AliG

Instead of putting PIR in the cavity why don't we put it on the inside

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Just thinking aloud.

 

My house has a 150mm cavity with 100mm PIR. We then have 37.5mm insulated plasterboard on the inside.

 

I am very suspicious that the PIR will never work as well as it should in the cavity due to air infiltration.

 

This got me to thinking, why not just build a 50mm cavity and put 100mm of PIR on the inside. Then airtightness should be much less of an issue as cold air would likely be behind the PIR.

 

It would also seem to be simpler to build. Steels in the wall would be better insulated and you could eliminate one step in the build process.

 

One negative I can think of is that there would a cold bridge where the first floor goes into the blockwork. But you could insulate the ceilings to deal with this.

 

What am I missing.

Edited by AliG

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The only downside I can think of is the challenge of fixing the pir internally.  100mm isn't a huge amount of insulation, but even that might be a pain in the ar$e to fix, assuming you fix with battens for plasterboard. By the time you've done all the additional fixing and battening, surely timber frame would be worth considering as an option (we've got 120mm in the frame, and an additional 40mm internally).

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Good points, I am assuming that the decision has already been made to use blockwork.

 

100mm isn't much, but I am guess that if it is all inside the blockwork it will perform as well as my 125mm split across the blockwork.

 

Fixing would be an issue though.

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Just build it 215mm single skin instead.  No point in putting in a cavity unless your in a high rainfall area.  

Why build in block then put plasterboard on the inside.  Make the cavity as wide as you can, 200mm + so all the insulation is in the one place.  Then wet plaster the block work and that's it all sealed up with no battens or extra long fixings to go through your plasterboard.  

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Single skin definitely works as a solution, I am assuming that there are other reasons that it isn't used, such as rain that you mention. I think full fill beads probably work well also as there are no fitting issues, but in Scotland that isn't allowed again because of rain.

 

I can see that when people started to use insulation in the cavity PIR would not have been available, so rockwool in the cavity and plaster on the inside would have been the obvious solution.

 

Now with PIR I don't see why the cavity isn't narrower and the insulation on the inside.

 

My issue with a wider cavity and putting all the insulation in there is that I don't trust bricklayers/builders to ever get it all tightly in there with no gaps. I had mine taped up and foamed in places as I reckoned that would make a big difference to air infiltration, but I still reckon it would all be fitted a lot better on the inside. You are right that you can fix air tightness with wet plastering, but I am not convinced that the insulation works well in the cavity.

 

If I look at areas of vaulted ceiling I have, PIR was cut between the rafter and fits perfectly. Then insulated plasterboard was put over the rafters. Why would a similar build up on internal walls not be a better way of getting the insulation tightly fitted.

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You could argue that this is all moot in that houses are built that way because builders don't like to change and if I am suggesting a change then that opens up the use of any different building method.

 

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Wouldn’t all your rooms get 200mm smaller 

ok in your case, as you have the land, but smaller sites wouldn’t want to lose that much floor area. 

 

Your suggestion really points towards ICF

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27 minutes ago, Russell griffiths said:

Wouldn’t all your rooms get 200mm smaller 

ok in your case, as you have the land, but smaller sites wouldn’t want to lose that much floor area. 

 

Your suggestion really points towards ICF

Don't think you would lose much floor space.  If I understand what the OP is getting at the wall thickness would be the same.  The position of the wall elements would swap:

 

Brick, cavity, insulation, block             becomes

Brick, cavity, block, insulation

 

The thickness of each remains the same.

