TF Internal PIR, to the sole plate or beyond?

[Thorfun]

we're getting ready to fit the 80mm PIR internally to our timber frame stud work and I'm wondering how far down it should go? does it just go to the sole plate or can I just take it all the way down to the block and beam flooring?

 

Here's a drawing from the architect and that shows it only going to the top of the slab which is the bottom of the sole plate.

 

 

but doesn't this assume that the ufh and screed is already in place? as we haven't done the screed or ufh yet and the block and beam is still where the level is at can I simply take it all the way down to the B&B? e.g.

 

 

and then simply run some DPM on top of the insulation under the screed and up the wall to protect the PIR from the screed?

 

ps. I know the drawing shows hollow core planks, that what was originally planned but we changed it to B&B.

Edited December 26, 2021 by Thorfun

[Thorfun]

I guess the other option is to do the UFH and screed before we install the PIR? but then we'd need an upstand of insulation around the screed anyway so by running the PIR to the block and beam means we won't need to do the insulated upstand?

[nod]

Just run it down to the bottom of the sole plate 

 

[Thorfun]

41 minutes ago, nod said:

Just run it down to the bottom of the sole plate 

 

Does that mean it has to be dead on accurate though? And I guess I’ll still need an upstand for the outside of the screed?

[tonyshouse]

I would like to see a thermal bridging analysis before answering, there seems to me to be bridging into the floor planks yet to be mitigated 

[nod]

35 minutes ago, Thorfun said:

Does that mean it has to be dead on accurate though? And I guess I’ll still need an upstand for the outside of the screed?

It doesn’t need to be totally accurate and yes you will need an upstand when laying the screed 

[Thorfun]

1 hour ago, tonyshouse said:

I would like to see a thermal bridging analysis before answering, there seems to me to be bridging into the floor planks yet to be mitigated 

Yeah, I think I see what you mean but I was planning on adding some insulation externally to cover the sole plate which would mitigate that cold bridge. 

[Thorfun]

1 hour ago, nod said:

It doesn’t need to be totally accurate and yes you will need an upstand when laying the screed 

So if I do need an upstand would if not be easier to just run the internal PIR down to the block and beam? Then I can run the floor insulation to butt up to it? 

[Iceverge]

TF over basement if I'm reading the drawing correctly.  Install PIR lower is better re the soleplate

 

What is the material sitting on top of the basement reinforced concrete wall below the sole plate. This will be your main path to heat loss. 

 

 

More importantly do you have a robust plan for airtightness at the junction. It's more important than thermal bridging. 

[Thorfun]

13 minutes ago, Iceverge said:

TF over basement if I'm reading the drawing correctly.  Install PIR lower is better re the soleplate

 

What is the material sitting on top of the basement reinforced concrete wall below the sole plate. This will be your main path to heat loss. 

 

 

More importantly do you have a robust plan for airtightness at the junction. It's more important than thermal bridging. 

yes, TF sitting above a basement. 
 

The sole plate is sitting on coursing blocks which have a reasonable lamda value 0.11W/mK (https://tarmac.com/products/blocks/toplite-coursing-bricks/#applications) so should help in reducing the thermal bridge. 
 

so there’s no ‘issues’ in running the PIR all the below the sole plate? It’s not detrimental in any way?

 

I have already run airtight membrane from basement as a Tony Tray around the block and beam which will join the AVCL on the ground floor which will sit in front of the PIR. another reason for running the PIR all the way down as I can just carry the basement Tony tray straight up. 

[Iceverge]

30 minutes ago, Thorfun said:

so there’s no ‘issues’ in running the PIR all the below the sole plate? It’s not detrimental in any way

Not that I can see. 

 

 

[Thorfun]

5 minutes ago, Iceverge said:

Not that I can see. 

 

 

I guess the only downside would be cost as the 80mm PIR is more expensive than a thinner upstand. But as our ceiling height is 2.6m downstairs I need off-cuts anyway. 

[Thorfun]

5 hours ago, Thorfun said:

The sole plate is sitting on coursing blocks which have a reasonable lamda value 0.11W/mK (https://tarmac.com/products/blocks/toplite-coursing-bricks/#applications) so should help in reducing the thermal bridge. 

whoops. apologies, the Thermal Conductivity of the 7.3N blocks is 0.19W/mK. still, not bad when compared to a dense concrete block of between 0.7 - 1.3W/mK according to this website https://knowledge.specifiedby.com/blocks-blockwork/

[tonyshouse]

I worry about the sole plate getting cold. Were it fall below dewpoint I think that condensation would form on top of the dpc!  Which is the analysis would be useful. 
 

there are ways of mitigating this problem 

[Thorfun]

10 hours ago, tonyshouse said:

I worry about the sole plate getting cold. Were it fall below dewpoint I think that condensation would form on top of the dpc!  Which is the analysis would be useful. 
 

there are ways of mitigating this problem 

yeah, we've had this conversation before which is why I'm planning on bringing the external insulation from the basement up to the sole plate level on the outside

[tonyshouse]

Good idea, I might consider extending it 300 or even 600mm above that in a wedge , full width at the bottom of the plate reducing to nothing at the top, any membrane running over it. Bit tricky to cut wedges 

[Thorfun]

just another question on this topic...ish.

 

I have a DPC below the sole plate that is hanging over the inside by quite a bit. can this simply be trimmed back to the sole plate/internal edge of the TF or is an overhang a necessary requirement?

[Iceverge]

Trim it back flush IMO. 

[Thorfun]

18 hours ago, Iceverge said:

Trim it back flush IMO. 

just for completeness I have decided to fold it up and staple it to the sole plate as shown in this diagram from NHBC:

 

 

not sure if it's 100% necessary but it won't do any harm if I do!