mass concrete house adding 150mm external insulation. What do you recommend internally?

usual_suspect

Hi,

I'm kicking off a bunch of renovation activities in the house and have 150mm (the max I can go) external insulation being prepped. However....the house is mass concrete and I'd love to put some more on. What is recommended for the internal side of the house. I'm relatively limited for space so likely a max of 50mm, however, also concerned about causing damp/condensation issues. I'm sure there's products out there so what would people recommend?

I've tried searching around boards.ie however, can't find the exact answer. I'm also failing with it on Google, ChatGPT, etc. etc.

Thanks in advance for any replies

Tefral

Are you meeting a good U-Value with the EWI?

If so, why would you put insulation on the internal walls? Think of them as a massive thermal battery. If you line the walls you take that away.

Have a look up about Thermal Mass.

MicktheMan

https://www.boards.ie/discussion/comment/121890289#Comment_121890289

+1

andyd12

https://www.boards.ie/discussion/comment/121890289#Comment_121890289

I would agree with this

Mellor

50mm on insulation internally with 150mm external is not a good build up

usual_suspect

Thanks all.

I've a bias coming from the house being so cold :) a desire to insulate as much as possible which overrides what I've read up on 'thermal mass'.

The last estimate had was an estimated "0.23 W/m2K" - I just need to learn how to translate that into layman's English! I get impression below 0.3 W/m2K is the target, but obviously the lower the better....

Is there any articles/papers/reports talking about what is best practise to get to the lowest u-value? Note: attic is being re-done, floors also - it's a full refurbishment job. Always amazed there isn't a few good 'proper' papers/reports explaining best practise - although this could also be my search skills failing me.....

Tefral

https://www.boards.ie/discussion/comment/121891435#Comment_121891435

You are effectively wrapping your house in a blanket, everything within that blanket will heat up. If you ensure you have good airtightness around opes your house will get very warm. You may also need to review your Ventilation strategy in tandem. Look into DCV etc.

Do not line your walls....

usual_suspect

I’ve a mechanical ventilation system going in also so keeping an eye on that aspect 👍

Shoog

Consider this, if the massed concrete house is similar to ours then external insulation will be relatively ineffective.

Why ?

If yours is a bungalow and has thick walls like our (80cm min) then the wall plate represents a significant amount of the wall surface (around 20%), unless you are able to effectively insulate the top this will undermine your external insulation to the point of been virtually useless. The solution is to add insulation to the top of the wall in a continous layer with the outside surface - which in our case would require the roof to be removed. You must also pay particular attention to the ventilation at the eves which must be improved to avoid the roof from roting out. Then there are the floors which will significantly compromise your attempts.

Unfortunately drylining with 100mm foil backed insulated platerboard is the only seriously viable solution for an old massed concrete house unless you are prepared to remove the roof. You will need to be absolutely scrupulous in ensuring that all seams are vapour tight otherwise your cold massed concrete wall surface will be a breeding ground for black mold and your life will be significantly shortened. Also if your ceilings are anything higher than 8ft then you should be considering dropping them - incorporating most of your loft insulation in the new void and adding vapour barriers to the top of the plasterboard.

Thats my opinion for what it is worth.

BryanF

Shoog, why can’t the wall plate be thermally insulated externally?

what about taking up the last ~3rows of tiles/slates and lap insulation with attic insulation?

WRT drylining preference - is there not a greater thermal bridging risk (than external insulation @ wall plate) where every internal wall meets an external wall?

Shoog

https://www.boards.ie/discussion/comment/121904389#Comment_121904389

You can take off the bottom rows of tiles but you have then made the roof timbers at the wall plate the coldest point of the roof structure almost guaranteeing condensation and rot. The real issue is you have turned a relatively complex and expensive job into a horrifically expensive and difficult job. There is an ssumption that external insulation is universally applicable but this an over simplification. If yours is a two story then the balance shifts significantly towards external insulation. If your house is built like ours then the roof comes down and touches the wall which creates huge difficulties in detailing this critical area.

Internal walls do create a cold bridge at the junctions, but it is less significant than it is painted as. We did what I advocate and the house remains fairly stable at 19c with minimal heating, though this is not on the massed concrete house. I thought about doing what you propose for years and could never get to a place where the losses at the wall to roof junction made it a viable option without replacing the whole roof.

In the end we dropped the ceilings and insulated the floors and it made a significant difference to the rooms we did it to.

I would say that you need an energy consultant or an architect with a good portfolio of renovating old houses to advise you of the best practice. Companies selling external insulation systems will sell you an external insulation system so their advise is all but worthless.

Mellor

https://www.boards.ie/discussion/comment/121891683#Comment_121891683

For the sake of accuracy. Insulating a house doesn't cause it to heat up. It doesn't generate heat. It just slows the rate that heat escapes.

https://www.boards.ie/discussion/comment/121891435#Comment_121891435

The minimum would be .35 W/m2K, for the weight weight average for the walls. The target should be as far past that as possible.

