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534 posts

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  #797784 11-Apr-2013 14:12
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Jaxson:
stuzzo:
It's not really about the internal temperature, that will only have a small effect. The temperature in a cold house will still be much above the outside temperature at the cold times.

The key thing is the air seal on the double/secondary glazing. It is denying moisture from being able to migrate into the window as condensation starts to form. There is not enough moisture in a sealed double glazed window for condensation to form but when the seal fails that's when it does.



Sorry mate, but that's just not the complete picture there.  Condensation will form most where you've got a cold surface and heavily moisture laden air.  The internal temperature of the room has a significant effect on the temperature of the internal most pane of glass.

Not all double glazing is filled with fully sealed air cavities, or a vacuum, or an inert gas etc.  Yes you can get condensation forming within the cavity, but the air most laden with moisture is going to be the air in the bedroom, more so than the air in the cavity.

Reducing moisture on windows specifically revolves around having the temperature of the inside pane of glass sufficiently warm enough as not to condense the moisture in the air upon contact with it.  You achieve this by heating the room, which could quite easily be 10 - 16 degrees in winter if not heated.  (For example a lot of people don't heat the room at night, but instead use an electric blanket).  In this case, you will get condensation on the windows, regardless of whether they are double glazed or not, as you have done nothing to heat the glass.

Of course, removing the moisture in the first place will also help, but my comments referred specifically to DIY double glazing.


"Don't expect unheated spare bedrooms to be condensation free just because they have double glazing."

That's what you said and it's wrong to my mind. The inner surface (to the room) won't get condensation, why?, because it is too warm as the air in the gap will be about of the average of inside and outside temperature. Condensation may form on the inside of the outer pane as the outer pane is cold (on the inside as well), that condensation must come from the air inside the room and the degree to which air can move from the room to inside the window is the key. Of course, secondary glazing that doesn't have a seal is effective to a point but that is because it creates still air that doesn't exchange with the room so readily.


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  #797787 11-Apr-2013 14:15
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stuzzo:
"Don't expect unheated spare bedrooms to be condensation free just because they have double glazing."

That's what you said and it's wrong to my mind. The inner surface (to the room) won't get condensation, why?, because it is too warm as the air in the gap will be about of the average of inside and outside temperature.



Where did the rooms heat come from?

 
 
 
 


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  #797791 11-Apr-2013 14:22
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Jaxson:
stuzzo:
"Don't expect unheated spare bedrooms to be condensation free just because they have double glazing."

That's what you said and it's wrong to my mind. The inner surface (to the room) won't get condensation, why?, because it is too warm as the air in the gap will be about of the average of inside and outside temperature.



Where did the rooms heat come from?


Even a cold sunless bedroom will still have an inside temperature way above outside on a cold night. This is from the thermal mass of the house and air circulation in the house. For example my coldest unheated room of a wooden framed house might get down to about 12 C when the outside temperature is zero.

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  #797799 11-Apr-2013 14:37
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stuzzo:
Jaxson:
stuzzo:
"Don't expect unheated spare bedrooms to be condensation free just because they have double glazing."

That's what you said and it's wrong to my mind. The inner surface (to the room) won't get condensation, why?, because it is too warm as the air in the gap will be about of the average of inside and outside temperature.



Where did the rooms heat come from?


Even a cold sunless bedroom will still have an inside temperature way above outside on a cold night. This is from the thermal mass of the house and air circulation in the house. For example my coldest unheated room of a wooden framed house might get down to about 12 C when the outside temperature is zero.


Not disagreeing with you at all on that assessment.  I'd expect that 12 degrees internal room temperature should see a bit of condensation forming for your average bedroom with one to two people sleeping in it. 

At 12 degrees internal room air temp, the window pane would be a little bit less than this also, colder still if only single glazed.  If the room was heated however, to say 20 degrees or so, then obviously this would raise the temperature of the internal window pane and you'd expect less condensation there.



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  #797858 11-Apr-2013 16:19
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A house at 12 degrees C is unhealthy, you are meant to heat it to at least 18 degrees C according to various health organization. The plug-n thermostat is great for that. BTW, the CPU fan on the heater kept the heater's own heat to the bottom half of the fins and away from the heater's thermostat so the added bonus was the thermostat sensed the true room temperature.

