Closing Cavity at Window Cills

creedp · 26-07-2010 11:32AM #1

I'd like an opinion on the best way to deal with closing a cavity at a window cill. I have a dense block/150mm cavity/150mm Quinnlite wall - the cavity being full filled with EPS bead. Originally my builder wanted to put a 25mm strip of PIR behind the granite cill and fill the remaining cavity with mortar - oh by the way he uses a slate to close the cavity. I was concerned that this caused a serious cold bridge and he said as a compromise he would double the width of the insulation strip but said he need to fill the remaining gap with mortar to support the cill.

Anyone got any views on whether this approach is appropriate or could suggest alternatives that minimised the cold bridge problem

Thanks

Viking House · 26-07-2010 12:12PM

As per the 2008 building regs window cills aren't allowed to bridge the cavity, the external blocks on both sides of the window should hold the cill in place, no slate or concrete should be required.

Poor Uncle Tom · 26-07-2010 12:36PM

One approach

creedp · 26-07-2010 01:03PM

Thanks for the 2 responses above. I didn't (and it looks suspiciously like my buiilder didn't either!) know that the 2008 Regs didn't allow this approach - will have to get back to builder pronto - should the supervising engineer not point this out to builder on inspection or is it a case or what you don't see don't matter ...

Poor Uncle Tom can I ask what is the blue material is in you diagram - I assume it is a strip of insulation but would like to confirm. Also when you say that the blocks at both sides should hold cill on place does this mean that this blue strip does not have any structural purpose?

thanks very much

Poor Uncle Tom · 26-07-2010 01:17PM

The blue material shown on the diagram is insulation (actually it is insulation in a propriety plactic cavity closer specifically for window and door opes) which is meant to close the cavity around the ope.

It has no structural capacity.

creedp · 26-07-2010 01:28PM

Thanks Poor Uncle Tom. Would it be possible to get some details as to where this material could be sourced. Maybe you could PM me if more appropriate. I would also be interested in using this to close vertical ope opening, currently quinnlite blocks and a 25mm strip of PIR is being used for this purpose.

btw I looked at the 2008 Regs and what was proposed for cill in a full fill cavity for me looks like the cill is resting on the inner leaf with a narrow strip of insulation inside the cill - is this the danger posed by the uninitiated trying to interpret Regs?

Thanks for your help

beyondpassive · 26-07-2010 02:01PM

Arty's 200mm Cavity Wall.jpg

originally we looked at building a ply box to close the cavities, a similar detail was used in the Denby Dale house.

I'm moving away from the play box at the moment, It really depends on the thickness of windows and if they have thermally broken frames. The required level of airtighness will also dictate the vapor tightness measures.

For a concrete cill, it will sit on the outer leaf and not cross the cavity, Passive windows can be heavy strapping the reveal may not be sufficient, you may need to sit on angles or a batten. EPS 300 can be used to fill the gap between frame and outer leaf with DPC taped to window frame, with render key over it.

beyondpassive · 26-07-2010 02:10PM

Poor Uncle Tom wrote: »

One approach

I wouldn't be a fan of this approach, the window sill is timber and sits on the outer leaf. This is appropriate for south east England, but not for Ireland. The cavity closer is a good idea, <SNIP> is one such product. The insulation is not sufficient, 50mm of PIR and 18mm of phenolic composite is only an effective u-value 25yr average of about 0.32WmK. The drylining is of no use as it inhibits the ability of the structure to buffer temperature swings.

Poor Uncle Tom · 26-07-2010 03:05PM

The diagram is about window cill location and closing the cavity at the cill. Not about the material of the cill or thickness of the insulation.

You know the forum charter by now, beyondpassive, please do not name a product on thread again.

masyva · 24-06-2011 10:29PM

Poor Uncle Tom wrote: »

One approach

Hi Poor Uncle Tom,

Did you attached a file/image to your message?

I can't seem to understand your post..

Thanks

Poor Uncle Tom · 25-06-2011 12:09AM

Yeah, but it's long gone now, I'll try to get another tomorrow.

Poor Uncle Tom · 25-06-2011 03:23PM

http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/cad_comm/Brick%20-%20Non%20Flanged%20Window%20(Sill).pdf

http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/cad_comm/Brick%20-%20Brick%20Mold%20Window%20(Sill).pdf

http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/cad_comm/Brick%20-%20Brick%20Mold%20Window%20(Sill).pdf

http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/cad_comm/Brick%20-%20VF%20Non%20Flanged%20Window%20(Sill).pdf

These are just some more details I found and are not the same as the original which was removed from it's original base which I had linked to.

Supertech · 26-06-2011 08:03PM

beyondpassive wrote: »

Arty's 200mm Cavity Wall.jpg

originally we looked at building a ply box to close the cavities, a similar detail was used in the Denby Dale house.

I'm moving away from the play box at the moment, It really depends on the thickness of windows and if they have thermally broken frames. The required level of airtighness will also dictate the vapor tightness measures.

For a concrete cill, it will sit on the outer leaf and not cross the cavity, Passive windows can be heavy strapping the reveal may not be sufficient, you may need to sit on angles or a batten. EPS 300 can be used to fill the gap between frame and outer leaf with DPC taped to window frame, with render key over it.

Beyondpassive do the details you show there not make the coursing of your blockwork above had level very odd ? It looks like you've staggered the inner and outer lintels ...

beyondpassive · 26-06-2011 10:39PM

Supertech wrote: »

Beyondpassive do the details you show there not make the coursing of your blockwork above had level very odd ? It looks like you've staggered the inner and outer lintels ...

Hi Supertiech, that details over a year old now, its been through a few iterations in the interim. The head detail matches the reveal, so as to recess the window frame. There are lots of improvements made to all aspects of that detail and the other 15 wide cavity details used for common house types.

Closing Cavity at Window Cills

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