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8 hours ago, AliG said:

My issue with a wider cavity and putting all the insulation in there is that I don't trust bricklayers/builders to ever get it all tightly in there with no gaps

I have done my cottage with PIR  on the inside and having worked with it a lot I am personally persuaded that unless it’s done with a great deal of care the end results are going to be poor. It’s just so easy for cold air to get in and around the wrong sides of the cut insulation, and on windy days I think your going to see even greater losses due  to wind wash. On any of my insulation build ups I starter with 50mm asthis is easy to cut, easy to fit and most importantly easy to foam up and BE sure it’s propperly done. I then cut back the foam and re do anything that has a gap or dubiously big air holes in the foam. Cut back again and tape all the joints. I then add the next layer of say 100mm with the knowledge that no cold air will be getting in and around this layer, again this will be foamed and taped to the enth degree as this will also be my vcl in most cases. Again I am then sure that no warm air is going to be getting into any of the gaps. I know this does not answer your question but I just can’t see pir as a safe way to insulate without a huge amount of care and attention and a LOT of money on foam and tape. I have used an insane amount of tape and foam over that last few years........   

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If you are going to do brick outer and lots of insulation on the inside, then you might as well do timber frame. At least you have a nice timber structure to support the insulation and fix the plasterboard to.

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1 hour ago, Russell griffiths said:

[...]

Your suggestion really points towards ICF

 

One of the principal ' worries ' we had about Durisol ( one of the ICF systems available) was the loss of internal space because of the width of the blocks. The argument runs: for any given area on which you can build, Durisol uses more space than average. Our blocks are 365 wide with the normal  plaster coating inside

 

But its simple. And, correctly stacked and poured,  properly plastered, delivers a reasonable U value.

 

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26 minutes ago, ProDave said:

If you are going to do brick outer and lots of insulation on the inside, then you might as well do timber frame. At least you have a nice timber structure to support the insulation and fix the plasterboard to.

 In hindsight I should have knocked the cottage down and started from scratch but hay ho..... what I have had to do is pretty much build a timber frame on the inside to support insulation and plasterboard. It’s going to work and work as well as it can but at a huge time cost. I do have a nice traditional looking  700mm-1000mm thick stone rain screen on the outside........  

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10 hours ago, AliG said:

Just thinking aloud.

 

My house has a 150mm cavity with 100mm PIR. We then have 37.5mm insulated plasterboard on the inside.

 

I am very suspicious that the PIR will never work as well as it should in the cavity due to air infiltration.

 

This got me to thinking, why not just build a 50mm cavity and put 100mm of PIR on the inside. Then airtightness should be much less of an issue as cold air would likely be behind the PIR.

 

It would also seem to be simpler to build. Steels in the wall would be better insulated and you could eliminate one step in the build process.

 

One negative I can think of is that there would a cold bridge where the first floor goes into the blockwork. But you could insulate the ceilings to deal with this.

 

What am I missing.

 

Fairly random thoughts on this.

 

In my continued thought experiments in search of an inexpensive high spec way to build a rentable bungalow, we considered another factor when thinking about block built walls with Built to exactly fit 8x4 sheets of insulation. We liked the insulation on the outside because it meant that it would be safe from damage ... even if a T is deliberately trying to wreck a breezeblock and plaster skim wall, it is relatively easy to repair, and things such as picture hooks are less troublesome, with less rigorous instruction needed.

 

But the equation is different in a home.

 

THat prompts the thought - Can your proposal be done using either thicker PIR-backed-IPB glued to the wall, perhaps with a counter-orientated layer beneath? PErhaps the lower layer could be taped for airtightness? FOr rally thick iPB, one could even use EWI type mechanical fixings if desired.

 

In a home environment you can be surer that the surface is not damaged e.g. Surface fixings and if needed physical fixings are now available that take heavy weights from PB.

 

In my most recent refurb and ran all my rewiring and plumbing under a floating floor inside the thermal envelope using short vertical chases, and it seems to have worked Ok so far. So a lack of service void in the walls is potentially manageable.

 

I think in principle that is one way the Americans do it, but they use cable duct on the surface.

 

For areas with heavy attachment needs, they could be studded or frame-and-fermacell-ed. Or we could go for the new age disabled / old people / short people friendly kitchens where wall cupboards are simply abolished.

 

I would say that could be made to work, but certain compromises would be needed.

