Insulation depth in cavity (help for a dispute with builder)

[sways81]

Hello. I am currently in a dispute with our builder regarding the level at which he has stopped the insulation in our cavity wall. The floor height finishes 150mm from the floor level in the picture attached but the insulation stops about 180mm from the floor leaving a 30mm thermal bridge around the perimeter. He is saying this is because he didn't want to go past the DPC level causing damp ingress. Does anyone have any views on this? I'm waiting for a response from building control but other advice is welcome.

Thanks in advance!

Simon

 

[Mottie]

It sounds logical to me - below the damp course and that insulation will suck up the damp. Is that the right insulation to use in a wall though - I thought wall insulation was solid sheets of stuff?

 

[sways81]

Thanks for the response. Yes, it is Knauf Earthwool dritherm cavity insulation and is designed for this purpose. I get the idea behind not going below DPC but then you have a thermal bridge which can't also be right? The screed insulation upstand wouldn't meet the wall insulation which is part of this problem.

 

[xdrive]

Thanks for the response. Yes, it is Knauf Earthwool dritherm cavity insulation and is designed for this purpose. I get the idea behind not going below DPC but then you have a thermal bridge which can't also be right? The screed insulation upstand wouldn't meet the wall insulation which is part of this problem.

I've also used dritherm earthwool insulation. 100mm for cavity.

That's what the inspector recommended as its cheaper than kingspan and does the same job

 

[Notch7]

You arent really using the correct terminology, there isnt a thermal bridge.

Where there is no insulation the cavity is just air.....which is not a thermal bridge

Usually cavity insulation is 450mm wide to fit between wall ties which is what I guess has been done.

If the insulation is lower and gets soaked at the bottom of the cavity, that would cause a serious reduction in u value of the wall.

Bear in mind you prob have 10mm floor insulation and 70mm screed to go on the block n beam floor.

If you are worried about thermal bridging at floor level put in a strip of 25mm celetex all the way around before the floor insulation goes down so it traps it in place and make it stick up so it can be trimmed to the top of the screed.

 

[sways81]

You arent really using the correct terminology, there isnt a thermal bridge.

Where there is no insulation the cavity is just air.....which is not a thermal bridge

Usually cavity insulation is 450mm wide to fit between wall ties which is what I guess has been done.

If the insulation is lower and gets soaked at the bottom of the cavity, that would cause a serious reduction in u value of the wall.

Bear in mind you prob have 10mm floor insulation and 70mm screed to go on the block n beam floor.

If you are worried about thermal bridging at floor level put in a strip of 25mm celetex all the way around before the floor insulation goes down so it traps it in place and make it stick up so it can be trimmed to the top of the screed.

Thank you for this information. My concern is that the finished floor height does not meet where the insulation finishes. The cavity (where there is no insulation) will have cold air and warm air will be drawn out from inside into it. I planned to put perimeter insulation up to screed level but this still leaves a gap. I’m sure there are some ways to sort this but if it is not done right then I would like the builder to help sort this. The photo attached illustrates more clearly where the floor finishes (bottom of door) and where there is this gap. This is the same height all around the perimeter.

Thanks,
Simon.

 

[sways81]

I've also used dritherm earthwool insulation. 100mm for cavity.

That's what the inspector recommended as its cheaper than kingspan and does the same job

Yes, it was half the price of the boarded stuff and easier to work with.

 

[Mottie]

Bear in mind you prob have 10mm floor insulation and 70mm screed to go on the block n beam floor.

I’m completely clueless but 10mm seems awfully thin to me.

 

[Notch7]

I’m completely clueless but 10mm seems awfully thin to me.

Blooming tablet keyboard

10mm would be rather thin.

100mm it should say

 

[Notch7]

Thank you for this information. My concern is that the finished floor height does not meet where the insulation finishes. The cavity (where there is no insulation) will have cold air and warm air will be drawn out from inside into it. I planned to put perimeter insulation up to screed level but this still leaves a gap. I’m sure there are some ways to sort this but if it is not done right then I would like the builder to help sort this. The photo attached illustrates more clearly where the floor finishes (bottom of door) and where there is this gap. This is the same height all around the perimeter.

Thanks,
Simon.

Yes but what arent considering is that internal blockwork will be thermalites or similar -these are insulating themselves, which is why cavity walls dont need much insulation to get down to the elemental u value as per b/regs.

So there wont be a thermal bridge.

I agree that the insulation should run a bit lower in theory, but in practice its going to make no appreceable difference to the u value.

I would prefer in cavity insulation to start a bit high, than sit down too low.




Its difficult to see the floor levels but the 2 bricks courses which must be about 150mm or so seem higher than the door cill.
I would think floor make up would be min 70mm insulation + 65mm screed + 15mm floor finish =150mm min.

 

[martin hill]

By the time the floor is built up with the different layers it be near as dam it to the dritherm. With plasterboard/ skim and skirting board you will be ok and the floor level insulation will be bridging that gap. No artist sorry but the bottom 3 the insulation/screed/flooring. yellow the plasterboard and skim. Red the skirting board so it be fine.

 

[Deanthebean]

Where's the vertical cavity closer between internal skin of lightweight blocks and external skin of masonry ?

 

[sways81]

Yes but what arent considering is that internal blockwork will be thermalites or similar -these are insulating themselves, which is why cavity walls dont need much insulation to get down to the elemental u value as per b/regs.

So there wont be a thermal bridge.

I agree that the insulation should run a bit lower in theory, but in practice its going to make no appreceable difference to the u value.

I would prefer in cavity insulation to start a bit high, than sit down too low.

Its difficult to see the floor levels but the 2 bricks courses which must be about 150mm or so seem higher than the door cill.
I would think floor make up would be min 70mm insulation + 65mm screed + 15mm floor finish =150mm min.

By the time the floor is built up with the different layers it be near as dam it to the dritherm. With plasterboard/ skim and skirting board you will be ok and the floor level insulation will be bridging that gap. No artist sorry but the bottom 3 the insulation/screed/flooring. yellow the plasterboard and skim. Red the skirting board so it be fine.

View attachment 174688

Thank you both for your advice (and the drawing - better than I could do) and apologies for not replying sooner. I think the problem was that the two bricks had large mortar joints as it is bigger than 150mm (about 180mm). To the door cill, it is 150mm. The floor will be 75mm insulation (to meet u-value), 60mm screed and 15mm floor. Building control has said in the most non-committal way, "strictly speaking the insulation should overlap the floor insulation so it appears unsatisfactory". They should have picked up on this and I think the builder got his levels wrong as to where the floor finishes. I ended up taking a couple of days off the builder's bill. Even though the impact of this might be minimal, it isn't right and we are paying a lot of money for his services - I'd expect myself to do something like this . I was more annoyed that the builder kept saying it was right and that it had just been pushed up at the exposed cavities for door openings which simply wasn't true. If he had been more forthcoming about this being an error we could have found a better resolution. Thanks again for all of your help!

Where's the vertical cavity closer between internal skin of lightweight blocks and external skin of masonry ?

Cavity closers are going to go in next and will slide in behind door frame. The door needed to go on quickly as our boiler would have been too exposed to the weather.

 

[Deanthebean]

This insulation milarky is catching everyone out.

I'm the eighties I was laying Lika6 lightweight blocks with the insulation in an inner core....

 

[John D v2.0]

floor to wall junction is a renowned thermal bridge, in theory you're supposed to run the cavity insulation a fair bit below the upstand insulation at the internal floor level, but if you're fitting in with existing floor levels it might be tricky. Even if you do that, it still has a path down to the foundations unless you do a row of insulating bricks