Dense Blocks used - is insulated plasterboard a must???

[daveb789]

Please help, have I got a case...

My scenario is I am having a 20m2 extension built and I used dense blocks rather than lightweight. It was a selection made following a conversation with the supplier that the existing house is dense block and is to be open plan through to the extension - therefore using lightweight blocks in the extension would not be worth the additional money they would cost as heat saving would be negligible, which seemed logical so I went ahead and got on with the order and work.
I didn't give this another thought untill the Building Inspector pulled me up on this point and said I'd have to install insulated plasterboard min 25mm insulation (so 37mm boards), I didn't think much of this until, to my horror, I inquired about the price of these boards!!
The wall construction is brick, 100mm cavity filled with 100mm drytherm, 100mm dense block.
My query is does anyone know how far off regs I am with this construction and do Building Control ever show any common sense in this scenario seeing as the house extension to the remainder of the house is open (to be open plan) and therefore will be seeing no benefit from my investment on the costly insulated plasterboard. Basically is there anything I can do????
Cheers for any advice.
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[^woody^]

Your new walls must conform to the current thermal standards regardless of what the existing structure is.

The only possible alternative would be to have a SAP calculation done to measure heat loss, and see if you can compensate elsewhere in the building.

Incidentally, if the merchant supplied/recommended the blocks with specific performance claims, and they are not suitable, then you may have a claim against him, if you wanted to pursue it

 

[theoldun]

The answer depends on what Dritherm you used?
Dritherm 32, 35 or 37
Regards oldun

 

[daveb789]

Used Dritherm 37

Was straight back onto supplier and to be fair they have said they'll do the insulation at cost, but the point is it's still at big expense for no gain.

 

[theoldun]

To the best of our knowledge the Building Codes in Scotland are now the same as the Building Regs south of the border..
With this in mind you require a U value on external walls of 0.28W/m2k, and your present construction falls way below that figure.
We list below what we consider to be your cheapest option to achieve required U value.
K=conductivity W/mk, R=resistance m2K/W
External air R =.0.06
102 brickwork K = 0.840 R = 0.12
Dritherm 37 K = 0.037 R = 2.70
100 dense block K = 1.28 R = 0.08
35mm Gyproc Thermaline Plus R = 0.79
Internal air pressure R = 0.12
Add all resistance = 3.87 less 8% cold bridge = 3.56m2K/W = 0.28Wm2/K
Thermaline plus will be cheapest way. Your dense block has knocked all the stuffing out.
Regards oldun

 

[daveb789]

Many thanks 'theoldun' for taking the time to calc this for me.
This is the solution Building Control have said - I could kick myself for listening to the supplier as I had no intention of using dense block - if only I have seen sense that day!!
I was hoping I could get round the problem I've caused myself by increasing insulation in the roof space (over spec'ing) seeing as the quilt insulation is alot cheaper and as the roof loses the most heat, but seems I'll just have to take my medicine on this occasion!

 

[daveb789]

I suppose now this post is on this forum may help any other diy'ers of 2 important lessons;
Don't act on a suppliers advice alone! and
Don't use dense block for cavity walls! Even if your extension is open plan onto a house that looses heat like a sieve!!

 

[daveb789]

35mm Gyproc Thermaline Plus R = 0.79

'theolden' I just have one small query, after looking online at this product; you quoted an R value of 0.79 but all sites I visited quoted Rvalue of 0.88, is there two simlar products available?
On the plus this product does appear to be cheaper than I have been quoted - so will put this to the supplier tomorrow and see what they come up with.
Thanks again.

 

[^woody^]

Do a SAP calculation, it is less stringent than generic u-values, and you can most likely compensate elsewhere. How do you think all these designer glass houses and extensions pass?

 

[daveb789]

Thanks woody,
Could you explain the basic's of a SAP calc, who does them, cost, when to do them, etc, etc.
I assume I complete ceiling detail/insulation and then do the test?
Although, I fear is this would mean the rest of the house is then considered and I know this would be likely to fail. I have done as much as possible (insulated roof & floor and cavity wall), but needs more investment like new windows to improve further which I can't currently afford but would like to do in the near future, and probably insulated plasterboard throughout!

 

[freddiemercurystwin]

No there's no test, its a calculation done beforehand to work out what insulation you need here or there especially if you know you're gonna be light somewhere, like your walls. Usually only necessary in highly glazed extensions or new builds. Anyway have a look here:

//www.diynot.com/forums/viewtopic.php?p=1925023#1925023

//search.diynot.com/forum_sear...reddymercurystwin&search=Search+Forum&stype=0

 

[theoldun]

Do not know what web sites you have looked at,
Google British Gypsum. Plasterboard and Accessories, Thermaline and they definitely state 35mm Thermaline Plus as having R value of 0.79, although if you convert the K value it does come out to approx 0.88.
If you want to go down the SAP route and lay out possibly £200, fill out a 5 page questionnaire and possibly cause your self problems, then by all means fill yer boots, however we would have thought that it made more sense to write out the figures we posted, and ask Building Control for their approval. The worst that can happen is that they say no, so then offer to go up to 40mm which will give you an R 0f 3.71 and a U of 27Wm2/K
Regards oldun

 

[mikric]

Have all your other elements of insulation been installed ie floor/roof etc ?
If your not to bad at maths/areas etc a notional building trade off calculation for an extension is relatively simple to do and you might get away with increasing insulation u-values elsewhere in the build. If you want to have a go just shout and I will try and explain how to do the calculation.

 

[^woody^]

If you want to go down the SAP route and lay out possibly £200, fill out a 5 page questionnaire and possibly cause your self problems

I don't know where you get that information from, or why you are trying to put the OP off a solution which is much less bother than your brute force, line everything on sight solution

A typical builder's way of thinking

 

[noseall]

than your brute force, line everything on sight solution

A typical builder's way of thinking

Yeah, but how good are you at arm wrestling eh Woodpulp?