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Hi everyone, 

 

My name is Andre and I’ve signed up as I’m in the process of a new build house. 
 

The new building regs are driving me mad and I’m the go between all parties, BC, architect, SAP calc guy, builder and engineer. 
 

Naturally it’s in my interest for everything to go right but my experience has been minimal thus far. 
 

Hoping to get some advice on issues I’m facing.
 

The biggest one so far is that they’ve built the pitched roof with a hybrid method. The SAP guy had put 40mm on top of three rafters and 150mm in between. So the builders followed this. I’ve come to find out that’s it’s meant to be the other way round. 
 

The council accepted the SAP calculations and I didn’t think much of it. It’s only when I researched something else that I came across the 1/3 - 2/3 rule and then dug deep into the rabbit hole. 
 

It’s really stressing me out. How can I remedy the situation without ripping the whole roof down?

 

I will naturally put a vapour control layer below the rafters. Will this be enough to correct the mistake? I don’t understand how the PIR in between the rafters being 150mm is an issue but 40mm is fine. The 40mm PIR above the rafters surely will keep these rafters warm enough for condensation not to occur?

 

any help would be much appreciated

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Welcome! 
Do you have diagrams of what was submitted to Building Control and/or the SAP calculator?

Someone should hopefully then be able to tell you if it’s ok or what to do.
 

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Of the parties mentioned Ultimately it’s BC that needs to be happy So much of materials submission is generic and Sap also 

Tick lists 

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Post individual questions in the relevant sub forums.

 

A lot of us here regard building regs as the minimum standard and aim to exceed them. 

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I have attached the sap report but also an online calculator which doesnt let me implement the buildup in the SAP report.

 

My main concern is condensation/rot. As this doesn't conform with the 2/3 external and 1/3 internal rule

 

 

IMG_5190.thumb.png.1e67ee5d9240a6f01e87bb841facada7.pngimage.thumb.png.2a4df0360f58d9eabf43a46f4e2e6116.png

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18 hours ago, Andre90 said:

The SAP guy had put 40mm on top of three rafters and 150mm in between. So the builders followed this. I’ve come to find out that’s it’s meant to be the other way round.

The sap report has all insulation between the rafters at 40mm thick.(Layer 3) Then a layer of 150mm insulation below that is layer 4, there are no rafters or wood present, so this must be below the rafter.

 

There is no insulation above the rafter.

 

Layer 5 and 6 are the wrong way around?

 

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19 hours ago, Andre90 said:

Hi everyone, 

 

My name is Andre and I’ve signed up as I’m in the process of a new build house. 
 

The new building regs are driving me mad and I’m the go between all parties, BC, architect, SAP calc guy, builder and engineer. 
 

Naturally it’s in my interest for everything to go right but my experience has been minimal thus far. 
 

Hoping to get some advice on issues I’m facing.
 

The biggest one so far is that they’ve built the pitched roof with a hybrid method. The SAP guy had put 40mm on top of three rafters and 150mm in between. So the builders followed this. I’ve come to find out that’s it’s meant to be the other way round. 
 

The council accepted the SAP calculations and I didn’t think much of it. It’s only when I researched something else that I came across the 1/3 - 2/3 rule and then dug deep into the rabbit hole. 
 

It’s really stressing me out. How can I remedy the situation without ripping the whole roof down?

 

I will naturally put a vapour control layer below the rafters. Will this be enough to correct the mistake? I don’t understand how the PIR in between the rafters being 150mm is an issue but 40mm is fine. The 40mm PIR above the rafters surely will keep these rafters warm enough for condensation not to occur?

 

any help would be much appreciated

What do your drawings say? That's what the builder should follow nothing else.

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Well my wonderful builder has built it the way he has now (40mm PIR above rafters). And to be fair I dont blame him, when he asked about insulation BC always would say "refer to SAP" so he got used to just following SAP as that is what building control was interested in. I didn't question it because I didn't know any better.

 

My question is, if I put 150mm PIR between the rafters as well as the 40mm thats above (hybrid roof), will a VCL work?

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Refer to the SAP for the type and thickness of the insulation - not the construction of the roof FFS!!!! - if your builder built from the SAP he should be given a kick up the hole!

