Cavity wall insulation

[psarinuk]

Hi all,

I've a single storey rear extension coming up. I advised the builder to keep a 75mm gap for cavity wall (to save space for the room). We are at damp course level now.

The buildings inspector has come around and said, "We like to see 100mm cavity, but as you have gone for 75mm, I would advise to use dritherm32 cavity wall insulation along with insulated plasterboards using dot and dab method".

Not only this is super expensive as I am finding out, the problem is that the dritherm32 is "slab" type of board that is quite rigid. The brickey is asking for a roll that is not rigid so its easier for him to insert between the wires holding the 2 walls as well as it helps by not displacing the blocks due to non-rigid nature of roll.

Can someone please advise if I can get a decently priced non-rigid/ semi-rigid type of insulation that comes like a roll and insulates well?

Many thanks in advance..

 

[Diyisfun]

I thought thats why plans were done & approved, so as insulation etc was specified. Prior to the build.
Sorry cant answer your question though.

 

[theoldun]

Hi all,



Can someone please advise if I can get a decently priced non-rigid/ semi-rigid type of insulation that comes like a roll and insulates well?

For full fill cavity. In a word. NO

Will run through it with you.
1. Assume this is on building notice, otherwise this problem would have been sorted before commencement.
2. Your bricklayer clearly knows nothing what so ever about insulation and U values to be asking stupid questions.
3. As your building inspector was in a helpful mood by advising you to use Drithern 32, (which is correct) we hope that he also advised you to use a block with a minimum R value of 0.11W/mK to achieve a U value as close as possible to 0.28W/m2K.
4. Have assumed in the above that external skin is brickwork.
5. It is standard and acceptable practice to make cavity 10mm bigger than thickness of insulation there by making it easy to fit insulation slabs.
6. Insulation slabs are made ridged to stop any sag. and are laid on the top of tie irons.
7. We never use 75mm cavity now as 75mm full fill does not really stand up to 0.28W.m2K when you take cold bridging into account.
Hope this helps and you understand.
Regards oldun

 

[psarinuk]

Thanks for your reply.
1. Yes it is on a building notice.
2. Amazed you find the questions stupid. At least, it helps lay men learn the stupid system. Also amazed that you like to answer stupid questions, maybe there is a link there.
3. I've asked the bricklayer to go for minimum cavity, as I want to maximise interior space. Dritherm32 is too expensive and rigid. My neighbours never used it in their recent extensions. We are using 7.3N concrete blocks. Are they any good for insulation?
4. Exterior is brickwork.
5. Again the bricklayer is probably going by my word to save space but I take your point.
6. I will pass this useful piece of information, the bricklayer was planning to install iron ties with a gap of 456mm so the slabs can slide between the them. I am not sure how he is going to do them now.
7. We can still make the cavity 100mm, especially if the building inspector is rigid on using an insulated plasterboard, which adds price and thickness. But if he lets us use normal plaster board, I will stick to 75mm for now like our neighbours' extensions which are "surprisingly" warm.

 

[^woody^]

You and your builder clearly don't understand building then, and you should not have done this on the building notice for that reason

Reducing the cavity is irrelevant to the thickness of the wall ... ie it's no good saying you want a thinner cavity when that means needing thicker blocks and then thicker insulated plasterboard .... and a wall thicker than one with a 100mm cavity!

If you wanted walls of "x" thickness then you should have designed, or got someone to design the wall for you so that it meets the u-value at minimum thickness

This is an excellent example of trying to scrimp on well specified plans, only to pay more later. And is all too common

And the builder should be charging more for now using rigid board

 

[psarinuk]

You are not very good at judging and you should not have tried for that reason. I feel sorry for your customers if all you care about is, "And the builder should be charging more for now using rigid board".

"Reducing the cavity is irrelevant to the thickness of the wall". You need to take a scale/ tape measure and do some basic maths.

Anyway, for those who are interested in the outcome, the buildings inspector has come around today and was content with the 75mm cavity insulation with "dritherm32 75mm". Normal plasterboard would be used and he has recommended thermal blocks for the inner work which I've ordered anyway.

The builder agrees with the way of installation of insulation as described on here.

So apart from more expensive cavity insulation, the 75mm cavity bet has paid off with "thinner walls"

 

[^woody^]

Yes, there is nothing like blindly chancing it through life and then gloating when it just happens to come together

BTW, how did you reason that dritherm32 is "super expensive" when it is about £1more per pack than normal quilt?

What cheap an nasty insulation did you plan to use initially?

 

[psarinuk]

I am sure gloating is what you like to do with your customers.

From my local builder's yard, normal insulation that is commonly used for cavity walls is £17 for 8.3 sq. meters. dritherm32 is £30 for 4.5 sq meters (approx).

It would be futile explaining it further to you when I see evidence for your level of maths.

BTW, I used 100mm QUINNTHERM for my floor. Without going into numbers, do you label it as cheap and (an) nasty?

 

[freddiemercurystwin]

Oh Jeez you didn't use Quinntherm did you? Please say you didn't use Quinntherm?

 

[^woody^]

BTW, I used 100mm QUINNTHERM for my floor. Without going into numbers, do you label it as cheap and (an) nasty?

Well, it seems that you know what you are doing with this building lark. And the grammar pedantry is coming along just fine too.

Over spec'd on the floor, under spec'd on the walls. I'll look out for the roof insulation post

 

[theoldun]

2. Amazed you find the questions stupid. At least, it helps lay men learn the stupid system. Also amazed that you like to answer stupid questions, maybe there is a link there.
.

Yes there is a link there and will tell you what it is.
We started work at the age of 15 and we worked for 5 years with three olduns. 75% of our time was with two bricklayers and 25% with a carpenter. We asked them many stupid questions. They in return, patently and with kindness answered our questions and taught us a trade for which we have been externally grateful.
For want of something better else to do, and to pay our debt to those three old boys, we now occasionally answer a post to try and give guidance to somebody with a problem, however in your case it does not seem to have worked.
We repeat your bricklayer has asked stupid questions.
Your bricklayer should have given guidance from day one.
It is not what size cavity or insulation that you want, or what your neighbour has done, but what is required to comply with L1B2010 Approved Document. Suggest you have a read of the document.
Regard 7.3N concrete blocks. If they have a density of between, say 1050 and 1900kg/m2 and a k value of say between 40 and 1.13W/m2, then congratulations, you could not have picked a worst block.
Tie irons are not iron, but stainless steel and they go at every 450mm the sane depth as an insulation slab.

You and your builder clearly don't understand building then, and you should not have done this on the building notice for that reason

This is an excellent example of trying to scrimp on well specified plans, only to pay more later. And is all too common

Agree with you 100% Mr Woody.
oldun

 

[psarinuk]

Lets hope its a joke freddy.

The building inspector and the builders yard both recommended quintherm or celotex or kingspan. There is no difference in price but my local yard only keeps quinntherm.

Any known issues?

Woody, I shall send you the completion certificate to add to my gloating and there would be no additional charge

 

[^woody^]

Oh Jeez you didn't use Quinntherm did you? Please say you didn't use Quinntherm?

Why not? It was cheap

 

[psarinuk]

Make up your mind, "Over spec'd on the floor" or "Why not? It was cheap"

no wonder the oldun has linked to another "stupid"..lol

 

[Rad1o]

How are you getting on with the building inspector?