Render before installing windows?

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Render before installing windows?

Postby canopy » Sun Feb 07, 2016 3:02 pm

Consider uPVC windows and AAC blocks with the windows recessed a bit so there is an external window sill. So instead of rain draining straight down the window frame to a flush exterior wall, it hits the top of the sill where render meets window frame. It seems like this point could allow water intrusion: see figure at left. So would it make any sense to sit the window on render instead of directly on the blocks to help insure the blocks stay dry: see figure at right. Curious what others have seen/done/recommend or not recommend for this.

sill.jpg
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Re: Render before installing windows?

Postby BKKBILL » Sun Feb 07, 2016 3:32 pm

I used Double 7.5 AAC blocks with air gap. The windows were installed flush to outside after rendering. With proper caulking don’t think a problem would occur either way but rendering after installing windows could make for a messy cleanup.
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Re: Render before installing windows?

Postby Sometimewoodworker » Sun Feb 07, 2016 5:54 pm

This is an answer
image.jpg
Use this


Use a correctly profiled window sill
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Re: Render before installing windows?

Postby canopy » Mon Feb 08, 2016 11:28 am

It dawned on me one possible disadvantage of putting the window on the render as in my second figure is that it creates a thermal bridge from the outside to inside through the render which may not be optimal.

The widened window sill piece idea under the frame would be an excellent solution to keep water off the seam. I'm only aware of one window manufacturer offering it so far and I bet sometimewoodworker you know who :).
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Re: Render before installing windows?

Postby schuimpge » Mon Feb 08, 2016 8:20 pm

canopy wrote:It dawned on me one possible disadvantage of putting the window on the render as in my second figure is that it creates a thermal bridge from the outside to inside through the render which may not be optimal.

The widened window sill piece idea under the frame would be an excellent solution to keep water off the seam. I'm only aware of one window manufacturer offering it so far and I bet sometimewoodworker you know who :).


Though there's no arguing with your reasoning, but IMHO, when you start to look at that kind of detail in your thermal isolation, you've quite lost the plot. Or is my POV an isolated one? :mrgreen:
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Re: Render before installing windows?

Postby canopy » Tue Feb 09, 2016 1:59 pm

schuimpge wrote:when you start to look at that kind of detail in your thermal isolation, you've quite lost the plot


If you have some experience or information to show heat transfer through render is negligible then perhaps. But otherwise the extent is quite unknown and the fact some specs like to see windows frames flush to AAC begs for at least some consideration. If you look at the picture in the 3rd post you will see good window frames go to a lot of trouble to create chambers to prevent heat exchange and they are just light plastics, not heavy mortar material.
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Re: Render before installing windows?

Postby Sometimewoodworker » Tue Feb 09, 2016 2:36 pm

canopy wrote:
schuimpge wrote:when you start to look at that kind of detail in your thermal isolation, you've quite lost the plot


If you have some experience or information to show heat transfer through render is negligible then perhaps. But otherwise the extent is quite unknown and the fact some specs like to see windows frames flush to AAC begs for at least some consideration. If you look at the picture in the 3rd post you will see good window frames go to a lot of trouble to create chambers to prevent heat exchange and they are just light plastics, not heavy mortar material.



The heat transfer through render is not unknown at all. You just need to do the math.

However as shuimpge said (and without bothering with the math) the area of render is so small that you can ignore it completely. Also windows and doors are likely to be fixed into a concrete frame that is set into the opening of the AAC blocks anyway as they gives you a much greater area for thermal conductivity to occur.

Having the window frames flush to the outside is related to shedding water.

Also the chambers in frames I posted have more to do with creating a strong light profile than preventing heat transfer.

The thermal conductivity of the PVC prevents heat transfer
Polyvinylchloride, PVC 0.19 at 25 degreesC




Thermal conductivity is a material property describing the ability to conduct heat. Thermal conductivity can be defined as

"the quantity of heat transmitted through a unit thickness of a material - in a direction normal to a surface of unit area - due to a unit temperature gradient under steady state conditions"

Thermal conductivity units is W/(m K) in the SI system and Btu/(hr ft °F) in the Imperial


Cement, mortar 1.73 at 25 degreesC


http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
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Re: Render before installing windows?

Postby canopy » Tue Feb 09, 2016 3:25 pm

The r-value of concrete is among the worst of all building materials. So while the area may be small relative to a wall face, its poor rating still triggers a bit of concern to me whether valid or not I don't yet fully understand. I wouldn't be too quick to chalk up chambers in window frames to structure only. The manufacturer of the below frame (Aluplast) states: "the five-chamber construction ensures high acoustic and thermal isolation".

aluplast.jpg
aluplast.jpg (21.99 KiB) Viewed 460 times
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Re: Render before installing windows?

Postby eyecatcher » Tue Feb 09, 2016 6:56 pm

Sometimewoodworker wrote:This is an answer
image.jpg


Use a correctly profiled window sill


That is the answer. Most of the plastic profile suppliers will have this..veka as an example.
This is a full sill; you can get a sub sill which half the length and less intrusive; much better in your case.(google subsill)

Another option would be to plant on a drip section, perfectly adequate; but reliant on a good silicone and a neat job.

Your detail however is not thought out very well. Certainly I would not render first, amongst other reasons, chances are your sills may not be level, you need to pack up the windows and you have a gap again.

In AAC blocks you could even cut your slope a tad shallower so it undercuts the botton of the window AFTER you have installed it.

Thermal bridging, I agree that this is a non issue. Thermal bridging is all about the cold weather not the hot weather, if you are concerned about heat getting through 5mm of render over 100mm thick then you may want to think about never opening your doors and windows. yes its the same. :wink:

The chambers in the plastic window sections are nothing to do with thermal bridging,(so dont believe anyone who says thats the case) they are designed to strengthen the profile. just look at the photo, each chamber supports something. when a manufacturer claims he has taken measures to include thermal bridging, you will see the biggest chamber full of expanded foam. Its just fortunate that the suppliers can use this "logical" b.s
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Re: Render before installing windows?

Postby Sometimewoodworker » Tue Feb 09, 2016 8:55 pm

canopy wrote:The r-value of concrete is among the worst of all building materials. So while the area may be small relative to a wall face, its poor rating still triggers a bit of concern to me whether valid or not I don't yet fully understand. I wouldn't be too quick to chalk up chambers in window frames to structure only. The manufacturer of the below frame (Aluplast) states: "the five-chamber construction ensures high acoustic and thermal isolation".

As eyecatcher has said the Aluplast statement is just marketing BS as far as the chambers being designed for thermal isolation.


And as to if the render has the remotest significance it doesn't. On a big window with quite thick render you are looking at an area of around 0.1 to 0.2 sq metres of render.

While you are perfectly correct about the high U-value of concrete (R-values are used for insulation and you need to state if it is an SI or US value as the U.S. Number is 6 times the SI value) compared with AAC blocks, please do the math to prove to yourself that it is insignificant.
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Re: Render before installing windows?

Postby canopy » Wed Feb 10, 2016 7:56 am

Here is a render first approach w/o subsill:

aspec.jpg
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Re: Render before installing windows?

Postby splitlid » Wed Feb 10, 2016 8:47 am

it doesn't really matter if you have the window front, middle or back.
the only difference will be the visual effect.
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