25mm polystyrene as insulation

Joined
19 Jan 2009
Messages
86
Reaction score
1
Location
Yorkshire
Country
United Kingdom
We're in the process of having a small extension done on our kitchen.

I have a question regarding the insualtion the builder is using in the cavity.

The outer skin of the wall is reclaimed random stone so the cavity gap is not uniform all the way down. The exisitng part of the wall has rock wool in it but i've found that he's fitting 25mm slabs of bog standard polystyrene in the new part. Is this really suitable/effective? or should he be using rockwool or kingspan?

thanks in advance

 
Sponsored Links
No. Totally ineffective. There are several options but 100mm cavity batts (mineral wool) would be the best solution.
 
Are Building Control involved? They should be! this could bite you on the ass if not. What else is he not doing?
 
Sponsored Links
Cheers for the replies

I spoke with the builder tonight re my concerns and he said that he was going to fill the void with vermiculite as it would absorb all the voids and was the loose full insulation they use for lining chimneys etc.

Does that sound reasonable or is it better to insist on just sticking with the rock wool as per the rest of the wall?

Cheers
 
how wide is the cavity?

Was he using 25mm because nothing bigger would fit?

I don't think vermiculite is moisture-repellent, which cavity fills usually are if they are touching the outer leaf.

Wasn't the insulation specified in the Building Control plans?
 
The extension doesn't fall under building control as it's literally extending the corner of a utility room of the back of the current kitchen. Probably 1m x. 5m

The builder came recommended from a friend who has done 2 extensions for him and and he's done about 5 extensions on their road as well so he's not a total unknown however this insulation didn't feel right with me.

Right well if the vermiculite isn't moisture resistant then I'm going to insist on using rockwool
 
Its an extension to the house, it needs Building Regs Approval.

Anyway if he uses a full fill insulation make sure its the right one, not the rockwool you use for loft insulation. Its called Rockwool Cavity funnily enough. crown Dritherm would also be suitable.

That said, I suspect that if you rang the manufacturer (either Rockwool or Crown) they would both tell you their products were technically unsuitable, as they have not been tested for a random skin situation. Should been partially filled with Celotex or Kingspan. Where are the wall ties?
 
As far as I am aware Vermiculite can be used as a loose fill cavity insulation provided it has been treated with water repellent.. However that said it is mainly used where a higher fire rating is required rather than an insulation factor
 
No expert, but I know that the risk of vermiculite is it absorbs water if untreated. It is frequently used in chimneys around flue liners as additional insulation, but it's also known that if it get's wet, it holds the water and will slowly release this into any walls it is in contact with.
 
If as the builder has told you, nothing thicker than 25mm board will fit the cavity, then he should not put anything in until the extension is built and the cavity fully closed. Then he should have it cavity filled with the silver polystyrene beads. These are treated to be fire retardant and have a light coat of adhesive to bind them in place once blown in.

Building control approval is certainly required whenever any structural alterations are carried out and this is no exception. any wiring must be notified under part P and will need to be past at completion.

If you do not have these then you could have problems if and whenever you come to sell.

You do not however need planning, which is determined by the size of the extension and maybe that is what is referred to when you mention the size of the extension. If you have any doubts (and by now you should have) contact Building Control your self.
 
Filling with treated vermiculite is ok. (treated and suitable for cavities) What I don't understand is why the polystyrene sheet. Why not just build and full fill with vermiculite. Ideally you'd check U-value but for a small section of wall it won't make that much difference.
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top