Floor insulation: Polystyrene sheet?

[Fevvahz]

I'm currently in the process of digging out and replacing the floors in my 1930's 3 bed semi. In accordance with current building regulations we need to add insulation. We're going to do hardcore, sand, damproof, concrete, then insulation with a hardboard floating floor, on which I'll be adding woodblock.

With regards to the insulation, our local building control has advised a minimum of 75mm of insulation. We've been looking at which products to use from Wickes, and on one hand there's products like Celotex PIR at about £45 for a 2400 x 1000 x 100mm, and on the other there's Wickes Polystyrene Insulation Board at £18.50 2400mm x 1200mm x 75mm. The price difference is huge.

My concern is the themal value of the materials. All I've found regarding extruded polystyrene in general is here "The thermal conductivity of extruded polystyrene is typically between 0.027 and 0.025 W/m K at 10 degrees C." Can anyone offer any advice as to a suitable product which doesnt cost the earth - or will polystyrene be fine?

 

[mointainwalker]

If your local BCO really has just said 75 mm insulation, then that is incomplete info.

As you have discovered PIR, extruded polystyrene and expanded polystyrene all have different U values.

75 mm PIR has a U value of 0.33
75 mm Ext poly................. 0.40
75mm Exp Poly................ 0.50

typical values, individual mftrs may quote slightly different.

Find out what the BCO wants ( in U value) and then you can translate that into whichever material you like at the appropriate thickness ( I believe)

So if you have height to spare you can use flooring-grade exp poly and save money. just be aware this stuff is much more "fragile" than the other two and if you put it down and then walk over it doing other tasks before laying the flooring-surface, you will have lots of broken bits around.

Having just re-read your post, i see you are going to float the insulation on the concrete and then the hardboard (?) on the insulation .

That sounds very wrong to me, but i'm less certain about those areas so will let someone else reply.

 

[Fevvahz]

If your local BCO really has just said 75 mm insulation, then that is incomplete info.

My thoughts precisely, I'll get back onto them about it. Although they've moved the goalposts already with regards the insulation around the perimeter of the slab.

Having just re-read your post, i see you are going to float the insulation on the concrete and then the hardboard (?) on the insulation . That sounds very wrong to me, but i'm less certain about those areas so will let someone else reply.

I've also confirmed this with building control. We can either have the insulation under the concrete slab, requiring a very accurate, very smooth pour, or we can have the insulation on top of the slab, which will enable to us to create an easily adjustable and smooth plywood floating floor on which to add the woodblock flooring. This is what they tend to suggest for garage conversions.

 

[mointainwalker]

I think once you have met regulations for insulaltion, thickness of flooring ( safety) the BCO isn't concerned if your decorative flooring is a disaster - that's down to you.

In your earlier post you wrote" hardboard" which wouldn't work, now you say plywood, that's o.k. ( dependant on thickness )

I get the impression this may be your first major DIY effort and think you will have a shock if you think it will be easy to get a flat(tish) slab and then an easy-to-flatten surface when working with a large area.n.

It will be easier to flatten "lumps" in the expanded poly and extremely difficult in the other materials and you need it flat because once you have put down a large sheet of ply, hauling it up and down to try to find the irregularity that is causing it to be skewwiff is not likely to be a lot of fun.

My advice would be to take maximum care at each stage to get things flat and level and not to think "Oh, I'll correct that slope/whatever with the next stage "

I have made this mistake and found out that pushing inaccuracies ahead of you usually creates a hell of a headache at the finish when you are forced to confront the sum of all problems

Good luck.

 

[DevilDamo]

Even though you're upgrading and not forming from new, would the P/A figure need to be taken into account?

 

[mattysupra]

you want the insulation under the concrete slab. Floating systems really are a pain in the backside and you will regreat it. Your new floorcovering will be alot better adhered to the concrete subfloor.

P.s you need to monitor moister levels to check there below 75%rh for floorcoverings and lower than 75% before you can install a floating subfloor or wooden floor.

 

[Fevvahz]

...the BCO isn't concerned if your decorative flooring is a disaster - that's down to you.

Don't worry, I'm not under that impression at all! Also my apologies - I meant ply - not hardboard in my original post.

I get the impression this may be your first major DIY effort and think you will have a shock if you think it will be easy to get a flat(tish) slab and then an easy-to-flatten surface when working with a large area.n.

It will be easier to flatten "lumps" in the expanded poly and extremely difficult in the other materials and you need it flat because once you have put down a large sheet of ply, hauling it up and down to try to find the irregularity that is causing it to be skewwiff is not likely to be a lot of fun.

My advice would be to take maximum care at each stage to get things flat and level and not to think "Oh, I'll correct that slope/whatever with the next stage "

I have made this mistake and found out that pushing inaccuracies ahead of you usually creates a hell of a headache at the finish when you are forced to confront the sum of all problems

While this is a DIY job in that I'm not paying a 3rd party company to do the work, I have an experienced builder helping me with it. We know what the layering should be, and we were given two different options by one of the BCO's at the local authority. As stated in my previous posts, she was vague regarding the insulation properties, hence my original post about the polystyrene type. The jobs that my builder has been contracted in - we've asked the guy who spec'd the job and have found that they simply spec 100mm insulation, which is quite expensive, simply to cover for building regulations. ie its overkill. We will be taking the utmost care at each stage - if your going to do it, do it right, and do it right first time.

Even though you're upgrading and not forming from new, would the P/A figure need to be taken into account?

Sorry 0 what do you mean regarding the P/A figure? Are you referring to calculations regarding thickness/efficiency of insulation regarding 'cold' border areas etc?

you want the insulation under the concrete slab. Floating systems really are a pain in the backside and you will regreat it. Your new floorcovering will be alot better adhered to the concrete subfloor.

I think its going to end up under the slab at this rate anyway. My builders just been on a job, another BCO from the same authority (who probably sits next to the woman I spoke to initially) has said it needs to be under the slab. Not only that but he insisted that a border of insulation is now only required where the slab meets a cold edge. Once we find out what the BCO's actually want, then we can proceed. Fortunately we're not on a tight schedule!

I appreciate your replies thanks!