Tefral

https://www.boards.ie/discussion/comment/121908295#Comment_121908295

Im sure most people understand the laws of thermodynamics Mellor. I wasnt insulting the persons intelligence. The concrete walls inside the thermal layer will reach equillibrium with the heat sources inside this layer, that was the connotation. Heatup / cool down appropriately etc.

Mellor

https://www.boards.ie/discussion/comment/121900578#Comment_121900578

I couldn't figure how you were calculating that the wall plate is 20% of the surface? Then I seen the 80cm thick dimension. That would be a massively thick wall for a house - I'd expect to see a wall that thick in a castle. I can't comment on your wall. But a typical mass concrete wall is nowhere near that. Council built mass concrete walls were ~250mm. Older cottages may have been bigger, say 350mm. I doubt the OP is looking at much more than that.

If however you do have wall that are 800mm thick. There is no reason you couldn't use external insulation. You simply bring the roof/ceiling insulation out to meet the EWI - which is the standard details, otherwise the thermal envelope isn't closed.

You can take off the bottom rows of tiles but you have then made the roof timbers at the wall plate the coldest point of the roof structure almost guaranteeing condensation and rot. 

No you haven't. The roof timber at the wall plate is inside the thermal envelope. It is sitting within the roof insulation. Outside the insulation is the cold side, and this part of the ceiling space is ventilated - specifically to prevent condensation and rot.

If yours is a two story then the balance shifts significantly towards external insulation. If your house is built like ours then the roof comes down and touches the wall which creates huge difficulties in detailing this critical area.

This is essentially how all walls roof junctions work. That standard detail for roof insulation solves the issue you have. The wall plate gets insulated, the roof insulation joins the wall insulation (wherever is it) to close the thermal envelope.

listermint

https://www.boards.ie/discussion/comment/121904435#Comment_121904435

Huh ?

No no no, the roof timbers moisture is kept in check by soffit ventilation. Well designed soffit ventilation can by typical up and over flow in cold attic, or tiled and or ridge ventilation for warm spaces.

There are various means to do this. But saying you get rotten timbers by EWI up to wall plate is inaccurate.

Shoog

Many of the older houses were built with massed concrete simply replacing the old fashion twin skin and rubble with concrete - hence 2ft thick walls like old cottages. All the principles of building old cottages were carried over. So it depends a great deal on what massed concrete principle was been used.

It's very dependent on the detailing at the wall plate. Many of these old massed concrete house have no eves ventilation.

They were designed based on the principle that a copious amount of unregulated stack effect would keep things dry. Every bit of house insulation and sealing puts the roof at risk.

Mellor

https://www.boards.ie/discussion/comment/121908908#Comment_121908908

Lack of ventilation causing issues is very very different to saying EWI cause cold wall plate timbers and rot. They are two unrelated issues.

If a roof has no eves ventilation, it is trivial to fix. And more importantly, if that's the actually issues. Insulating internally doesn't avoid it. You've just as much removed the unregulated stack effect, and sealed off the envelope.

BryanF

https://www.boards.ie/discussion/comment/121908908#Comment_121908908

the language suggesting ‘horrifically expensive’ is ott. And ‘guaranteeing condensation’ is rubbish, the whole point of Insul (& vb)continuity is to avoid such issues. ‘If’, there is a thermal bridge at wall plate, then it’s worth sorting as part of an ewi treatment.

separately With any upgrading works one of the first things to sort is the attic ventilation.

Shoog

https://www.boards.ie/discussion/comment/121934472#Comment_121934472

Such a strategy is a two part job with two skilled teams doing the work, its going add a considerable amount of extra cost and complexity to any external insulation job - something someone considering it needs to at least understand and consider when seeking quotes. If he doesn't then there is a very real chance that the contractor may not address the issues and leave the client in a bind a few years down the line.

Most people will get a big shock when they seek quotes for such a plan.

Mellor

https://www.boards.ie/discussion/comment/121934578#Comment_121934578

It's not significantly extra work. It's not a difficult detail. A laypersons or DIYer might miss it, but a neither should be having a crack at these things.

The solution you proposed is slightly less work, mainly because it's half a job. It doesn't solve or avoid the issue of eves ventilation. So you are "almost guaranteeing condensation and rot" and are in for a shock when it needs to be replaced in a few years.

usual_suspect

Thanks for the super interesting discussion. I can add only the mass concrete walls of the house are far from 800mm and much closer to Mellor's estimate off 250mm (I'd say more like 200mm).

However, it does lead to a great note around insulating the top of the wall. We are having the attic insulation re-done and I'll be making sure that this section is adequately completed at the same time.....

listermint

https://www.boards.ie/discussion/comment/121937745#Comment_121937745

The 2 should meet. That's the most ideal scenario.