[Greenbuilding] Airtight windows

[John Salmen]

I will hazard some comments here - dangerous ground for me on this one. RH
is as the term implies is a relative humidity. Large open celerestory
residential structures can get complicated in terms of air flows and what
humidity is contained, where it is stored and where it is released (and
how).  They get even more complex with levels and clerestory heights and
exterior wall exposures.  

 

Modelling air flows gets even harder but even simple modelling gives some
decent information as to what basic provisions can be made. It is not rocket
science just providing for some flexible options.  if there is a source of
dry (dehumidified air) ceiling ducting can be used to air wash windows but
you need some source of dried air and movement and hopefully some materials
that can hold moisture without degrading.  I usually include some ducting or
plenum in clerestory structures that allows for some forced or even
facilitate passive air movement that can move a conditioned air to a top of
building. Doesn't have to be integrated but if it is it can alter a wall or
window condition and air movement within a room. Room to room passive wall
air transfer grills are also really effective if sized properly for the room
(undercutting doors just doesn't do it) and there is some draw that provides
effective air movement. Properly sized grills can provide for air transfer
with even limited draw (low cfm).

 

In moving air across surfaces - those surfaces also need to be able to do
what they are capable of in terms of moisture capacity. Large areas of wood
(ceilings, floors) are quite amazing in what they can hold and release in
terms of moisture safely with limited air flow.  Cementious products like
stucco are also quite forgiving.  Drying air flows across exterior walls
that subsequently load interior surfaces with moisture can be quite
effective in managing humidity despite high 'relative' humidity readings.
But it is all about materials and surfaces and their capacity - Gypsum,
paper surfacing and layers of latex paint are not so amazing and studies
have shown interesting growth between layers of paint...

 

Moisture is essentially about materials and their capacity to hold and
release moisture without significant deterioration (meaning some
deterioration that compromises the health of the material (or occupant).
Window manufacturers gave up the battle on that and loaded interior wood
with exterior grade fungicides that may or may not outlast the warranty
depending on the application - but may look ugly over time.  If air movement
can be directed to 'delicate' conditions (such as wood next to glass) and
effect some drying and subsequently moved across 'less delicate' surfaces
that can deal with a moisture load then you can cope with a relatively
higher degree of humidity (which can also be a level of comfort).

 

 

 

 

 

From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org]
On Behalf Of John Straube
Sent: October-28-12 7:46 PM
To: Green Building
Subject: Re: [Greenbuilding] Airtight windows

 

Hi Sacie
I assume you have double-glazed low-e argon Pellas, with wood frames?
If you have condensation problems it is very likely too high RH.  I am
curious about your comment that the RH is within the recommended range.
This could be ture, but is that recommended range might be 20%RH or 50%,
since I have seen both "recommended" for the Kansas climate. Have seen some
significant condensation on low-e Argon filled windows in a large multi-unit
building about 1.5 hr drive from Kansas City which was reported to always be
bad at this time of year.
Condensation like this tends to be worse early in the season as the moisture
from building materials is released.  In my airtight home this delay is very
noticeable (eg RH is stubbornly high until January I find), and I have to
ventilate aggressively during this time period to keep my RH within safe
levels.
As rough guidelines, when it is 30F outside, I would target 30%RH and when
it is 10F, I would look for 20%RH. It takes a while to ventilate down to
those levels, but once you get there it tends to stay.

PS I have triple glazed foam filled fiberglass framed windows. Condensation
can be made to occur on these rather high performance units merely by
letting the RH ride up to, say, 50to 55% during 30F nights.  The
condensation quantity is small but totally visible.
PSS Bob's claim that windows leak outward in cold weather is not true.  It
is often true for the second storey of homes, but rarely true for the 1st
floor of 2 stroey homes and almost never true for windows on the windward
side during windy weather.




Dr John Straube, P.Eng. 
www.BuildingScience.com

On 12-10-28 1:20 PM, Sacie Lambertson wrote:

On Sun, Oct 28, 2012 at 10:36 AM, Bob klahn <Home-NRG at dnaco.net> wrote:

Lance,

One guideline that has been implied but not stated in any reply I've read is
that whatever approach you choose, the innermost pane must be the
air-tightest to limit condensation.  The old "rule of thumb" was five times
tighter than subsequent panes, if there is any leakage.  This allows any
leakage (drying) to be to the outside.

Re Bob's note on tight windows, we're now moving into the winter heating
season.  Our windows are starting to condensate at the base on the inside.
Many of you have suggested this is the penalty of winter.  Isn't there
anything I can do about it?  With the windows I have?  Or, are there windows
out there that don't condensate in the winter?  Are there windows that don't
leak cold air to the inside from the outside glass per Bob's dictum?  Or is
this not a matter of tightness, but rather of the differences in
temperature?

thanks,  Sacie






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