Best quote so far for closed-cell spray foam is £20/m2 at 75mm depth. Given 75mm PIR boards are around £42 each and cover 2.88 m2, they work out at £15/m2. When you add the cost of glue, expanding foam for gaps, tape etc, it doesn't seem crazy expensive to have it spray foamed instead.

Am I missing something? 

I'm planning on doing something similar but also with an MVHR - does this eliminate or mitigate the internal-to-external issues?

 

Regards

 

Tet

On 29/08/2022 at 21:57, larry said:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9580

Thanks, seems longer than 8 years ago I read that.

On 18/10/2022 at 15:52, Tetrarch said:

I'm planning on doing something similar but also with an MVHR - does this eliminate or mitigate the internal-to-external issues?

 

Regards

 

Tet

 

ditto, starting on a turn of the century house with solid walls.

 

current plan in 100mm celtex bonded to internal skin, cut out window reveals/cill/top and get 100mm celtex in as well. then 12.5 plasterboard and skim.

 

mvhr to take care of humidity.

Interesting topic to read, especially re: breathability of solid walls + risks of using solid PIR.

 

In my own case, I'm insulating an attic space room exterior wall. I've battened out roughly 30-50 mm from the wall (uneven, as they just threw whatever stones they could find together, lol). I then put up 2x2 timbers and will put a 50mm PIR in between, likely them putting 15mm PIR over the top of it all to minimise any cold bridging (20-25mm is starting to eat away the room space too much for my liking, and 15mm seems like it'll do more than enough to prevent cold spots).

 

As this is in an attic space where the roof is still original slates without a felt layer, it can get quite windy in there (and thus, in the cavity I created). I'm presuming this is more than enough 'breathability' to not worry about condensation risk behind the insulation, as was talked about earlier?

5 hours ago, nostos156 said:

Interesting topic to read, especially re: breathability of solid walls + risks of using solid PIR.

 

In my own case, I'm insulating an attic space room exterior wall. I've battened out roughly 30-50 mm from the wall (uneven, as they just threw whatever stones they could find together, lol). I then put up 2x2 timbers and will put a 50mm PIR in between, likely them putting 15mm PIR over the top of it all to minimise any cold bridging (20-25mm is starting to eat away the room space too much for my liking, and 15mm seems like it'll do more than enough to prevent cold spots).

 

As this is in an attic space where the roof is still original slates without a felt layer, it can get quite windy in there (and thus, in the cavity I created). I'm presuming this is more than enough 'breathability' to not worry about condensation risk behind the insulation, as was talked about earlier?

 

The felt, or lack of, shouldn't provide the ventilation (originally I suspect the underside of the slates would have been lime torched), the ventilation should be coming from the eaves, so providing there's a clear path to/from them, you should be fine.

 

Conversely, somewhat playing devils advocate, that ventilation could also be considered thermal bypass, you're essentially building a PIR tent. 

2 hours ago, jayc89 said:

 

The felt, or lack of, shouldn't provide the ventilation (originally I suspect the underside of the slates would have been lime torched), the ventilation should be coming from the eaves, so providing there's a clear path to/from them, you should be fine.

 

Conversely, somewhat playing devils advocate, that ventilation could also be considered thermal bypass, you're essentially building a PIR tent. 

 

I've been doing some quick reading on thermal bypassing and that looks like another minefield. Ultimately, there's only so much you can do within a given budget for an old house. The outside walls are solid breathable stone meant to wick away moisture from any ingress and so on, and it's probably far riskier to stop that on one side by not bothering with a cavity. It'll render the PIR not as effective as you say, but trying to retrofit modern standards meant for completely different housing types in much older construction is always going to less efficient.

 

As long as it doesn't lead to condensation and rot risks by having the ventilation then I'll take that as a win.

running the values for my solid wall buildup (225 brick,100mm celotex, plasterboard, plaster) through the calculator using -10c external 26c internal still puts the dew point well inside the insulation so I don't think any air gaps between the bricks and insulation is needed.

 

That looks like external insulation?

well spotted, its internal but the calc had it wrong way.

 

4 hours ago, Dave Jones said:

well spotted, its internal but the calc had it wrong way.

 

 

Very similar to mine, but I have a 25mm service void too

 

7 hours ago, Dave Jones said:

running the values for my solid wall buildup (225 brick,100mm celotex, plasterboard, plaster) through the calculator using -10c external 26c internal still puts the dew point well inside the insulation so I don't think any air gaps between the bricks and insulation is needed.

 

 

What calculator is this?

30 minutes ago, nostos156 said:

 

What calculator is this?

 

https://www.changeplan.co.uk/u_value_calculator.php

 

If you register an account you get the graphics too. 

The heatloss calculator on the same site is excellent 

Gave the calculator a go, I presume the dew point being inside the insulation layer is what you want? Interesting that I can't add studwork within the thicker PIR layer either.

Okay I figured out how to add studwork properly, so it has changed the values a bit.

 

 

Had a little play with this model.

If you put the level of pir as low as 5mm it still puts the dew point in the insulation

Edited February 20, 20233 yr by bassanclan

1 hour ago, bassanclan said:

Had a little play with this model.

If you put the level of pir as low as 5mm it still puts the dew point in the insulation

 

Yeah, honestly I think this site is better as it will tell you exactly if you are getting condensation and how much: https://www.ubakus.de/en/r-value-calculator/?

8 hours ago, nostos156 said:

 

Yeah, honestly I think this site is better as it will tell you exactly if you are getting condensation and how much: https://www.ubakus.de/en/r-value-calculator/?

 

I've modelled my detail on both sites and both look pretty similar to me. I think the dew point on change plan is accurate. 

10 minutes ago, jayc89 said:

 

I've modelled my detail on both sites and both look pretty similar to me. I think the dew point on change plan is accurate. 

 

and its in english!

Surely you want the dewpoint outside the build up?

6 minutes ago, Roger440 said:

Surely you want the dewpoint outside the build up?

 

i think the point is, insulation is sealed so its hard for moisture from inside the house to travel through it and meet the dewpoint. Worse would be if the dewpoint made it through the insulation and into the blockwork/plasterboard etc.

10 hours ago, Roger440 said:

Surely you want the dewpoint outside the build up?

Ideally yes.

What generally happens is that the dewpoint is spatially at a suitable interface as it needs a 'nucleus' to start forming, and can be both above and below the dew point temperature, at the same time.

15 minutes ago, SteamyTea said:

Ideally yes.

What generally happens is that the dewpoint is spatially at a suitable interface as it needs a 'nucleus' to start forming, and can be both above and below the dew point temperature, at the same time.

 

But that's not possible with IWI, is it (dew point outside)? It will always be internally, somewhere, and the least worst location would be within the insulation, assuming it's foil faced, taped etc, so internal air can't reach it? 

8 minutes ago, jayc89 said:

But that's not possible with IWI

Exactly.

That is why the most suitable type of insulation has to be used.