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100mm vs 150mm insulation UFH under slab.


Eric

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We have to make a decision tomorrow on our insulation to go under the slab, it’s 100mm on the drawings but is there any advantage to go to 150mm?

 

Scotframe 6” closed kit, 180m2 ground floor, 100mm subfloor, £1350 in difference between them.

 

Any thoughts/advice appreciated ?

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there's lots of discussions and posts about this subject on here. a quick search would reveal the threads.

 

generally 100mm is just about building regulations and most on here have 200mm at a minimum. and those that have put only 100mm in have regretted it later down the line. 

 

but you don't mention if your insulation is PIR or EPS as the lambda value of each differs greatly. but regardless it's generally considered that the more the better to a certain point.

 

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Yes and you have only this one chance to do it, especially if you are having underfloor heating.

Someone else here can do the calculation for you, but I am guessing you get the money back in about 5 years, and as fuel prices rise you benefit more (lose less).

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Not sure 150mm is enough! I have 200mm PIR  and some here have 300mm EPS always go more with insulation, you only buy it once, electricity/oil/gas you buy forever and at increasing cost.

Edited by joe90
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50% less heat loss through the floor.  The effect of better insulation with UFH will be greater than without, as the floor temp at the insulation level will be higher.  UFH temp at insulation approx 35 deg, with radiators it will be closer to 18 deg.

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Just now, Roundtuit said:

Under the slab, or under the screed? 100mm is pretty poor if you're building to a good insulated, airtight standard, and if you are, 150mm isn't 'top spec' either.  I assume we're talking PIR?

Under the slab & yes PIR.

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If your planning to stay in your new home for years and years then the thicker the better. We have about 200mm but in a timber suspended floor. AND if your are going to have UFH the thicker the better.

 

Good luck

 

M

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Going through similar ourself. 100mm PIR gets a u value of around 0.18 (I think) , 125mm gets you a 0.14, 150mm drops it to around 0.11ish. 

 

Law of diminishing returns & cost needs to be factored in, and its prob several years+ payback. Anything more then 150mm is overkill for 90% of people (IMHO)... But this is a forum for enthusiasts! 

 

However, if you can... I'd always opt for more. 

 

We are just facing a £4500 bill (builder supply, fit & overheads) to upgrade from 100 to 150mm in our bungalow, as build already well under way! Probably will, but its a tough call balencing all other aspects of the build costs :(

Edited by Andehh
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49 minutes ago, Andehh said:

We are just facing a £4500 bill (builder supply, fit & overheads) to upgrade from 100 to 150mm in our bungalow

This sounds a bit steep …… but maybe you have a very BIG bungalow ? 

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18 minutes ago, Andehh said:

Big, but not that big... 250sqm! He gave us a spectacularly keen price to start off with, so not unexpected some of the upgrades come at a premium. 

At a rough guess, that's maybe about £2k in materials to upgrade...

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Hmmm... Does seem excessive that doesn't it? I'll query it... He was actually quite reasonably priced on some other tweaks we made (roof lights & internal brick walls...) so maybe I'm missing something! Thanks :)

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It can make a huge difference to just go hybrid, adding 100mm of EPS under the 100mm of PIR ;)  Chesp to achieve if budget is tight with still high performance and good results. 
Folk here who have done passive rafts are all using EPS @ 300mm NOT PIR.

Good thing with using EPS as the underlay, is being able to do that with 25+75 and raising the DPM up atop the 25mm to keep it completely  free from punctures.

100 EPS + 100 PIR would be a good outcome and save you a considerable uplift in costs. 

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18 hours ago, joe90 said:

Not sure 150mm is enough! I have 250mm PIR  and some here have 300mm

 

 

Are you sure you have 250mm of PIR? That would perform way in excess of 300mm of EPS and 300mm of EPS is enough for the most demanding passiv house insulated slabbers.

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35 minutes ago, epsilonGreedy said:

 

Are you sure you have 250mm of PIR? That would perform way in excess of 300mm of EPS and 300mm of EPS is enough for the most demanding passiv house insulated slabbers.

Sorry typo, 200mm PIR ? (Corrected)

Edited by joe90
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10 minutes ago, Conor said:

50mm EPS and 150mm PIR.

EPS on the bottom?

 

The thickness of insulation must provide diminishing returns, but I think the theoretical calculations say that 200 is twice as good as 100. As an extreme example, at 1000 I don't think the last 100mm will be making any difference.

Extending that logic, it would be best to put the better insulation on top....I think.

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7 minutes ago, saveasteading said:

The thickness of insulation must provide diminishing returns, but I think the theoretical calculations say that 200 is twice as good as 100. As an extreme example, at 1000 I don't think the last 100mm will be making any difference.

It is also to do with the overall losses of the building.

Floors are one of the biggest, single elements, and if UFH is fitted, the hottest.

The really big one is that upgrading is almost impossible, so better off getting it in early at the design stage, rather than regret it later.

 

 

Edited by SteamyTea
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1 minute ago, saveasteading said:

At what thickness do you think it becomes uncommercial

That has changed significantly over the last 3 months.

 

I think the only real way it to compare embodied energy and embodied carbon dioxide equivalent emissions.

Cash values are a bit of a nonsense really, especially when inflation erodes the price paid.

 

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3 hours ago, saveasteading said:

At what thickness do you think it becomes uncommercial?

 

Payback is relatively easy to figure out. Just plug the U values into a model of your house with a predicted unit value of heating.

 

In order to balance the relative benefits of our house I used 10c/kwh and increased the payback time until we hit passivhaus levels of annual heat demand. It was 25 years IIRC.  

We ended up at 200mm EPS in the floor, 250mm EPS beads in the walls, 400mm cellulose in the attic. 

 

Time over again that would be at least 300mm EPS floor. 300mm in the wall, 400mm in the roof.  Maybe more............. 

 

To paraphrase Kate Moss, 

 

"Nothing heats as good as egomaniacal-insulation feels" 

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