 

Ferdinand

Edited by Ferdinand

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1 minute ago, Ferdinand said:

 

Fairly random thoughts on this.

 

In my continued thought experiments in search of an inexpensive high spec way to build a rentable bungalow, we considered another factor when thinking about block built walls with Built to exactly fit 8x4 sheets of insulation. We liked the insulation on the outside because it meant that it would be safe from damage ... even if a T is deliberately trying to wreck a breezeblock and plaster skim wall, it is relatively easy to repair, and things such as picture hooks are less troublesome, with less rigorous instruction needed.

 

THat prompts the thought - Can your proposal be done using either thicker PIR-backed-IPB glued to the wall, perhaps with a counter-orientated layer beneath? PErhaps the lower layer could be taped for airtightness? FOr rally thick iPB, one could even use EWI type mechanical fixings if desired.

 

In a home environment you can be surer that the surface is not damaged e.g. Surface fixings and if needed physical fixings are now available that take heavy weights from PB.

 

In my most recent refurb and ran all my retiring and plumbing under a floating floor inside the thermal envelope, and it seems to have worked Ok so far, so a lack of service void is potentially manageable.

 

For areas with heavy attachment needs, they could be studded or frame-and-fermacell-ed. Or we could go for the new age disabled / old people / short people friendly kitchens where wall cupboards are simply abolished.

 

I would say that could be made to work, but certain compromises would be needed.

 

Ferdinand

I read all the above 

maybe worth comparing isotex /durisol against  block  construction  ,as you would be hard plastering them on inside as well and you can hang anything anywhere on them 

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One MAJOR issue with putting insulation on the inside only is what's known as Interstitial condensation. In any building you’ve the movement of moisture through the structure. This happens most when drying out but happens throughout the lifetime of a build but at a reduced rate. Airtight layers are designed to control this moisture flow but they still allow it.

What happens is in a traditional cavity wall the outer block is cold and the inner block is warm. This cold moisture travels through the air across the cavity like the way moisture condenses on grass on a cold morning. The moisture travels inside and hits its dew point. The dew-point in a traditional cavity wall is in the cavity so when it condenses it drains out the cavity. With the insulation on the inside only then dew point will be inside the building. You’ll have moisture condensing inside leading to big damp problems, mould growth, rotting of the building fabric, etc. It’s a big problem.

 

In timber frame buildings the issue is solved by ventilating the outside of the building and having it airtight. If you don’t have a timber frame with an airtight layer or if the airtight layer is punctured with a big hole the warm air from inside the house escapes through this hole, condenses to water and can damage the building structure over time.

 

When drylining an old stone cottage you always have to get an interstitial condensation test done. This test tells you the maximum amount of insulation you can have before you move the dew point to far inside.

If you just want to use large sheets of PIR on it’s own then external insulation is the way to go not internal.

 

EDIT: I mentioned mould growth and rotting of the building fabric. Other issues include (which I've just googled):

  • Mould growth, which is a cause of respiratory allergies.
  • Mildew.
  • Staining.
  • Corrosion and decay of the building fabric.
  • Frost damage.
  • Poor performance of insulation and reduced thermal resistance of other elements of the building fabric. This in turn can reduce the temperature of the building fabric, exacerbating the condensation problem.
  • Migration of salts.
  • Liberation of chemicals.
  • Damage to equipment.
  • Electrical failure.
Edited by Dudda
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17 minutes ago, Dudda said:

... You’ll have moisture condensing inside leading to big damp problems, mould growth, rotting of the building fabric, etc. It’s a big problem.

 

 

Do you mean it is a big problem in theory or is there plenty of field evidence? I do not imagine there are many brick an block builders pioneering unusual insulation build ups with an empty cavity and 150mm of internal PIR.

 

I am leaning towards a 100mm cavity filled with beads and then 25mm or 40mm of internal PIR sheet between battens with regular plasterboard on top. Would this put me on the safe side of condensation worries?

