Cill Thermal Bridge

Splinter Cell

Hi all,
Making good progress with construction details, thanks to some great help from you guys. A question about cill thermal bridge details - I see 2 different recommendations on various websites including environ.ie:
1. Rear of cill sits on inner leaf with narrow strip of insulation behind and concrete infill flush to face.
2. Cill sits on outer leaf only, with insulated cavity closer behind, inner leaf continues height of cill above outer leaf.
Builder prefers option 1 (structural integrity, weight of window), engineer prefers option 2 (less thermal bridge). Both make sense to me, what do you guys think? Wall is 100 block, 40 cav, 100 PIR, 215 block. Thanks.

sinnerboy

the cill should rest at least 25mm onto the inner leaf
1. for stability
2. to fire stop the cavity

Carlow52

Splinter Cell wrote: »

Hi all,
Making good progress with construction details, thanks to some great help from you guys. A question about cill thermal bridge details - I see 2 different recommendations on various websites including environ.ie:
1. Rear of cill sits on inner leaf with narrow strip of insulation behind and concrete infill flush to face.
2. Cill sits on outer leaf only, with insulated cavity closer behind, inner leaf continues height of cill above outer leaf.
Builder prefers option 1 (structural integrity, weight of window), engineer prefers option 2 (less thermal bridge). Both make sense to me, what do you guys think? Wall is 100 block, 40 cav, 100 PIR, 215 block. Thanks.

Just posing the question here: under either option where will the window be fitted vis a vis the outer leaf/cavity/inner leaf? and will your 'standard' cill work or will they be specials?

Splinter Cell

sinnerboy wrote: »

the cill should rest at least 25mm onto the inner leaf
1. for stability
2. to fire stop the cavity

Thanks again Sinnerboy. With a 215mm std block inner leaf, how does 25mm for the cill, 90 pir, 100mm soap bar behind the cill sound? Also, what if the course directly under the cill was a lightweight block, would this be a good compromise to improve thermal bridging?

PS second time in a week that you have agreed with the builder rather than the engineer

Splinter Cell

Carlow52 wrote: »

Just posing the question here: under either option where will the window be fitted vis a vis the outer leaf/cavity/inner leaf? and will your 'standard' cill work or will they be specials?

Not got around to that yet . I do believe the window will be in the cavity, front flush with the inside of the outer leaf. I need to check about std cills but I imagine that there are std cills for 150mm cavity given how popular full fill bead insulation is becoming. On the my check list now .....

Carlow52

Splinter Cell wrote: »

Not got around to that yet . I do believe the window will be in the cavity, front flush with the inside of the outer leaf. I need to check about std cills but I imagine that there are std cills for 150mm cavity given how popular full fill bead insulation is becoming. On the my check list now .....

Thanks, if window is positioned as u surmise, how will it be fixed?

Not much grip on full fill:D

Splinter Cell

Carlow52 wrote: »

Thanks, if window is positioned as u surmise, how will it be fixed?

Not much grip on full fill:D

Not an expert here I am afraid, but I will be finding out and let you know. Not going full fill myself, 100mm PIR in 140mm cavity. Been thinking about cavity closer at reveal, thinking about 150mm L shaped cavity closer block, i.e. 50mm extends into cavity and 90mm PIR strip trapped between it and DPC/outer leaf.

sinnerboy

Splinter Cell wrote: »

Thanks again Sinnerboy. With a 215mm std block inner leaf, how does 25mm for the cill, 90 pir, 100mm soap bar behind the cill sound? Also, what if the course directly under the cill was a lightweight block, would this be a good compromise to improve thermal bridging?

PS second time in a week that you have agreed with the builder rather than the engineer

Maybe I AM your builder ... how do you know I'm not ? :pac::pac::pac:

With a 215 inner leaf there is lots of scope to place insulation behind as you say

Splinter Cell

sinnerboy wrote: »

Maybe I AM your builder ... how do you know I'm not ? :pac::pac::pac:

With a 215 inner leaf there is lots of scope to place insulation behind as you say

Not unless I married your sister

ardara1

sinnerboy wrote: »

the cill should rest at least 25mm onto the inner leaf
1. for stability
2. to fire stop the cavity

HI Sinnner -

And herein lies the problem - if there is ANY contact with cill and inner block - the detail fails - especially at such half decent U-values. (Even when using an aerated block)

Close the reveals with a proprietary closer - extend 50mm past the cill proprietary closer (rebate around horns of the cill entering block work) Move the cill off the inner leaf - insulate to back of cill. I'll try to load an image later (When I wake up!)

soldsold

Anyone have thoughts/ concerns about using 18mm waterproof plywood as a cavity closer? i have seen this on a couple of selfbuild websites.

Knauf insulation website also has this detail suggested for wide cavities.

sinnerboy

Splinter Cell wrote: »

Not unless I married your sister

That would be big of me












(geddit - bigamy :pac: )

sinnerboy

ardara1 wrote: »

HI Sinnner -

And herein lies the problem - if there is ANY contact with cill and inner block - the detail fails - especially at such half decent U-values. (Even when using an aerated block)

Close the reveals with a proprietary closer - extend 50mm past the cill proprietary closer (rebate around horns of the cill entering block work) Move the cill off the inner leaf - insulate to back of cill. I'll try to load an image later (When I wake up!)

