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Wall Insulation - Internal or External


canalsiderenovation
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Still comparing and analysing quotes from builders. It's made difficult in that the two we have narrowed it down to work in very different ways. One is using automated QS software so I have a huge spreadsheet to navigate with lots of jargon I don't understand the other tends to just give a narrative and price but not huge amounts of detail so I am basically trying to translate both into one document and compare everything. Not helped by the fact there is a significant price different.

 

I think I'm clearer on floor insulation, next bit comes to our wall insulation of the existing. We know from our EPC when we purchased it there is cavity wall insulation but we have asked builders to quote for some improvement to wall insulation as we are upgrading the existing floors, triple glazed windows, new roof etc.

 

There is lining paper on some of our existing walls and we know the plaster will need to come off. When we stripped some of the wallpaper a couple of years ago, all 2-3 layers from the 70s that were soaked in dog wee a lot of the plaster came off in chunks then so it needs replacing which both buildings have allowed for. In terms of insulation:

 

One builder has quoted for on the existing to 'dab a 62mm insulated plasterboard on all external walls'.

 

The other building is focusing on insulating from the internal walls. I'm copying and pasting bits from their quote which says 37.5 mm Recticel Eurothane PL insulated plasterboard in 1200 x 2400 mm sheets fixed to softwood with 3 mm gap at all joints filled flush with neat Thistle board finish plaster in one coat work and 90 mm wide jute scrim embedded at all internal and external angles and angles between walls and ceilings.

 

Either way the who external walls need to be rendered again to match the new large extension. 

 

So should I be favouring internal or external wall insulation. Any help would be great ??

 

 

IMG_20200412_172740.jpg

Edited by canalsiderenovation
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I quite like external wall insulation as it wraps the whole house in a tea cosy.  You can take it down below ground if it is EPS.  Neither of your quotes sound like external, just internal plasterboard / PIR laminate.  Make sure you are comparing like for like.

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31 minutes ago, Mr Punter said:

I quite like external wall insulation as it wraps the whole house in a tea cosy.  You can take it down below ground if it is EPS.  Neither of your quotes sound like external, just internal plasterboard / PIR laminate.  Make sure you are comparing like for like.

 

I was under the impression the first one was external unless he means insulating internally of the external walls.....

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Reading that both are INTERNAL one using 65mm dot and dab fixed the other 37mm boards fixed to a softwood batten 

Have you not supplied a specific guide as to what they are quoting for ?

 

this could be where your price differences are coming from, between those two different specs could be £2-4 grands worth of insulation do this a few times on different aspects of the build and you can get vast differences. 

 

I think you need to get a set spec done done and get that priced by both parties or your not comparing apples with apples. 

 

Edited by Russell griffiths
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1 hour ago, Mr Punter said:

I quite like external wall insulation as it wraps the whole house in a tea cosy.  You can take it down below ground if it is EPS.  Neither of your quotes sound like external, just internal plasterboard / PIR laminate.  Make sure you are comparing like for like.

Plus one with external

We do both 

External is much better and quick 

83CBFAB4-FF02-4BE1-8D3B-359A0FB73BDF.jpeg

AD6D2859-B51F-40EA-9748-76B49F44E78F.jpeg

80D0F183-2D6C-423A-9A19-01AD5D069112.jpeg

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39 minutes ago, Russell griffiths said:

Reading that both are INTERNAL one using 65mm dot and dab fixed the other 37mm boards fixed to a softwood batten 

Have you not supplied a specific guide as to what they are quoting for ?

 

this could be where your price differences are coming from, between those two different specs could be £2-4 grands worth of insulation do this a few times on different aspects of the build and you can get vast differences. 

 

I think you need to get a set spec done done and get that priced by both parties or your not comparing apples with apples. 

 

 

We have a spec for the majority, though for some elements we had asked for their advice (this has been helpful as we have been able to analyse and review the finer details with the help on here however this is also where we have variation as they seem to each have different opinions on which is the better approach). E.g one proposes an overlay system to our existing internal floor for insulation as we have the ceiling height and doesn't seem keen on excavating the floors however this would only give us 75mm of insulation. The other. The other builder said he would prefer to ecvavate the floors to give us at least 100mm and potentially more (I have a separate thread on this). 

 

I'm sure you are right now and they are both internal which I think is correct now reading back the quote.

 

Ironically the builder quoting for excavating the floors and putting at least 100mm of insulation in and doing 65mm of wall insulation is the cheaper one. The more expensive builder is the one wanting to do an overlay floor insulation of 75mm and the wall insulation of 37mm boards/softwood baton). The extra money for the more expensive builder seems to be on project management, foreman fees, administration and a very expensive welfare cabin as oppose to an improved spec....

