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. 

[saveasteading]

1 hour ago, JohnMo said:

saved more than that by shopping about,

Of course. But that 50 % is off a published list price for a few boards. My estimates will always assume expert buying.

I'm assuming both, with the rest from waste control, optimising the design, detailing.

And minimal skip cost....or have you a secret of using offcuts?

50 +20.%   Almost free!

 

Actually this is the one area where my estimate seems high. 

I'll report back when decisions are made...this isn't going in for about 8 weeks.

 

Any more experience of the unfaced  stuff from anyone?

 

14 hours ago, G and J said:

would not contemplate a step in the DPM.

My instinct too.

 So prob the 150mm goes down as 2 x 75, on dpm, and following whatever dips and ridges. Then tape and dpm. The 'ponds' can be reassessed for 25mm boards or screed and I'm tending towards just screed it.

 

PIR at about £80/m3 and add tape and labour. 

Screed material I'm only guessing at £150/m3????  Extra labour about 15 minutes.

 

Ahhh. Best do a level survey.

 

[JohnMo]

1 hour ago, saveasteading said:

50 % is off a published list price

No, that is off the lowest price I could get locally.

[SteamyTea]

Have you work out the price by U-Value, rather than thickness.

 

Regarding unfaced sheet. Unfaced are used a lot in the composite plastic industries because it is easier to adhere to.

For any sheet insulation there will be a certain amount of compression when the initial pour of concrete is added over the top.  There has been some discussion in the past about whether it should be polystyrene first  then PIR, or the other way around. I can't remember what the outcome was, but seemed to remembered it was counterintuitive. The insulation manufactures should have that information.

There will also be a long-term creep as well. PUs can keep shrinking for years. They also absorb moisture (as do most materials) that can affect the k-Value quite significantly. I think polystyrene is a bit better in this respect (though it does depend on density.

 

It really comes down to what you want from your floor insulation.

I am with @JohnMo in this respect. Get the very best performance you can financially justify, you can put it right later.

[dpmiller]

Another vote for a layer of EPS. Graphite if you can, it's even more gooder.

[G and J]

How long before some clever wotsit comes up with a self levelling compound, made in a lorry on site and pumped in, that also functions as a DPM.  

 

Until then we’re still talking sheets. I accepted a slightly higher wastage (~4%) as I didn’t want small bits and when I planned it out that required a couple of near half sheets spare.  
 

Some of that had been used elsewhere like the former for the shower tray to exclude the screed.  
 

Unusually for me, the sheets went down almost exactly to plan.  I think the only change was to make the PIR layer slightly easier to get as tight a fit as I could. 

[JohnMo]

7 minutes ago, G and J said:

made in a lorry on site and pumped in, that also functions as a DPM

You can already get it it's called epoxy resin. But plastic sheet is way cheaper.

 

Can remember my dad doing a house up in the 70s, it had some pumped under the ground floor to just below floor level, black but I think that was bitumen based. 

[G and J]

I’ve heard of ash felt floors being laid in old places which as well as being water resistant, apparently made the places warmer than they were (probably was bricks direct on damp earth or the like).   It would be interesting to see how homes are built in 50 years time given the pace of change. 

[G and J]

17 hours ago, SteamyTea said:

Have you work out the price by U-Value, rather than thickness.

 

Regarding unfaced sheet. Unfaced are used a lot in the composite plastic industries because it is easier to adhere to.

For any sheet insulation there will be a certain amount of compression when the initial pour of concrete is added over the top.  There has been some discussion in the past about whether it should be polystyrene first  then PIR, or the other way around. I can't remember what the outcome was, but seemed to remembered it was counterintuitive. The insulation manufactures should have that information.

There will also be a long-term creep as well. PUs can keep shrinking for years. They also absorb moisture (as do most materials) that can affect the k-Value quite significantly. I think polystyrene is a bit better in this respect (though it does depend on density.

 

It really comes down to what you want from your floor insulation.

I am with @JohnMo in this respect. Get the very best performance you can financially justify, you can put it right later.

Because I’m not on site today I just calculated that had I gone for all PIR instead of 120mm each of polystyrene and PIR the payback (assuming underground temp of 8C, COP of 4, peak time electric cost, etc.) would be in the region of 117 years.  Hmmmm.    
 

Perhaps a good illustration of the diminishing returns of insulation.

[saveasteading]

18 hours ago, SteamyTea said:

Get the very best performance you can financially justify,

10 year return would be my judgement, if that fits your advice.

The "very best" implies value for money by my way of thinking. Eg passivhaus to me isn't the best, but a benchmark that is admirable but seldom the best thing to do measured by money or carbon.

[JohnMo]

1 hour ago, G and J said:

ould be in the region of 117 years

Are you sure your maths is correct? Have another coffee and retry?

 

You would need to use your mean UFH flow temperature, not room temperature for the heat loss downwards. That would be around 24 to 25 averaged across the heating season

Edited 1 hour ago by JohnMo

[saveasteading]

Thanks all. What have I learned so far?

 

Eps is much easier to handle and cut neatly. As a good fit is a strong benefit  this looks like a real plus for it.

The unfaced pir appears to be aimed at other uses than construction....keep it in mind for other purposes.

I'm reminded that pir shrinks. Perhaps not in an underfloor situation. And it is a pain to cut. Gaps are inevitable, and the double layer,  staggered joint, principle is pretty well essential. Factory cuts from one corner for the first layer, then reverse for the second.

It's for minor benefit, but why not?

 

AND as the division walls go in, it's apparent that there will be a lot of cutting-in. And that some spaces may be relatively consistent in level so tweaking to an optimum depth isn't so tricky ...perhaps each space will be insulated to the same thickness with any steps being at doors.. I may buy some 25mm eps to make this simple and best value.

 

Waste.   I hate waste in principle and in cost. The unused material and then the skip. Perhaps each room needs a plan.... offcuts get taken to where they will fit neatly.

Fortunately our workers understand this and our timber waste so far is tiny. I will work it out. Guessing 2%.

 

 

 

 

 

 

 

[JohnMo]

36 minutes ago, saveasteading said:

Eps is much easier to handle and cut neatly

But you also need to buy the correct grade for appropriate compression strength. Not all are equal.

 

37 minutes ago, saveasteading said:

And it is a pain to cut.

Not sure I get that, why is it a pain? I have cut PIR and EPS,both are equally a pain or easy.

 

 

[saveasteading]

22 minutes ago, JohnMo said:

why is it a pain?

Other people are saying that. And I've only ever done small amounts using a handsaw or slicing opposite faces.

 

You've had a lorryload. Presumably not all laid as one area, so you've done it.  So any recommendations? What tool? On the floor or a big bench?

 

I think the gross wall perimeter is about 400m. Half of that to cut,  x 2 layers. A lot of cutting.  So that needs to be efficient and precise.

If it's worth buying or hiring the best tool it would be good to know.