If you retrofit double glazing with plastic sheeting and your greatest concern is reducing condensation for health as well at visual, then you want the plastic on the outside. This is because the plastic has a higher thermal resistance, so the greatest thermal gradient will be across the plastic. Plastic outside the cavity will be warmer, plastic inside the cavity will be colder. You get the same total effect, but the cavity warmer means the cavity has less condensation.

For the ultimate thermal performance, have plastic inside and outside. But then the glass temperature will be midway between inside and outside instead of slightly higher (compared to plastic only outside) so will get marginally more condensation (compared to plastic outside).

Keep in mind that polycarbonate degrade with UV, and crack from oil. Acrylic is better and cheaper, just not as strong as polycarbonate (but still very strong).




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  #797913 11-Apr-2013 17:31
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Niel: A house at 12 degrees C is unhealthy, you are meant to heat it to at least 18 degrees


It was an example for the discussion....ie even when unseated it won't drop to outside temp.

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  #798227 12-Apr-2013 09:24
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A guy at Vic wrote an interesting thesis on retrofit double glazing: http://researcharchive.vuw.ac.nz/bitstream/handle/10063/1248/thesis.pdf




Solution Architect @Firstgas Group
All comments are my own opinion, and not that of my employer unless explicitly stated.


 
 
 
 


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  #798260 12-Apr-2013 10:05
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Yep, consumer summarised it - Nick Smith is a minister of parliament.

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  #798287 12-Apr-2013 10:29
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lokhor: A guy at Vic wrote an interesting thesis on retrofit double glazing: http://researcharchive.vuw.ac.nz/bitstream/handle/10063/1248/thesis.pdf


Yeah, top of the page of the DIY double glazing thread:
http://www.geekzone.co.nz/forums.asp?forumid=48&topicid=64936&page_no=3

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  #798316 12-Apr-2013 11:28
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On the condensation aspect in the above article by Nick Smith: 

"This research found almost all manufacturers advertise their secondary glazing products as an
effective way to stop or reduce condensation problems in winter. Secondary glazing achieves this
by increasing the temperature on the internal pane of glass. Due to this increase in temperature,water vapour in the air is less likely to form condensation on the interior glazing surface. This is
because the window surface temperature is more likely to be above the dew point of the air."

Agreed, that is surface 4, the surface inside the room.

"One problem with secondary glazing however is that, when the secondary glazing is closed, normal
room air is trapped between the double glazing space, this having the same dew point as the
room air. One of the effects of secondary glazing is to substantially reduce the temperature of
the inner surface of the original single glazing, thus greatly increasing the risk of condensation in
between the panes."

The outer internal surface, surface 2 is now colder and more likely to form condensation except the trapping of the air in the gap means access to moisture from the room is limited.

He then does some calculations to show the surface temperatures.

"In fact, the danger of condensation within the air gap can only be reduced by ensuring that both
the original windows and the secondary double glazing are efficiently sealed, and providing a
desiccant within the air gap..."



"Alternatively, the space between the panes of glass can be ventilated to the outside air to prevent
condensation forming on the inside of the outer pane (BRANZ, 1993)."

Possibly, though I have read that air humidity increases quite markedly in the early morning which is supposed to be the cause of condensation on the outside surface of IGU windows and, of course, it detracts from the thermal performance.

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  #798336 12-Apr-2013 11:56
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I used to design heating installations in the UK - I know the climates and buildings are different but the concepts are the same.

Heating 101:
Every surface in your home has a uValue (as we called it in England).  This value represents it's ability to transfer heat through its structure - glass by far is one of the worst which is why you lose so much heat through your windows in proportion to their size (most heat goes through the roof but that has a bigger surface).  Double glazing and loft insulation reduce these problems.

So by measuring the area of each surface and using the uValue of that surface you end up with a calculation as to how much heat a room will lose even if completely sealed - which they are not so then you multiply by the number of air changes per hour.

You end up with a value of KW/h to keep the room at a constant temperature which you increase by a percentage to allow for warming up a room from cold to acceptable ambient - usually 18C for living areas and 20C for bathrooms.

So if you need 4KW/h of heat, but have a heater that can only produce 2KW/h then the room is going to get cold.  If you have a large area, instead of one 4KW/h heater install two 2KW/h heaters at each end.

If you are running a heatpump at 30C and it is not reaching that temperature - your heatpump is either drastically undersized or faulty - or both!  If you are trying to heat the whole house with a 4.5KW/h heatpump and given NZ building standards - it's not likely to work. 

Heatpumps although efficient when correctly installed and sized have limitations.  The first and most important is the outside temperature - the colder it gets outside the less efficient they are and when they reach maximum efficiency they will not be able to warm the air inside sufficiently.

