Explain to me like I’m five years old…

[BadgerBodger]

why there is such a strong aversion to PIR on this forum. 
 

In context. For my barn conversion I’m looking at the following. 
 

300mm EPS below slab

external walls to be panelised stud liner system installed in steel frame with breather membrane to outside 120mm PIR between studs and 50mm to face with VCL and service void to face before PB

 

roof will be breather above rafter, 150mm insulation below with VCL and service void to face before PB 

 

Additional perspective is that the architect proposed a hygroscopic solution but in my view this was not cost effective (surface area is huge and additional load to roof would have been problematic) and did not resolve the cold bridging issues I have with the steel frame being used to brace the external walls. 
 

 

[SteamyTea]

10 minutes ago, BadgerBodger said:

why there is such a strong aversion to PIR on this forum

I think it is because it is hard to fit it without a lot a time and carefulness.

 

Large steel buildings have roofs that are PU/PIR sandwiched between steels sheets, I think @saveasteading may know more about this.

[Russell griffiths]

Very time consuming to cut and fit

expensive

even if fitted well it shrinks over time leading to a poor fit. 
if your doing something from scratch you should be able to design it out and save some coin. 

 

[nod]

If pir is fitted correctly and taped 

It far superior to beads Beads settle get everywhere Sure they are cheaper and quicker Mass produced homes are full of the bloody things 

Pay your Brickies enough money to make a neat job Or do it yourself 

 

[Nickfromwales]

3 hours ago, BadgerBodger said:

300mm EPS below slab

and

3 hours ago, BadgerBodger said:

roof will be breather above rafter, 150mm insulation

?

 

Passive in the slab and a bit poor where the heat rises and wants to escape?

[saveasteading]

2 hours ago, SteamyTea said:

think @saveasteading may know more about this

You called?

PIR is twice as effective as eps. 

In floors it is easy to lay as sheets so I don't have any criticism 

 

But it is horrid to cut and fix between joists, making it hard work and less effective.

In steel buildings the PIR is extruded into a sandwich between 2 sheets of profiled steel, forming a single item, and no gaps, and there is no better way for big steel roofs and walls.

 

It burns. But not a lot in properly designed and constructed. buildings.

EPS is much more flammable and also melts readily. 

 

PU also burns. It is cheaper in cladding than pir but is for agriculture not industry.....animals and crops are not valued as much as people.

It would be OK under a slab but I dong think is available.

 

Why don't people like PIR? It was badly used on Grenfell. 

In everyday use though it's as I've said.

 

For your floor slab I can't see a reason for 300 eps rather than 150 pir.

But I now try to avoid the brands Celotex and Kingspan for moral reasons. Others are available.

 

I've seen sheds after fires. Eps was awful, and spread the fire fast and widely.

Fibreglass doesn't burn, and turns to sand, harmlessly in extreme heat. Some mineral wool has more resistance.

Pir in a composite  steel casing pretty doesn't burn because no air reaches it. But it has been known to fall off walls during fire and fire fighters stay clear.

[BadgerBodger]

7 hours ago, Nickfromwales said:

and

?

 

Passive in the slab and a bit poor where the heat rises and wants to escape?

I have to build up the levels to achieve the floor level and I’m using UFH. Figured we could use cheap EPS to hit two birds with one stone. Labour cost is the same. Small uplift in material cost for EPS/stone.
 

I’m also limited to the amount of weight I can add to the existing roof. Maybe I will get 160mm but that will be the limit. It will be PIR in the roof build-up. Celotex advise resultant u-value with 150mm will be 0.13. The installation risk of fitting between irregular timber is why I have chosen to go under rafter only as opposed to between and under. 

 

Ok. So. I continue with my quest. I’m well aware of the Grenfell issues and the associated risks. I’ve been working with the various insulation materials for the last 17 years and in my view PIR always results in the technically most effective solution (not withstanding installation issues). 

 

As I’m going to be the installer for the main build and the builder is only doing the extension where it is a full fill cavity wall, I’m happy. 
 

