Thermal Calcs - SAP 2009

[Zebra]

I would only use thermafleece if I were rich and breathability was a concern - in my place it isn't thanks to the materials used elsewhere.

It's quite a small area, so the extra cost is not such an issue as it would be in a larger house.

I stuggle with idea that 50mm of wool and tri iso is going to outperform 25+50mm of kingspan/celotex/ecotherm but I could be wrong.
What U value are you quoted for the multifoil and has it been verified by BRE?

Well I was quoted U-value of 0.29 for 25+50 of celotex/kingspan, and U-value of 0.15 for 50mm of wool with Triso, but as you say, this is unverified. How would I find out from BRE for a particular build-up? That was what I was hoping to do with this software but Triso isn't on it.

I would go with kingspan/celotex/ecotherm between the rafters and foamed into place then board at right angles with insulation backed plasterboard to minimise potential gaps (assuming your rafters are set at spacings that make that practical).

Thanks for your suggestions. What sort of foam? Expanding foam? Won't it stick to the rafters and mark them? Don't quite understand, probably being daft but - board at right angles? you mean under the rafters? In which case it doesn't matter what the spacings are?

 

[AMc]

You need to get an airtight seal between the foam board and the rafters or the insulation loses most of it's effectiveness - I was told 75% by my electrician which came from a Kingspan rep.
The simplest way I know of is to squirt expanding foam in the gaps - being careful not to over fill (it keeps expanding for an hour or so) and low expansion foam is probably a good idea - my builder said he'd seen over zealous application blowing tiles off the roof!

Yes, the foam will stick to the rafters, which is kind of the point - there are solvents that can be used to totally remove expanded foam but I'm not sure when that would be appropriate to do or why you'd want to I thought you were boarding over the roof structure?

You need an airtight seal and you aren't going to get that by scribing the board. The board has no flexibility so if you try and stuff it into a gap smaller than it you crack it, fracturing the insulation and losing performance - you can also break the foil on the outside suface creating a pathway for moisture.
You might be able to create a seal stuffing in fleece beside the board but I'd be surprised if it worked well.
You could perhaps line the rafters by stapling on plastic sheet first so the foam creates a seal between the board and plastic but doesn't directly contact the rafter?

The reason to board at right angles is you end up with less likelihood of the joins between the insulation and boards coinciding, providing a gap from the inside of the roof to the outside. If your rafters are at helpful spacings then this is worth doing, but if they're widely spaced or at awkward centres then you could end up with a lot of waste - in that case I'd use separate layers of insulation and plasterboard.

 

[AMc]

Regarding Certification it seems it's BRA you need not BRE or BCO :roll:
If you look at the PDF links on the right hand side of this page you can see the maufacturer has had the material and application tested.
http://www.ecotherm.co.uk/our_products/pitched_roof_insulation/eco-versal.aspx

If you don't do this then Building Control may give you a hard time about your solution - however my inspector came and looked at the installation but because my plans with SAP calcs were approved he didn't actually ask to see any documentation.

 

[Zebra]

Yes, the foam will stick to the rafters, which is kind of the point - there are solvents that can be used to totally remove expanded foam but I'm not sure when that would be appropriate to do or why you'd want to I thought you were boarding over the roof structure?

Just didn't want to permanently mark the rafters, even if I won't see it. If it can be removed at some future time with a solvent then I guess it would be OK. Thanks for your help. I see what you mean with boarding at right angles.
To what extent do Building Control have to be involved if it's a renovation of an existing structure? They've already said about the thermal insulation of the upstairs walls that as it's not the whole building, I don't have to give a Notice. You wouldn't need to notify Building Control if you were going to insulate your loft, would you? So why do they have to be involved if it's a pitched roof?

 

[AMc]

If you're worried about the rafters (which I admire in principal) then I would imagine stapling plastic sheet to them before you insert the board and foam would be just as effective and would prevent them from being damaged. The foam should push the sheet against the rafter firmly enough to create a seal.

I don't know at what point you are expected to get a building notice or submit full plans and SAP calcs - my work was major involving a complete reroof, rebuilding a massive glass vinery, new floors, heating, wiring etc. so I was never in any doubt I would need full approval.

Technically I believe if you are renewing more than 25% of the thermal element - roof, wall, floor - you are supposed to either meet or attempt to meet to show why you can't meet the specifications for performance laid out in PartL1B of the latest building control regulations.
Anything less than that is considered a repair and you can do whatever was there already again.

If you want to be sure then ask building control and try to get an email response that confirms you don't need permissions - then you are at least covered if you sell or there is later some dispute. I needed building control approval but not planning but I still sent drawings to planning and got an email confirming I didn't need planning consent which was reassuring.

 

[MatthewC]

To what extent do Building Control have to be involved if it's a renovation of an existing structure? They've already said about the thermal insulation of the upstairs walls that as it's not the whole building, I don't have to give a Notice. You wouldn't need to notify Building Control if you were going to insulate your loft, would you? So why do they have to be involved if it's a pitched roof?

Building Control told me that I should have notified them about re-doing my pitched) roof and adding insulation (on the joists not rafters) as I was "improving a thermal element". They said that had I done the roof first and then done the insulation later, notification would not have been needed. At least, I think that's what they said.... sounded daft to me as it has exactly the same end result.

 

[Zebra]

They said that had I done the roof first and then done the insulation later, notification would not have been needed.

This is the situation I'm in - done the roof, now want to do the insulation. Perhaps it's because insulating an existing roof doesn't need notification and once you've done the roof it becomes "existing?"...

So did you get in trouble with Building Control? Slapped wrists? Sent to the naughty step?

 

[MatthewC]

They said that had I done the roof first and then done the insulation later, notification would not have been needed.

This is the situation I'm in - done the roof, now want to do the insulation. Perhaps it's because insulating an existing roof doesn't need notification and once you've done the roof it becomes "existing?"...

So did you get in trouble with Building Control? Slapped wrists? Sent to the naughty step?

Nope! I said that honestly we had done the roof before I started doing the insulation (which I still haven't finished, actually). Also I have a very sensible and pragmatic inspector who is not going to cause trouble to no benefit when you have the end result that he would have wanted anyway - I think he gets into trouble because of this!

I think the point is about renewing a thermal element, as opposed to repairing or modifying it. Clearly, when you remove all the slates and the plaster from the lath ceilings, you are definitely renewing not repairing!

 

[AMc]

My understanding - if you need to refix a few tiles you can do like for like repairs (renewing less than 25% of the thermal element i.e. the roof).
If you are reslating the whole roof then you should meet the current regs at that point - that includes following guidance on ventilation and structural performance as well as complying with L1B for thermal performance.

Technically I would think any breach was when you re-roofed without insulating - you might have needed a building notice but it sounds like they aren't too worried.
IMHO Building Control are concerned by people who disregard L1B entirely and much more concerned with unsafe structural repairs and unsanitary sewage work.

 

[skier-hughes]

IMHO Building Control are concerned by people who disregard L1B entirely and much more concerned with unsafe structural repairs and unsanitary sewage work.

You talking about me :wink: