Celotex installed in cavity block wall build

[Hastings]

I have just seen an extension under construction where the celotex is being fitted with no taping of the joints. It is being installed against the inner one of two 4" concrete block walls, just like you would fit glass wool bats - each sheet resting on or against the next at the edges, allowing draughts through.

Surely a mistake?

Edited March 20, 2019 by Hastings

[Declan52]

They have a tongue and groove so if installed correctly then you don't need tape.  But it all comes down to how they are put in.  Put it in wrong and even tape won't solve the heat loss. 

[Miek]

More important (IMO) is to make sure there are no snots or imperfections between the PIR and the block work creating a void which would allow convective heat loss up through the wall to the roof. The boards need to be tight on to the blocks which is where mineral wool batts have an advantage as they have some give. 

[Cpd]

Loosely fitted rigid insulation in a hidden cavity......business as usual I’m afraid.  

[nod]

I can’t Rembrandts ever seeing cavity batts taped onsite I

[ADLIan]

Seen it done but it not in most MIs or BBA cert instructions. Plus a foil tape on the external face creates a localised VCL on the COLD side of the insulation which can lead to condensation.

[MarkyP]

I saw a new build estate locally under construction with PIR cavity walls and stopped for a look, the insulation was ill fitted, gaps everywhere. I think its a case of out of sight, out of mind  on such builds. But the thermal bypass must be tremendous. This insulation will be coupled with aerated block work to the inner leaf which will crack behind the dot and dabbed plasterboard = a nice steady breeze blowing through the plugs and under the skirting from the outside. Far, far better to go fully filled with a decent thickness of mineral wool. Makes it harder to leave gaps.

 

We had some rubbish, really sloppy brickies on our blockwork extension but I'd gone for a wide fully filled cavity on the extension, even these were able to keep the insulation batts tightly fitted without too much mental strain, albeit with me checking it constantly.

[nod]

1 hour ago, ADLIan said:

Seen it done but it not in most MIs or BBA cert instructions. Plus a foil tape on the external face creates a localised VCL on the COLD side of the insulation which can lead to condensation.

Back to the same thing

Private BC

Wont Rick the boat 

this site has 500 houses with 

220 still to go 

[nod]

Just now, nod said:

Back to the same thing

Private BC

Wont Rick the boat 

this site has 500 houses with 

220 still to go 

Also these are all scoring 3 in a air test 

Dont rock that boat ?‍♀️ 

[Declan52]

42 minutes ago, MarkyP said:

I saw a new build estate locally under construction with PIR cavity walls and stopped for a look, the insulation was ill fitted, gaps everywhere. I think its a case of out of sight, out of mind  on such builds. But the thermal bypass must be tremendous. This insulation will be coupled with aerated block work to the inner leaf which will crack behind the dot and dabbed plasterboard = a nice steady breeze blowing through the plugs and under the skirting from the outside. Far, far better to go fully filled with a decent thickness of mineral wool. Makes it harder to leave gaps.

 

We had some rubbish, really sloppy brickies on our blockwork extension but I'd gone for a wide fully filled cavity on the extension, even these were able to keep the insulation batts tightly fitted without too much mental strain, albeit with me checking it constantly.

Far better to go with a good brickie who will do it correct.  It isn't much work to run the trowel down the wall to clean the snots of. If you can its better to hit it with a brush as it will help to seal up all the motar so well. 

You need a much wider cavity if you go for batts. 

[Hastings]

Thanks everyone - very useful info.

[ADLIan]

With glass wool batts you can squeeze them together to make sure no gaps

[Carrerahill]

2 hours ago, nod said:

Back to the same thing

Private BC

Wont Rick the boat 

this site has 500 houses with 

220 still to go 

Why I would never buy a house built after about the 1960's - unless it was a self builders and done well.

[nod]

I think that’s why we are all self building 

[MarkyP]

PIR boards have been shown in studies to lose their thermal properties due to loss of the pentane gas over time, becoming worse than the best mineral wool (lambda in the region of 0.032). Also the required void in a partially filled PIR cavity acts as chimney and /convection and air movement space, fully filled removes this problem and will make the wall far more wind tight. Also lots of discussion and reports over the years that foam boards are not brilliantly dimensionally stable, they can shrink and bow with ageing (a reason most system suppliers dropped foam boards as an option for EWI). 

[Temp]

45 minutes ago, MarkyP said:

PIR boards have been shown in studies to lose their thermal properties due to loss of the pentane gas over time, becoming worse than the best mineral wool (lambda in the region of 0.032).

 

My understanding is that the lambda quoted for CE marked PIR (eg that complies with Harmonised European Standard BS/EN 13165) is the average value predicted over 25 years. In addition the degradation mostly happens over the first 8 years and then slows. So for most of the 25 years it will be close to the quoted figure. 

[Dreadnaught]

1 minute ago, Temp said:

My understanding is that the lambda quoted for CE marked PIR (eg that complies with Harmonised European Standard BS/EN 13165) is the average value predicted over 25 years. In addition the degradation mostly happens over the first 8 years and then slows. So for most of the 25 years it will be close to the quoted figure. 

 

Very interesting. Hadn't realised that at all.

[MarkyP]

13 minutes ago, Temp said:

 

My understanding is that the lambda quoted for CE marked PIR (eg that complies with Harmonised European Standard BS/EN 13165) is the average value predicted over 25 years. In addition the degradation mostly happens over the first 8 years and then slows. So for most of the 25 years it will be close to the quoted figure. 

It's a topic which has been discussed in great technical depth over on GBF over the years. I recall a study showing that it degraded to 0.028 within the first 10 years, but even this would be within controlled conditions- a board with a perfect foil face and I wouldn't be surprised if edge sealed in some way. Would be good to know what a board with 100% air rather than pentane would achieve, that would perhaps be a reasonable backstop worst case lambda.

 

[Hastings]

On 20/03/2019 at 21:04, Declan52 said:

They have a tongue and groove so if installed correctly then you don't need tape.  But it all comes down to how they are put in.  Put it in wrong and even tape won't solve the heat loss. 

 

I just looked again at the site and the PIR sheets are not t&g edge, just standard edges.

Would building control approve such a poor design?

Edit:

Just heard from the architect that it was specified because it's easier to install than the T&G version (due to the cavity ties) and BC approved it, therefore no problem.

There is no hope for humanity.

Edited March 22, 2019 by Hastings