As I saw someone else write on a thread recently. I always have the builder lay 6 course of block, flush up the cavity side and then point the internal face. Install insulation once the block has gone off, tape the joints, then face up. When coursing above the insulation, cover with a piece of timber or hessian, then brush off before more insulation. 
 

 

 

[BadgerBodger]

And thanks all for the input.

[DownSouth]

Can I hijack this post? Our garage ceiling spec (habitable room/workshop bit) was 140mm PIR - but as the joists are deep and we have rock wool left over we’re wondering whether to use wool instead. I can’t find a calculator which shows how much we’d need compared to PIR. Any thoughts on how to work this out? And does it matter if it’s a mix of PIR/wool next to each other in the same ceiling?

[SteamyTea]

2 minutes ago, DownSouth said:

Any thoughts on how to work this out

Only one 

 

U-Value = 1/ (thickness / k-Value)

 

 

[saveasteading]

1 hour ago, DownSouth said:

Any thoughts on how to work this out?

The figures are in the suppliers' literature.

You need about double the pir thickness. So fill the space then put another layer over the top.

 

If using pir, ask your builders merchants for pir that isn't kingspan or celotex or St Gobain. As you don't want to use suppliers who were responsible for Grenfell. There are plenty.

 

[Nickfromwales]

13 hours ago, BadgerBodger said:

21 hours ago, Nickfromwales said:

 

I have to build up the levels to achieve the floor level and I’m using UFH. Figured we could use cheap EPS to hit two birds with one stone. Labour cost is the same. Small uplift in material cost for EPS/stone

Ah, makes sense, sorry. 
 

Focus on taping up well to prevent ANY airflow between boards and rafters.

If you can add 25mm to the interior face to stave off the repeat cold bridging then that would be the cherry on the cake, and PIR weighs next to nothing so shouldn’t affect those maths.

[saveasteading]

If i had a lot of depth to fill, i would likely use eps on the bottom rather than subbase or pir.

There are diminishing returns with thickness so eps might be optimal there.

[BadgerBodger]

9 hours ago, saveasteading said:

If i had a lot of depth to fill, i would likely use eps on the bottom rather than subbase or pir.

There are diminishing returns with thickness so eps might be optimal there.

This is exactly what I’m doing. There is some Hardcore in there which needs knocking about once the foundation modifications are complete and then compacting. Sand blinding followed by EPS 100. Cheap and cheerful. 
 

10 hours ago, Nickfromwales said:

Ah, makes sense, sorry. 
 

Focus on taping up well to prevent ANY airflow between boards and rafters.

If you can add 25mm to the interior face to stave off the repeat cold bridging then that would be the cherry on the cake, and PIR weighs next to nothing so shouldn’t affect those maths.

 

I’m going under rafter only. Only need to tape to purlins. BUT also using a full VCL membrane the warm side before batten for service void and board. 
 

Debating whether to use reflective membrane or not. I think no need considering foil face on the PIR

[saveasteading]

2 hours ago, BadgerBodger said:

no need considering foil face on the PIR

That only works as a heat reflector if it is onto an air space eg void. Years ago they said it was otherwise but the claim seems to be dropped now.

The same applies to a membrane. 

[BadgerBodger]

1 hour ago, saveasteading said:

That only works as a heat reflector if it is onto an air space eg void. Years ago they said it was otherwise but the claim seems to be dropped now.

The same applies to a membrane. 

Interesting. So considering that there will be a service void on the inside face and a cavity on the outside, this will be of some benefit? 
 

[saveasteading]

4 hours ago, BadgerBodger said:

this will be of some benefit? 

Yes. small but it all adds up.

[Temp]

We have a stick built roof with PIR between rafters.  Builder had to cut hundreds of pieces to fit. Horrible task with unavoidable gaps. I'd never build that way again. Nothing wrong with PIR itself. I'd consider building  a warm roof with PIR above rafters next time.