Floor slab insulation. Test my logic please?

[saveasteading]

We will have 150mm of insulation under a 60mm, or so, screed.

I'm looking at costs and practicalities.

the existing slab has a variation of 30mm in level and is about 18m x 18m.

 

150mm to 180mm of insulation is preferable in performance and cost to making the difference up in screed.

 

I'd prefer 2 x 75mm thickness to allow overlaps, with the top face taped. Then add 25mm where necessary

but perhaps  1 x 100 everywhere and a top layer of 50 or 75 as appropriate.

 

dpc and/or tape over steps may be tricky.

 

I started off considering PIR to the top and EPS to the bottom for economy. On the basis that EPS is half as good and half the cost.

But as we have an exceptional HP/A,  the centre of the building isn't going to lose much heat downwards. And, as previously discussed on here, the performance is not linear and insulation diminishes with thickness.

 

Listed costs for PIR are very similar on some sites (who will match each other) with others a lot dearer (but will match) so I'm using these for the exercise.

I'm optimistic the area will encourage a discount. The sites quote different delivery charges at surprisingly small sums (order of £150ish) so their target buyer appears to be for small works,

 

As such, EPS is half the cost for thin boards but 2/3 for thicker boards.   But assuming half price, the saving is £1,100. 

 

The Seconds company seem to be the only supplier of unfaced boards. That can't be so?

It is much cheaper and matches the eps prices.

There is no benefit from a foil facing in the dark with no air spaces. Or is there strength or the tape sticks better?

 

Seconds quote thicknesses as eg 90-100mm or 140-150mm. Does this imply that most of the product has been rejected for being too thin?

Or maybe some is thin, and they don't know what will turn up,  so this is sensible advice.

But what do I order if I dont know the actual thicknesses. Maybe they can tell me at the time.

 

I'd like, as always, to support local merchants rather than distance. so am hoping they can match these prices. maybe they can source seconds when requested.

I wonder if it would be double handled at the yard or come direct from the factory.

 

Any thoughts and experiences welcomed, both science and site, as are questions.

 

[Mr Punter]

The foil tape on the joins will not last long, especially if the boards have any dust from cutting etc.  Thin polythene will work fine.  150 is tricky to cut but if it is one big area may be OK.  I would buy foil faced.  Unfaced will turn to crumb / powder when walked on.

[saveasteading]

48 minutes ago, Mr Punter said:

I would buy foil faced.  Unfaced will turn to crumb / powder when walked on.

Hadn't thought of that. Just how soft / fragile is it?

what about handling it?

 

But the cost difference appears to be £1,000 maybe more if in 2 layers. 

There will be lots of cutting as the rooms are divided. But that would help in  defining areas and promptly covering  in polythene.

[Roger440]

Your comment about non faced only being available used/seconds is something i have wondered before. Where does it come from? I think its imported.

 

Ive bought a heap of stuff from seconds, and it does indeed vary slightly in tickness. Certainly more so than when ive bought "proper" stuff. Mine was nominal 100,, foil faced, but many were 5mm plus less.

 

I found the "cold weather" tape sticks to unfaced OK. Maybe not for long, but that wont matter if you are putting screed over it.

 

I dont think id be concerned about using unfaced PIR here, assumimng the slab you are putting it over is dry and stays dry?

 

[G and J]

I was dead set on 250mm of polystyrene with the DPM effectively sandwiched in the middle.   Our slab however came up higher than planned so we ended up with 240mm of space so we converted to a PIR layer atop the polystyrene.   
 

I sort of priced various options using online prices and almost ended up designing by spreadsheet.  
 

Then I talked to my favourite building supplies bunch.  A few mildly confused emails led to a clarifying phone call and it being pointed out that if I used 120mm of both then stocks are held locally and a good price could be had. Suddenly it got easier.  We got prices matching or beating the best I could find online. 

So given my experience I’d have a coffee with the most helpful local dudes and see what they suggest. 
 

I would not contemplate a step in the DPM.   I’d put a flat layer of polystyrene down then the DPM then vary the above later. 
 

I found to get good straight cuts in PIR I had to saw and 120mm is not much more nasty than 50mm.  Both I found horrible.  But I did notice that when the foil face became detached the sheets were much weaker and had to be handled more carefully.  I suspect one gets less horrid dust with foil faces too. 

[G and J]

PS.  Regardless of economy polystyrene is so nice and easy to work with - the spring in it means you can easily get it really tight - for me that makes it worth serious consideration.  PIR May have a better lambda but the better fit of polystyrene must compensate a bit methinks. 

[JohnMo]

Stop faffing, get PIR, spend once, save for ever more. Once in you cannot upgrade.

 

I shopped around, bought online saved a small fortune, it came on lorry direct from factory - I had a full lorry load of 100mm thick, so 2 layers.

[G and J]

We’re self builders.  
Faffing is what we do. 
Why do the obvious when one can avoid doing anything for weeks while researching?

😕 

[saveasteading]

12 hours ago, G and J said:

coffee with the most helpful local dudes 

That's arranged. But on site so he can see the vast amount already from them and the future scope.

And he can go away with some homework.

Interesting about the stocks. The cost £/m3 seems to be in just 2 bands. 25mm to 50mm is dearer. Above that it is nearly identical whatever thickness as if the foil had no value.

But the local merchant buying in a whole load of one size would get them a good price so I'll look I to that.

Another benefit would be phased  delivery. When we need it and less handling. And we can order just enough and a few more later if needed. 

I'm thinking 5% waste....any thoughts?

 

56 minutes ago, JohnMo said:

Stop faffing, get PIR, spend once, save for ever more. 

Faffing is what I do. Efficient, holistic design, negligible waste but without stinting on quality.

If I can make £1,000 by faffing I will do so.

Or put another way: 20% saving on materials: imagine that on the whole project.

The floor insulation value with 150mm of whatever on 150 concrete and HP/A of 0.1 gives us plenty and I'll take the cash now thanks.

 

[JohnMo]

25 minutes ago, saveasteading said:

thinking 5% waste..

I thought that, and god you had a load of excess, I would allow 1-2%.

 

26 minutes ago, saveasteading said:

another way: 20% saving on materials

I saved more than that by shopping about, without compromising on quality. What was on the plan was bought, but at good rates. Saved 50%+ on insulation alone by shopping about.