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Comes down to who is the better builder ultimately which may be hard to judge. Are you confident that they are both competent and do good work? We got burned on our renovation by going for a smaller sole trader who turned out to be a cowboy. It was ridiculously stressful to sort out the mess. 

 

My view is coloured by this experience, but just reading the post, it looks like you're first builder seems to have a well thought out spec and seems to have priced to manage the job. May be that it's unnecessary expense and the other guy is good, but you're more likely to hit bad builders where the spec and pricing is left woolly and open. 

 

Sorry, you probably know all this already, but after the experience we had I feel the need to over-share! 

 

On the actual insulation proposals, Dot and dab for internal wall insulation is in theory less effective than battens because you get an air gap between wall and insulation. However this depends on the design and approach of each method. Also with retrofit IWI watch out for interstetial condensation, especially the floor joists which can get cold and attract moisture. 

 

As others have said, EWI much better if possible.  

 

 

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I think both builders are competent. We have seen both of their work mid build, spoke to current customers and found previous customers via social media too.

 

Both are local (within 3 miles) and have completed lots of different properties ranging from extensions to working on complete builds from scratch. Both are familiar with our architect, which isn't difficult given he also lives less than 3 miles away) so everyone is local. One is about 15 years younger so the detailed software, spreadsheets etc, to town on on the detail, younger team, quite a few trainees (the initial quote we had was done by a trainee QS has since been revised resulting in a second quote (which has come down massively but still a fair bit different). The older one I don't think has the range of office staff etc.

 

Re the IWI, interesting re the dot and dab and the effectiveness re battons. This is the exact information that is useful to know. I need to understand which is the better system to then go back to them both and price for the same spec. I wonder if they both picked IWI as it is cheaper.....

Edited by canalsiderenovation
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8 hours ago, Archer said:

On the actual insulation proposals, Dot and dab for internal wall insulation is in theory less effective than battens because you get an air gap between wall and insulation.


You will have a gap regardless - battens will be thicker if anything, unless you’ve got 25mm insulation between the battens and then insulated plasterboard over. 

 

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8 hours ago, PeterW said:


You will have a gap regardless - battens will be thicker if anything, unless you’ve got 25mm insulation between the battens and then insulated plasterboard over. 

 

Pictured is one of our jobs that we are doing at the moment 

Five offices being converted into houses While there is cavity walls with insulation 

Like yours it’s not sufficient 

As Peter rightly pointed out an air gap gives you far better performance 

pictured 50 mill i stud 15 mil off the block work 50 mil insulation Then plaster boarded 

The couple of brackets seen are temporary and will leave the studs standing independent giving excellent acoustics also 

35E9B179-D5B1-43D1-89DC-E34F3659EBFE.jpeg

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Sorry, to be clear, the issue with the D&D installation method isn't to do with the thermal performance of the air gap itself. It's to do with airtightness, and in particular the risk of air getting in behind the insulation layer, particularly in retrofit situations where the wall may have gaps in blockwork or mortar. This article has a full explanation and some possible ways of preventing which include a parge coat to the wall prior to installation. 

 

https://www.designingbuildings.co.uk/wiki/Defects_in_dot_and_dab

 

The air gap shown above in Nod's image is beneficial, but only if the wall behind and any penetrations through are air sealed. If using steel studs then something to mitigate cold bridging would be needed otherwise you'll lose something like 50% heat through the stud. If using timber then it's less of an issue but you can either batten over the insulation or have a double layer as Peter described. 

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On 12/04/2020 at 23:01, canalsiderenovation said:

I think both builders are competent. We have seen both of their work mid build, spoke to current customers and found previous customers via social media too.

 

Both are local (within 3 miles) and have completed lots of different properties ranging from extensions to working on complete builds from scratch. Both are familiar with our architect, which isn't difficult given he also lives less than 3 miles away) so everyone is local. One is about 15 years younger so the detailed software, spreadsheets etc, to town on on the detail, younger team, quite a few trainees (the initial quote we had was done by a trainee QS has since been revised resulting in a second quote (which has come down massively but still a fair bit different). The older one I don't think has the range of office staff etc.

 

Re the IWI, interesting re the dot and dab and the effectiveness re battons. This is the exact information that is useful to know. I need to understand which is the better system to then go back to them both and price for the same spec. I wonder if they both picked IWI as it is cheaper.....

 

Sounds like you've got it covered then and maybe the cheaper guy is the better pick. Once you've thought through the approach it might be worth asking them both to quote for EWI as well to see the difference in price. The tricky bit with IWI is ensuring insulation continuity and vapour sealing internally. That's all in the detailing and quite fiddly to do properly, especially between floors, and at joist-ends.

 

Having said all that, I'm a hypocrite and insulated the northern external wall on my house internally finishing the insulation at floor level. No problems so far. 

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