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Put the 150 between the rafters - leave a 50mm ventilation gap above and the 40mm below or use insulated plasterboard!

Edited by ETC
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19 minutes ago, ETC said:

Put the 150 between the rafters - leave a 50mm ventilation gap above and the 40mm below or use insulated plasterboard!

so you are saying to have external and internal PIR as well as in between the rafters?

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I’ve never heard of anyone building from the SAP, they usually build from the construction drawings from your architect…

 

Edited by Jilly
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1 hour ago, Jilly said:

I’ve never heard of anyone building from the SAP, they usually build from the construction drawings from your architect…

 

I realise this myself now hence why I am trying to find a solution to the mistake that has been made.

 

Since whats been created is a hybrid roof, does anyone have any knowledge of what would happen if I was to put 150mm in between the rafters? Aparently its fine to match the insulation of the warm deck in between the rafters, so me putting 40mm in between is perfectly ok. Only problem is that would only achieve a U value of 0.25wm2

 

To put it simply the mistake thats been made is the 1/3 to 2/3 rule has been done the other way round.

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You need to go back to your architect and get them to advise you on a course of action that they are happy with. The builder then needs to apply the fix which is either undo what’s been done and build it as per the construction drawings or apply whatever solution the architect has advised you to do. This might also mean employing a separate professional to look at the problem too and advise a solution. 

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1 hour ago, Andre90 said:

I realise this myself now hence why I am trying to find a solution to the mistake that has been made.

 

Since whats been created is a hybrid roof, does anyone have any knowledge of what would happen if I was to put 150mm in between the rafters? Aparently its fine to match the insulation of the warm deck in between the rafters, so me putting 40mm in between is perfectly ok. Only problem is that would only achieve a U value of 0.25wm2

 

To put it simply the mistake thats been made is the 1/3 to 2/3 rule has been done the other way round.

`You might have to get a condensation risk analysis done, to answer that question properly. 

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your right to be concerned, id say the builder did it deliberately as it would have been a lot cheaper method.

 

Dew point will definitely now be inside the timbers. 

 

Easy fix though, remove covering and just add another 150mm insulation on top then re-cover.

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2 hours ago, Dave Jones said:

Easy fix though, remove covering and just add another 150mm insulation on top then re-cover.

 

How is that easy?  It will need scaffold and will raise the height of the roof and affect fascia / soffit and abutments.  Maybe I misunderstood you?

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23 hours ago, Mr Punter said:

 

How is that easy?  It will need scaffold and will raise the height of the roof and affect fascia / soffit and abutments.  Maybe I misunderstood you?

This is the exact issue I have. I was thinking to cut the 40mm PIR from the inside and render it useless (for airflow) and then put 200mm PIR in between the rafters

 

My rafters are 200mm deep but because the PIR has been added on top with an extra batten, that would leave more than 50mm for airflow.

 

its such a waste of insulation though. 

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On 28/07/2024 at 11:39, Mr Punter said:

 

How is that easy?  It will need scaffold and will raise the height of the roof and affect fascia / soffit and abutments.  Maybe I misunderstood you?

 

which is why the builder did it like he did.

 

As you had an architect i expect they did crystal clear fully detailed drawing of how to build it leaving no margin for doubt!!!

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You can ask the insulation manufacturer eg recticel to do a risk analysis they do this for free. Tell them what you have already and where you live and wait a couple of weeks if you can for a very detailed report and suggestions. 
my architect also got the insulation the wrong way round and BC didn’t query it. Luckily the architect got a few more things wrong like no fire engine turning circle so I went through everything double checking and he refunded the SAP fee. 
we’re still not out of the ground yet so hopefully no more hidden architect surprises. 
You can try another manufacturer as well for speed of reply kingspan and unicel etc will all do a risk analysis and the products are interchangeable. 

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What do your drawings state? That is the builders go to document. If they haven't built to drawing, the build is wrong.

 

It is the builders responsibility to fix and make good, so your build complies with the approved drawings.

 

In planning you are only allowed to build to the agreed specifications, deviation requires approval.

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