Windows vs Cavity wall position

[Jimbo37]

I'm editing drawings (in the absence of my architect, who is poorly), to widen the cavity between block leaves from 100mm to 150mm. I noticed a couple of  descrepencies in and around the window reveals

(1) window position different between the lintol & cill (see image 1)

(2) block return at return different between the blockwork and the detail drawings (see image 2 & 3) 

My questions, please

 (1) should the block returns be on the inner or outer leaf at reveals, or can I choose? (I have a preference for outer leaf, as there is a 140mm inner leaf on back wall - but maybe this is not the only consideration)

 (2) should windows be fixed to inner or outer leaf, or can I choose?

 (3) when I widen the cavity to 150mm, there will be some of the insulation of steel insulated lintol exposed - can I skim this, or is there further detail required?

TIA, James

IMAGE 1

 

IMAGE 2 - Blockwork showing return on outer leaf at reveals

 

IMAGE 3 - Jamb Details showing return on outer leaf at reveals 

 

 

 

 

Edited February 23, 2023 by Jimbo37

[Dave Jones]

not too clear from your description, you want to make sure your cavity closer or insulation completely fills the cavity. Don't return any blocks across the cavity. Ideally you want 100% of the window set into the cavity insulation but unfortunately this would leave a gap so you have to pull it forward a little. May need longer cill to accommodate the set back position. Minimum of 70mm set back.

 

Lastly if its not too late ditch hd 90 and use a thermally broken one instead.

[Jimbo37]

Thanks Dave,

I forgot to delete the images that came after image 3, which didn't help clarity (& cant do it now)

A couple of follow ups, please

 - "Don't return any blocks across the cavity" - see image 3 above, do you think this is not good?

 - You say "pull it forward a little" - is this outward, on to outer leaf?

 - Can you recommend a thermally broken lentol - I'm new to this

 

[JohnMo]

Easy way to look at things.

 

Follow the line of insulation around the building (window is classed as insulation) there should be no break in the line upwards/downwards or vertically. It should be continuous across all elements of the building.  The more insulation the better.

 

Your block/brick returns make a gap in the insulation, so are no good as they act as a bridge from the heated inside to cold outside of the building.  The cavity should continue to the window, either full fill that area with insulation or use a cavity closures.  The window should straddle the insulation, so it gives the line of insulation mentioned above.

 

Be careful with your choice of aluminium windows, as there are mostly poor performing ones out there.  Make sure you get a whole window U value, before your commit to buying.  Many try to give you just the glazing u value of 1.2 to 1.4.  but have seen whole window U values for double glazed higher than 2, because the frame isn't well thermally broken.

[Jimbo37]

43 minutes ago, JohnMo said:

Easy way to look at things.

Follow the line of insulation around the building (window is classed as insulation) there should be no break in the line upwards/downwards or vertically.

Yep, obvious when you have been told - thank you.

[Iceverge]

6 hours ago, Jimbo37 said:

to widen the cavity between block leaves from 100mm to 150mm

 

Is this a new build?  You can go wider.  It's very cheap to do now. Also bin the cavity insulation boards and use full fill mineral wool batts or EPS beads. 

 

 

6 hours ago, Jimbo37 said:

should the block returns be on the inner or outer leaf at reveals, or can I choose? (I have a preference for outer leaf, as there is a 140mm inner leaf on back wall - but maybe this is not the only consideration)

 

None ideally. 

 

 

6 hours ago, Jimbo37 said:

when I widen the cavity to 150mm, there will be some of the insulation of steel insulated lintol exposed - can I skim this, or is there further detail required?

 

Separate concrete lintels for each leaf. 

 

Excellent case study here from green building store. 

 

https://youtu.be/gqxEUpBRNic

[Dave Jones]

18 hours ago, Jimbo37 said:

Thanks Dave,

I forgot to delete the images that came after image 3, which didn't help clarity (& cant do it now)

A couple of follow ups, please

 - "Don't return any blocks across the cavity" - see image 3 above, do you think this is not good?

 - You say "pull it forward a little" - is this outward, on to outer leaf?

 - Can you recommend a thermally broken lentol - I'm new to this

 

 

crossing the cavity with blockwork creates a massive cold bridge, you dont want that.

 

Pull the window towards the outside so it just (20mm) covers the brickwork so you can make a weathertight seal with compriband (silicone has a bad U value)

 

All the normal lintel manufacturers make them as new builds must have them now on to meet SAP.

 

https://catnic.com/products/lintels/thermally-broken-lintels

 

 

[Dave Jones]

14 hours ago, Iceverge said:

 

 

Separate concrete lintels for each leaf. 

 

 

 

ok if its render but useless for brickwork.

[Dave Jones]

18 hours ago, JohnMo said:

 

Be careful with your choice of aluminium windows, as there are mostly poor performing ones out there.  Make sure you get a whole window U value, before your commit to buying.  Many try to give you just the glazing u value of 1.2 to 1.4.  but have seen whole window U values for double glazed higher than 2, because the frame isn't well thermally broken.

 

exactly this building regs is 1.4 max and thats whole window including frame. These are not common at the moment check carefully.

[dpmiller]

so remember all that this is NI, we're still on the OLD regs, and check reveals are a must...