A1 - just 1 of many clashes between structural vs thermal performance

Several Structural Engineers I have reviewed acceptable/agreeable/adorable details have commented that there is evidently no structural engineering input into them

So - i would run the risk of some mould in January vs structural failure

muffler

sinnerboy wrote: »

That would be big of me












(geddit - bigamy :pac: )

Bad jokes forum this way

---

>

Splinter Cell

sinnerboy wrote: »

That would be big of me

(geddit - bigamy :pac: )

Groan

Splinter Cell

ardara1 wrote: »

HI Sinnner -

And herein lies the problem - if there is ANY contact with cill and inner block - the detail fails - especially at such half decent U-values. (Even when using an aerated block)

Close the reveals with a proprietary closer - extend 50mm past the cill proprietary closer (rebate around horns of the cill entering block work) Move the cill off the inner leaf - insulate to back of cill. I'll try to load an image later (When I wake up!)

Hi Ardara1,
Interested to see how your image will compare with thermabate, close-r details on usual suspects web sites. While I can see the obvious thermal benefits, I struggle with the window securing - i.e. how is the window mounted and will the window load the cill no matter how careful the install? I assume these cills are the shorter type used in TF houses?

Splinter Cell

soldsold wrote: »

Anyone have thoughts/ concerns about using 18mm waterproof plywood as a cavity closer? i have seen this on a couple of selfbuild websites.

Knauf insulation website also has this detail suggested for wide cavities.

Hi SS,
I had a (quick) look at this website, are you referring to the plywood window former? I think this is removed later when the window is installed ... maybe not, will take a proper look later. Or are you referring to something else and I missed it?

soldsold

Looks like you are right about 18mm plywood being a temporary measure!

A few wide cavity closer discussions/ sites:

http://www.tonyshouse.info/mitigation.jpg
Looks like PU-backed plasterboard is the cavity closer on all 4 sides.

Pics on: http://www.tonyshouse.info/air%20sealing%20windows%20and%20doors.htm

Another discussion here:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1386&page=1

A sketch of bespoke cavity closer (from this discussion):

http://www.greenbuildingforum.co.uk/newforum/?PostBackAction=Download&AttachmentID=251

Cavity closer details from above:

1.[FONT=&quot] [/FONT]Return the inner leaf of blockwork as normal, but leave a gap up to, but not greater than the thickness of the window/door frame.
2. Ensure that a good quality vertical 225mm DPC is lapped on top of any cavity tray at DPC level on the outer leaf of masonry. Leave 50mm exposed clear of the masonry.
3. Lap the same vertical DPC into a bed of the inner leaf of masonry at Lintel level.
4. After the Masonry has hardened, push 120mm wide strips of PUR rigid insulation into the gap between the vertical DPC and the inner leaf of masonry. Use a thickness 10-20mm less than the gap width.
5. Use temporary wedges to force the insulation tight to the outer leaf and fill the remaining gap with Expanding foam.
6. The position of the window relative to the vertical DPC will vary with external finish. For render, I place the window flush to the DPC which is not trimmed until render is completed.
7. Windows can be fixed by using flat thin gauge steel. These are fixed to the outer profile of the window frame prior to installation and then screwed to the inner leaf. For those who don't like this minimal bridge you could probably get Nylon bars.

Note: On drawing above the dpc all the way up from the damp course is just for doors, not windows.



I'm pretty much lost in all this, I need to study it a bit more (or spend 2 minutes looking at one done).

Splinter Cell

Lots of different options here for cavity closers. Hard to make out any real detail about cill placement though. Carlow52 has me worried about non std cills now for wide cavity so I did a bit of research this evening. Assuming 300mm is std for 100mm cavity, only located 1 supplier of 350mm cills, up in Antrim. Notice a lot of posters on this forum have used 150mm full fill or 140mm AB platinum, where did some of you guys place your cills and what depth were they? Options seem to be 25mm on inner leaf as per Sinnerboy or on outer leaf only as per Ardara1 .....

sinnerboy

Look at metal cills . They solve the cold bridge issue without causing the structural issue

The reason you don't see them more - people don't like them - not "housey" enough .

You would need to ensure the cavity is closed with fire resistant insulation ( all around opening not just the cill ) .

http://www.atil.ie/fire/rockwool/TCB_Cavity_Barrier.pdf

http://www.birchfieldinsulation.co.uk/type_r_fire_stop.htm

http://www.theglazine.com/imagesseven/prof22cill.JPG ( pvc cill )

.

Splinter Cell

sinnerboy wrote: »

Look at metal cills . They solve the cold bridge issue without causing the structural issue

The reason you don't see them more - people don't like them - not "housey" enough .

Googled metal cills and looked at a few - not "housey" enough for my wife anyway! Mentioned something about dead and body .....

Thanks for the links to the fire resistant cavity closers, will ensure to include these in the details.

soldsold

Re: 18mm plywood window box:

http://www.knaufinsulation.co.uk/selfbuildinsulationcom/self_build_solutions/external_walls/mc01-full-fill_with_built-in_g.aspx seems to clearly show the plywood box as a permanent fixture.

It is an easy and cheap detail but is it recommended?

S.