Builder does not appear to have fitted sufficient insulation in extension roof.

[The Dude]

Hi,

 

We are building a permitted development extension. The builder fitted 100mm celotex in between joists and plasterboard directly to 8" joists which has subsequently been plastered. Photos were taken at the time of installation to show to building control.

 

The building inspector says this may not be sufficient insulation and wants to see the photos.

 

Can anyone advise what options I have if the insulation is deemed insufficient? I don't want to rip out a plastered roof but I would like to comply with regulations rather than wing it with a half certified project.

 

Cheers.

[JohnMo]

27 minutes ago, The Dude said:

rather than wing it with a half certified project

No such thing, it's certified or not certified. 

 

Did you give drawings to builder?

[Conor]

100mm won't be enough. You'll need to add 50mm under the PB. As ever, what did the drawings show and did the builder follow them?

[nod]

Ask BC what thickness of insulated plasterboard will he or she be happy with 

Then overboard with insulated PB 

A better option than taking the ceiling down 

[Redbeard]

So the builder has left a 100mm ventilation gap?! Guess you don't need to worry too much about the ventilation, provided he has built adequate 'ins' and 'outs'.

 

Was the instruction to the builder clear? "Build me an extension which meets Bldg Regs ", for example?

 

If so....

 

I like to have something to at least 'cloak' the thermal bridge of the rafters, so how about pull down the ceiling, add 50mm between and 25mm below the rafters (U val probably less than 0.13W/m2K compared with Bldg Regs target of 0.16. Then you know you have surpassed the Regs req't. Then you ask the builder to pay for re-boarding and plastering.

 

By the way,

7 hours ago, The Dude said:

The building inspector says this may not be sufficient insulation

It definitely isn't. In come circs the BCO might agree to accept it, but that just means you are getting away with more heat-loss than you should have. Not good.

[The Dude]

5 hours ago, Conor said:

100mm won't be enough. You'll need to add 50mm under the PB. As ever, what did the drawings show and did the builder follow them?

 

No drawings - trusted builder to get it right.

 

I know - my mistake.

[Jilly]

There should have been an insulation inspection requested before boarding out which could have picked up the error… BC will come out fairly quickly, so that’s annoying to find out. Only rely on photo evidence if you know the inspector and they have explicitly said they are happy with this and what they are expecting to be demonstrated. Double check everything which hasn’t been signed off yet, just in case there are more oversights. They are likely to want to do more visits now. 
Check the ventilation requirement above the rafters if not having a warm roof and watch out you keep within PD roof height. If you decide to add insulated plasterboard below, ensure there will be enough headroom. 
What a pain. Not your fault as such, but is your responsibility, I hope you can find a compromise with the builder, as he will argue you didn’t provide construction drawings. 

[joe90]

1 hour ago, The Dude said:

No drawings - trusted builder to get it right.

Which is why I always did full plans and not building notice, everything agreed before starting.

[DevilDamo]

On 08/08/2024 at 13:29, nod said:

Ask BC what thickness of insulated plasterboard will he or she be happy with

Not now an option since 1st October.

[nod]

7 hours ago, DevilDamo said:

Not now an option since 1st October.

Why?

[DevilDamo]

It is now the responsibility of dutyholders, especially Principal Designers. I am sure you will still find the odd RBI giving such advice but they shouldn’t be.

[The Dude]

I have a meeting with building inspector tomorrow.

 

If I require another 50mm of Celotex below joists can I simply ad this to the existing skimmed plasterboard, re-board and re-skim?

 

Celotex themselves produce plasterboard with a 50mm Celotex backing.

[joe90]

On 10/08/2024 at 20:56, DevilDamo said:

It is now the responsibility of dutyholders, especially Principal Designers.

So is that the builder in this case as no architect or plans agreed and the customer is not qualified in these matters?

[Dave Jones]

22 minutes ago, The Dude said:

I have a meeting with building inspector tomorrow.

 

If I require another 50mm of Celotex below joists can I simply ad this to the existing skimmed plasterboard, re-board and re-skim?

 

Celotex themselves produce plasterboard with a 50mm Celotex backing.

 

send the bill to the architect if he hadn't drawn it correctly or at all. If he did the builder pays.

[joe90]

On 09/08/2024 at 08:19, The Dude said:

No drawings - trusted builder to get it right.

 

[Dave Jones]

if construction drawings weren't paid for or used then its all down to the OP.

 

penny pinching.

[joe90]

On 09/08/2024 at 09:37, joe90 said:

Which is why I always did full plans and not building notice, everything agreed before starting.

 

[Mike]

On 09/08/2024 at 09:19, The Dude said:

No drawings - trusted builder to get it right.

Some builders can't get it right even when there are drawings, unfortunately.

[joe90]

4 hours ago, Mike said:

Some builders can't get it right even when there are drawings, unfortunately.

True but that firmly puts the fault on the builder who has to then correct it at his cost.

[Dave Jones]

On 09/08/2024 at 08:19, The Dude said:

 

No drawings - trusted builder to get it right.

 

I know - my mistake.

 

it shouldnt be a massive job to overboard with 50mm anyway. 

 

Cant really blame the builder the regs are constantly changing and are different for different jobs. 

[DevilDamo]

10 hours ago, joe90 said:

So is that the builder in this case as no architect or plans agreed and the customer is not qualified in these matters?

Yes, contractor. But the client does also have their own responsibilities making sure those who are appointed are competent. 

[The Dude]

Turns out there is 50 mm of celotex above the joists and the building inspector is happy!

[Redbeard]

If you want to dot 'i's and cross 't's it may be worth asking Celotex (or the tool on Celotex's web-site) to do an interstitial condensation risk analysis (or someone on here who uses Ubakus - I am sorry, I don't) of that lay-up. If I am understanding it right you are describing a 'hybrid warm roof', with some insulation between the rafters and some above (with membrane, counter-battens and slate battens above that). The 'probably-get-it-right-without-doing calcs' rule of thumb is that two-thirds of the insulation value should be *above* the rafters and one-third between. You have the opposite, if I understood right. The theory is that, if you have it the wrong way round, there is a potential risk of interstitial condensation at the 'interface', because the 100mm is doing its job of keeping the heat away from the interface.  Do it the other way, 50mm between and 100 above, and enough heat escapes thro the 50mm to keep the interface above the interstitial condensation threshold. That's the 'rule of thumb'. Much better is a calc.