[Greenbuilding] Humidity between multiple windows

[Bob Klahn]

Eli,
The old rule of thumb for minimizing / avoiding condensation between 
windows (e.g. between prime and storm windows) has been to be sure that 
the innermost window was five times tighter than the outer.  I suspect 
that five was likely a swag, but the principle is sound.  Essentially, 
you increase the drying potential to the outside.

If summers are humid the flows could tend to reverse but, unless you 
refrigerate the living space, condensation shouldn't be an issue.

IMHO, the small amount of leakage involved in maintaining this balance 
is a reasonable compromise to avoid the moisture damage, which can get 
far more expensive.

I hope this is helpful.
Bob Klahn

On 5/9/2011 10:33 AM, elitalking wrote:
> The issue of adding storm windows or even a second window in a wall 
> thickened by exterior applied insulation is humidity in the space 
> between windows.  I have just completed an exterior applied insulation 
> resulting in a thickened wall.  The current 1995 vinyl windows are 
> around R2.  If  the same window was added near the new exterior side 
> of the wall, the cumulative R value would be R4, reducing conductive 
> heat loss by half out the windows. The question is can the humidity 
> issue be managed?  One possibility is to add desiccants in the spaces 
> we determine have problems.  This could be cycled with freshly dried 
> descant by opening the interior side window to access the space.   In 
> my case, these are mostly double hung.  Though I now understand that 
> casements are tighter, casements would not lend themselves to opening 
> with another window in the way.  Also, when the house is warm and dry, 
> the interior side window could be intermittently opened for the 
> purpose of drying middle space.  It seems to me that there is 
> potential in multiple windows to achieve the high thermal performance 
> if there are suitable ways to manage the humidity issues between 
> them.  The benefit is leaving the existing windows in place to add to 
> the cummulative total and the new window can be the modestly priced 
> average.
>
>
>
> Eli
>
> ----- Original Message ----- From: "JOHN SALMEN" <terrain at shaw.ca>
> To: <bill.allen at verizon.net>; "'Green Building'" 
> <greenbuilding at lists.bioenergylists.org>
> Sent: Friday, May 06, 2011 2:47 PM
> Subject: Re: [Greenbuilding] Exterior insulation retrofit
>
>
>> Problem with cladding on wood windows is the condensation that occurs
>> between the cladding and the wood - especially true for aluminum 
>> cladding.
>> Rot was a problem which led manufacturers to treat the interior wood 
>> with a
>> fungicide/preservative. The other problem with wood is stability for 
>> opening
>> windows. To be structurally stable wood has to be either a solid 
>> premium old
>> growth product (pine, fir, hemlock, cedar) or built up of components of
>> those materials to make them structurally strong. Warpage and 
>> movement can
>> still be a problem leading to leaky seals.
>>
>> I think fibreglass is a better option (despite the fact that I own a
>> millwork shop) as it uses less material to better effect. If you want or
>> need the look of wood then I use it in small amounts for sill or 
>> liner and
>> casings (where you can use less threatened species - maples, alders, 
>> etc.)
>>
>> Lots of fibreglass manuf. out there now with r5 windows (triple glaze).
>> Venting windows (operable) should be casement type and kept small to
>> minimize leakage. Biggest problem remains in the installation of 
>> windows as
>> they have to float in an otherwise rigid assembly so you need a proper
>> layering of gaskets and seals to allow movement and prevent 
>> infiltration. If
>> they are too rigidly installed they tend to deflect with building 
>> movement
>> and break seals (allowing infiltration).
>>
>>
>>
>> -----Original Message-----
>> From: greenbuilding-bounces at lists.bioenergylists.org
>> [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of
>> bill.allen at verizon.net
>> Sent: May-06-11 10:52 AM
>> To: Green Building
>> Subject: Re: [Greenbuilding] Exterior insulation retrofit
>>
>> All,
>> Thanks for your comments on this thread.  The one thing no one 
>> commented on
>> was the windows.  Do folks have experience with R5 windows?  Can you
>> recommend any mfg's?  I am thinking Marvin Ultimate clad (wood with 
>> aluminum
>> cladding on the outside) but they cost an arm and a leg, even more so 
>> with
>> triple glazing to reach R5. Alternatives?
>> Thanks,
>> Bill
>> -----Original Message-----
>> From: Sam Ewbank <g.l.ewbank at gmail.com>
>> Sender: greenbuilding-bounces at lists.bioenergylists.org
>> Date: Fri, 29 Apr 2011 09:32:02
>> To: Green Building<greenbuilding at lists.bioenergylists.org>
>> Reply-to: Green Building <greenbuilding at lists.bioenergylists.org>
>> Subject: Re: [Greenbuilding] Exterior insulation retrofit
>>
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