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I was trying to think about condensation issues and that makes a lot of sense.

 

Also reading the earlier replies, it would be difficult to put services on the outside walls.

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30 minutes ago, Dudda said:

One MAJOR issue with putting insulation on the inside only is what's known as Interstitial condensation. In any building you’ve the movement of moisture through the structure. This happens most when drying out but happens throughout the lifetime of a build but at a reduced rate. Airtight layers are designed to control this moisture flow but they still allow it.

What happens is in a traditional cavity wall the outer block is cold and the inner block is warm. This cold moisture travels through the air across the cavity like the way moisture condenses on grass on a cold morning. The moisture travels inside and hits its dew point. The dew-point in a traditional cavity wall is in the cavity so when it condenses it drains out the cavity. With the insulation on the inside only then dew point will be inside the building. You’ll have moisture condensing inside leading to big damp problems, mould growth, rotting of the building fabric, etc. It’s a big problem.

 

All useful, thanks.

 

In my setup suggested above I would probably think about 2 layers of PIR on the inside with the lower on taped, so that the membrane is on the inside of say 50mm of PIR and therefore warm (may need a calculation), or indeed one could put some insulation on the outside with a rain screen, but that extra complication would defeat the whole point.

 

My current restoration approach has been to dry line and insulate, usually with 50mm PIR, put a membrane inside the insulation under traditional pb, then install a background ventilation system to manage the internal humidity in case there are any problems. So Traditional Drylining with belt and braces.

 

(Add: I have done this mainly on solid walled 1910-ish properties; when I have done the calls on cavity walled properties say 1960-70 it has not been cost effective to do the dry lining as cavity injection plus other efficiency measures reduces bills such that the dry lining then becomes unviable.)

 

F

 

 

 

 

Edited by Ferdinand

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11 minutes ago, epsilonGreedy said:

 

Do you mean it is a big problem in theory or is there plenty of field evidence? I do not imagine there are many brick an block builders pioneering unusual insulation build ups with an empty cavity and 150mm of internal PIR.

 

I am leaning towards a 100mm cavity filled with beads and then 25mm or 40mm of internal PIR sheet between battens with regular plasterboard on top. Would this put me on the safe side of condensation worries?

The bead filled cavity makes a lot of sense to me. We didn't explore it because as I understand it full fill is not allowed in Scotland due to driving rain issues.

 

I suspect this is an area where testing would prove the issues unfounded. The issue is more likely with rockwool and not EPS beads.

 

Personally one of the reasons we went with blockwork is that I don't like looking at the perpends required to ventilate a timber frame cavity.

Edited by AliG

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Just now, epsilonGreedy said:

Do you mean it is a big problem in theory or is there plenty of field evidence? I do not imagine there are many brick an block builders pioneering unusual insulation build ups with an empty cavity and 150mm of internal PIR. 

 

It's a big problem in reality. A lot of the issue is hidden behind the plasterboard or insulation so you mightn't know you have it until it's too late. People generally don't leave a cavity empty so it's not common on cavity buildings but does happen when people dryline too much on an old building. By this I mean they use to much internal insulation without getting an Interstitial condensation test done and move the due point to far inside.

 

1 minute ago, epsilonGreedy said:

I am leaning towards a 100mm cavity filled with beads and then 25mm or 40mm of internal PIR sheet between battens with regular plasterboard on top. Would this put me on the safe side of condensation worries?

That's fine. Rule of thumb is once most of your insulation is outside or in the cavity you've the due point far enough outside. In your case 100mm outside (cavity) and 40mm inside is fine. If you'd only 50mm in the cavity and say 100mm inside you could potentially have an issue. The likes of Kingspan or other insulation companies have a technical department who calculate these tests for you for free. I've used them several times for different projects. You just email them your wall buildup and then email you the result. I then have to include it in my records to show we designed out condensation risk to the building fabric.

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The dew is due to condense at the dew point.

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