annual heat loss from those south facing windows
is about 3500 kWh per year. Increasing the total R value to 8.5 for
1/2 the time ( might be optimistic) would save over 1100kWh per
year. ( 16607kWh/year x 22% south window heat loss x R5.7/R8.5x.5 of
the time)
We need to figure out how to simulate the effect of an R 4 blind
without spending a bunch of money.
Oops. I need to redo those numbers. I just remembered that Hot2000
is already assuming some kind of R1 window covering. Corrected for an
assumption that we already have R1 window coverings, the potential
saving goes down to about 700kWh per year. Still nothing to sneeze at.
Peter Amerongen
Answer from Rob Dumont
Hello from a previous user of interior insulation.
1. The lower corner of the windows are the coldest spots, and it does not take much interior insulation to cause condensation.
Assuming an interior temperature of 20 C and a relative humidity of 30% in winter, the dew point for that interior air is about 2 C.
If the humidity goes up to 50%, the dew point is about 9 C.
Any point on a window that is colder than 2 C will have condensation if the RH is 30%, and any point on a window colder than 9 C will have condensation if the RH is 50%. (The dew point is the temperature at which a given sample of air will start to condense.)
2. Back in the 70s I had a small house with lots of south windows. I experimented with interior rigid insulation on the windows.
Some observations:
You need a very tight air and vapour barrier on the warm side of the insulation. I did not have either, and condensation would readily form on the window. I used a 2 inch thick piece of beadboard as the insulation. The windows were double glazed sealed units with an R value of about R2. Even more condensation would form on the window when the insulation was removed, as the warm, moist air could then, unimpeded, hit the window. The condensation would run onto the sill. I actually got quite sick from the mold that grew on the lower corners of the windows. Never again.
3. A friend used rigid insulation on the inside of his windows and the rigid insulation was left on during a sunny day. The inner window cracked from the temperature stress. My windows did not crack because they were tempered glass (patio door pieces.)
4. I very strongly doubt that an interior curtain can have an R4 value. Convective air leakage around the perimeter of the curtain will readily move air past the sides, top and bottom of the curtain and dramatically lower the R value.
5. I do think that exterior insulation is a possibility. However, Harold Orr put it in perspective for me. An exterior door mounted on the outside of the window has a materials cost of about $200 and probably about $50 worth of labour to install and weatherproof. At $250 the cost is about $14 per square foot, and that would not include the cost of any actuator to allow the exterior insulation to be controlled from indoors. We did have a house in Saskatoon that used a sliding shutter on the south side with barn door hardware. It worked all right, but the sliding shutter used a lot of valuable real estate on the south wall. See the attached photo.
6. In Europe exterior shutters are very common. Roll-up shutters are seen a lot in Germany. I don't know the R value but they do provide some R value and also sun control.
If one could come up with a low cost shutter that didn't cause moisture problems...
Hope this is of value.
Rob Dumont