In summary, if you are cold you need to do one or more of the following;
1) increase the amount of heat you produce - more heaters or larger heaters
2) reduce the heat loss - double glazing, insulation
3) control air movement - install draught excluders, close doors between rooms replace downlights with LED ones that can be sealed and insulated over.

Condensation, I'm don't have a great deal of knowledge on as it's less of a problem in the UK - however this is what I understand;
The amount of water vapour in the room/area is generally only a problem when it reaches the dew point.  When this happens the water vapour is condensed and moisture is left on the surface. 

Glass (and aluminium/metal window frames) having the worst uValue is usually the coldest surface in the room which is why you see the condensation there first.  Double glazing as mentioned above creates an air barrier between the outside pane and the inside pane which reduces the transfer of heat and cold between the two panes.  As long as the inside pane temperature is above the dew point you won't get condensation.

Reducing relative humidity with a dehumidifier will not only remove some of the water vapour but will also improve the efficiency of the heating (that's another story).

At the moment I don't have a dehumidifier - I use DVS (old DVS-1).  The rooms that have DVS outlets generally remain dry, the upstairs bedrooms that do not have it get condensation.  It is my intention to get a dehumidifier to remove some of the moisture from the bedrooms and so hopefully reduce the condensation.  Even keeping them warm with electric heaters is not sufficient.

I hope my information helps - if I have made any errors - please correct me, particularly on the condensation.




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  #798478 12-Apr-2013 15:56
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Yes StarBlazer, NZ condensation issues are much greater than UK. Normal humidity here is over 80% (interesting that LCD panels are specified for operation only below about 70%...). If it gets cold enough to freeze, then the humidity will drop because it solidifies and falls to the ground (also known as snow ;-). Something that most NZ homes do not see unlike UK (we do have snow, just not where most people live).




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  #798484 12-Apr-2013 16:05
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Niel: Yes StarBlazer, NZ condensation issues are much greater than UK. Normal humidity here is over 80% (interesting that LCD panels are specified for operation only below about 70%...). If it gets cold enough to freeze, then the humidity will drop because it solidifies and falls to the ground (also known as snow ;-). Something that most NZ homes do not see unlike UK (we do have snow, just not where most people live).


If you grow up in Manchester (like me) then summer is mist and drizzle - also known as Manchester sunshine - guess what a Manchester tan is!  That's right - wet.

In NZ I guess also where you live will affect the humidity levels you experience.  Ironically, they are now saying that we are sealing our houses too well and they don't breath properly and so the moisture levels in the building increase because there is insufficient air flow! 

You can't win unless you pay a small fortune for a fully integrated HVAC system where the air is conditioned (not necessarily cooled - just the correct humidity), cleaned and ducted to every room creating an equal temperature and pressure throughout the house.




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  #798566 12-Apr-2013 19:12
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StarBlazer:
, they are now saying that we are sealing our houses too well and they don't breath properly and so the moisture levels in the building increase because there is insufficient air flow! 



I haven't heard building scientists say this. The reason houses were not built very air-tight in the past is probably because the principles weren't understood and also the materials enabling it weren't available. Even Branz has recognized that the primary cause of moisture issues in NZ climate conditions is external moisture. This was the cause of the leaky homes crisis, external moisture infiltration.

Sure you need to ventilate but this is mainly for human habitation and as mentioned in the thread, can be done well via good housekeeping, extraction and airing the house each day.

In very cold climates such as Canada dew points can be well within walls and having the interior of external walls airtight can actually protect from moisture damage as it prevents warn internal air from moving into the walls and condensing. The same principle applies in NZ but milder conditions make the effect less of a concern.

Air moving through the building envelope is always bad and is likely to increase moisture issues rather than prevent them. An example is solar drive where sun on wet walls can drive moisture into a leaky envelope.

Buildings do need to allow moisture to escape when necessary and this can be done via vapour permeable materials such as the ubiquitous gib board and a sound approach to external claddings.

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  #799085 14-Apr-2013 10:34
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stuzzo: 
The only circumstance where they might be likely to decline a wall insulation request would be something like brick veneer where the insulation would be likely to interfere with the drainage plain. In other cases, where you make an alteration you are generally required to bring the area up to current standards which might require adding building paper or other moisture barrier.


That had been my assumption before I was turned down. I wasn't expecting to have the application declined on a 1927 timber house, but it was. 

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