Can a cold bridge trigger a BC fail?

[epsilonGreedy]

I understand that cold bridges are a bad thing that can drag down a building's overall thermal performance but I would like to know whether cold bridges are an independent pass/fail item of inspection. Can a building control inspector refuse to give a newbuild a final sign-off because of a cold bridge or must a building's overall thermal performance derived from the final EPC score?

 

I do not have a problem at the moment but the type of window cill I order will influence the degree of cold bridging at windows. I would like to fit a special type of artstone window cill that reaches back 70mm into my 100mm cavities. This will provide a nice support plinth for my sash windows that will be inset a full break reveal i.e. tucked behind the facing bricks. These cills are 100mm high unlike the industry standard 150mm, so this reduces the cold bridge area somewhat.

 

To compensate for the reduced cavity depth at the cill I was planning to insert 30mm of PIR foam sheet between the rear of the cill and the inner block wall. If the R value of PIR is (guess) about twice that of a U37 cavity batt then the effective cavity insulation at the cill is still about half that of full fill U37 cavity batts.

[A_L]

33 minutes ago, epsilonGreedy said:

I would like to know whether cold bridges are an independent pass/fail item of inspection.

 

No, they are not. If using non-approved details a method using the area of external fabric elements is normally used. In a heavily insulated building it can easily add 30%-50% of the fabric heat loss to the total heat loss.

[tonyshouse]

I doubt that they will be sharp enough to pick it up 

[epsilonGreedy]

8 minutes ago, A_L said:

In a heavily insulated building it can easily add 30%-50% of the fabric heat loss to the total heat loss.

 

 

Oo-err that much! I will have 12 sash windows with an average cill width of 1m x 0.1m high

 

I need to get a provisional SAP calc done, I know @PeterWhas previously expressed surprise I don't have one. I am beginning to think my private building control inspector has put me on "as built".

 

So time to talk to a SAP calc expert and get an idea of the thermal performance delta between conventional cills that do not extend back into the cavity and these specials for sash windows. The problem with a cold bridge is I guess that they drag down the temp of surrounding building fabric hence the heat loss is greater than that implied by the internal surface area of the components that creates the cold bridge.   

[MikeGrahamT21]

Knowing BC, they probably don't even know the meaning of a cold bridge!

[A_L]

11 hours ago, epsilonGreedy said:

Oo-err that much! I will have 12 sash windows with an average cill width of 1m x 0.1m high

 

To be clear, it is the total area of external walls, ceiling, floors, windows. The actual U-values and relative proportions of these are irrelevant.

[epsilonGreedy]

23 minutes ago, A_L said:

To be clear, it is the total area of external walls, ceiling, floors, windows. The actual U-values and relative proportions of these are irrelevant.

 

 

Now I am more confused ?  Prior to your first post in this thread I thought a few window cill related cold bridges won't have much effect on overall thermal performance because the 1.5 m2 of window cill cold bridge area is small compared to the overall 200 m2 of wall area. Then you warned that cold bridges can have a large influence on thermal performance in a passiv house and even though my house is a long way short of passiv standards I speculated that maybe cold bridges radiate cold spots outwards much further than their actual material area.

 

Anyhow this morning I used an independent u-value calculator online and found that 170mm of concrete (the cill) + 30mm PIR + 100mm light block gives a u-value of 0.55 which is better than a double glazed window.

 

 

Edited March 2, 2021 by epsilonGreedy

[ADLIan]

As mentioned above most BCOs probably wouldn't recognize a thermal bridge if it fell on them. Advise you to get a SAP assessor involved asap. Part of the assessment is to look at all of the linear thermal bridges (around windows, junctions of elements etc). Do not let the assessor use the overall default y-value for the effect of thermal bridges!

 

You cannot estimate the effect by calculating the U-value at this point - 2 dimensional thermal modelling is used to assess the junction.