insulated shutters

[Dave Howorth]

Thanks Mark, thanks David. That information clarifies things a bit. I guess it's the old problem about the publication of negative results that makes it difficult to find details!

[Rob Rickey]

This is something I'm interested in, too. Back in New Hampshire, my wife made insulated roman shades for our draughty windows. These feature a sheet of aluminised mylar and some polyester insulation (think multi-foil), with strips of magnets along the edge to snap them to the architrave. They helped, but I didn't know how to measure the success. Second idea is the supply air window, which works with a flow of air (rather than trying to eliminate it). Secondary double glazing plus stack ventilation. Less of a moisture problem, and when the sun is shining you get negative u-values, but the overall case has not been proved as far as I know. Anyone with info?

[Dave Howorth]

I think the notion of supply-air windows is enticing, as is dynamic insulation (or is that active insulation, I can never remember?). But I've never seen a convincing explanation of how flow is controlled when the wind varies, nor of how the heat is recovered from the air that is expelled somewhere else. Then again, I've no experience of the technique! There are a lot of ideas that work well in some circumstances but are questionable at Passivhaus levels.

With regard to draughty windows, I think the standard advice is to cure the draughts before adding the shutters/blinds. But oddly, I had a suggestion today from somebody I respect to make a hole in the wall behind an internal shutter, to let the water vapour escape outside instead of condensing. Which still seems to make a lot of sense a few hours later.

[Chris Herring]

Supply air windows have been discussed previously on the forum eg www.aecb.net/forum/index.php?topic=1086.0  There are other threads too.

Don't quite understand the leaky window/hole in wall, Dave.  Fine with shutters shut, but what about when the shutters are open? Think I am missing something. 

[Dave Howorth]

Sorry, Chris, I should have been more explicit. The idea was to make a hole in the wall that is normally filled by a plug of insulation. The plug to be removed when the shutter is in place, ideally being displaced automatically by the act of closing the shutter.

[Mark Siddall]

Dave,
Was thinking - have you considered the pane temp of the glass resulting from the insulated shutter? One concern would be that the internal glass temp could rise to a point whereby thermal stress causes the glazing to crack, esp with triple glazed low-e. This kind of thing is known to happen when low-e is on mid pane - my concern is that you could induce the same result by using internal insulated shutters. My suspicion is that this fracture damage to the glazing may be more likely to happen in the mid morning when the occupant has not opened the shutters. Any thoughts?

Mark

[Dave Howorth]

No, I haven't considered that Mark. I think experiment would be the simplest way to determine if there is a problem and if so, it would be another factor tending towards use of external shutters (or no shutters!). I am planning to have a reflective foil surface on the outside face of the shutter, which should help to reduce heat buildup if the sun shines. Opening the shutter on time is another way to avoid problems, of course!

[Mark Siddall]

Dave,
Placing a foil on the outer face of the internal shutter could make the problem worse (more energy bounces back towards the glass).

Mark

[Dave Howorth]

I don't think so, Mark. The reflected photons still have the same frequency that enabled them to pass through the glass on the way in,so they can go out too. The ones that cause heating are those that are absorbed and re-emitted at lower frequencies/energies at which the glass is opaque and/or are emitted in other directions. Or at least, that's what I suppose.

But experiment is the best way to determine the facts. It's beginning to be cold enough to do some now.

[Dave Howorth]

A long time ago in a topic far far away I wrote:

Normally the pressure on both sides of a seal is going to be pretty much the same, and the standards for rating such seals and for airtightness testing describe what to expect in general. But while the pressure on both sides of a seal may be roughly equal, there may be considerable differences in the partial pressure of water vapour (serveral hundred Pascals). In many ways the gases in the atmosphere behave independently, but do they here? Will water vapour pass through a seal according to its partial pressure difference or just as part of the total pressure difference?

I recently came across a new paper that addresses this subject. It's "Revised Model for Molecular Diffusion and Advection" by Corey et al in Vadose Zone Journal 9:85-94 (2010) http://vzj.scijournals.org/cgi/content/abstract/9/1/85

The one sentence summary as I understand it is that the answer to my question is not either/or, it's both at the same time.