25mm or 50mm internal wall insulation for this room size?

[House man]

The room is 4.3m x 3.6m

 

The rest of the home is large enough to put 50mm PIR boards for IWI but was thinking if it is advisable to go for 25mm to not make the room too small?

 

Thoughts? 

 

Cheers

[JohnMo]

At least 50mm is my vote 

[Ferdinand]

Are you going to miss or notice losing an extra 1/160th (=0.625%) of your room dimension?

 

(If you need to get rid of a 3.56m wide Jackson Pollock to make that possible, I'll take of off your hands, and collect at no charge)

 

Why not go for 75mm PIR everywhere ? 😛

[House man]

5 hours ago, Ferdinand said:

Are you going to miss or notice losing an extra 1/160th (=0.625%) of your room dimension?

 

(If you need to get rid of a 3.56m wide Jackson Pollock to make that possible, I'll take of off your hands, and collect at no charge)

 

Why not go for 75mm PIR everywhere ? 😛

 

Thank you for your response

 

I would have thought theres a point at which if you added any further insulation it would not help. Ive done a new extension and added 50mm in the cavity with air gap. However i dont believe any cavity job can be complete due to all those wall times that go in, hence the IWI

 

One other thing about IWI is that are you not losing heat through the battens between the PIR insulation? 

[JohnMo]

Would have thought the internal insulation would have gone on as full sheets then the battens then plasterboard or no battens and just plasterboard on top of the insulation. 

 

There is a diminished returns past a certain point, but I think with 50mm cavity insulation your still way of building regs I would have thought.

 

Have you completed u value calculations?

[Ferdinand]

20 hours ago, House man said:

 

Thank you for your response

 

I would have thought theres a point at which if you added any further insulation it would not help. Ive done a new extension and added 50mm in the cavity with air gap. However i dont believe any cavity job can be complete due to all those wall times that go in, hence the IWI

 

One other thing about IWI is that are you not losing heat through the battens between the PIR insulation? 

 

Yes, I'm teasing a touch.

 

I'd suggest that the "limited return" point will be somewhere beyond where you are now, so I'd go for at least 50mm of PIR and perhaps a bit more given energy prices and that I only get to do it once.

 

Framing or dot and dab is the choice of method. If you are already inside an insulated wall then your cold bridge is well limited. 

 

I'd suggest the 62.5mm or 72.5mm PIR backed plasterboard, or perhaps use separates if you can get a really good price on the PIR (but will require framing) - the last one I did a few years ago I found non-foiled 50mm PIR at £10 a sheet, which was OK.

 

If you dot and dab remember to make sure your glue lines prevent air circulation in the gap.

And you do need to check the requirements for a thermal element under Regs.

ATB.

 

F

[House man]

On 11/09/2022 at 18:55, Ferdinand said:

 

Yes, I'm teasing a touch.

 

I'd suggest that the "limited return" point will be somewhere beyond where you are now, so I'd go for at least 50mm of PIR and perhaps a bit more given energy prices and that I only get to do it once.

 

Framing or dot and dab is the choice of method. If you are already inside an insulated wall then your cold bridge is well limited. 

 

I'd suggest the 62.5mm or 72.5mm PIR backed plasterboard, or perhaps use separates if you can get a really good price on the PIR (but will require framing) - the last one I did a few years ago I found non-foiled 50mm PIR at £10 a sheet, which was OK.

 

If you dot and dab remember to make sure your glue lines prevent air circulation in the gap.

And you do need to check the requirements for a thermal element under Regs.

ATB.

 

F

 

Thanks for the reply,

 

Wondered if you lose any heat through the framing battens?

[Temp]

If Building Control are involved they will have a view on the minimum thickness.