Insulated Slab Question

[Shaun McD]

Hi all, 

 

We are starting to get into the working details on our new build, and have yet to make a firm decision on traditional strip V insulated slab foundations. I have only ever known of insulated slabs detailed like below for TF, (stolen from a Kore manual)

This detail calls for services to be within this slab, plumbing, underfloor etc. 

 

I have been recently told of another approach, not sure how common, which has the same as above, but opposed to embedding underfloor and other first fix services within the slab, calls for a 20mm insulation board to be placed on top of the slab, then a finished floor to be poured, containing the underfloor etc.

The reasons I was told that this approach was preferred;

1) This allows the wall installers to place anchors/stays etc. on top of the primary slab without concern of hitting services within the slab,

2) The introduced layer of insulation, while thin at 20mm, provides sufficient assistance to heat up times AND thermal store of the primary slab, i.e. allows to heat up quicker, and cool down quicker.

 

I am pretty conflicted on this logic, while in part I see some merit, I can also see how the primary slab will always get some heat, which will never be encouraged to transmit into the house. 

Would love any feedback/advice, is this approach common, new etc. and what people perception is?

 

[jack]

Welcome Shaun.

 

1 minute ago, Shaun McD said:

This detail calls for services to be within this slab, plumbing, underfloor etc.

 

I don't think that's right. Lots of people on BuildHub (myself included) have an insulated raft, and I believe that the only thing that's run through it (horizontally) is UFH loops. You definitely don't run plumbing or electrics through it.

 

The other approach you've been told about is effectively the old-fashioned screed on a slab approach, but with only minimal insulation between screed and slab. I can't see any advantage to this.

 

4 minutes ago, Shaun McD said:

1) This allows the wall installers to place anchors/stays etc. on top of the primary slab without concern of hitting services within the slab

 

You don't put UFH loops near walls, so with even a moderate amount of care, you shouldn't hit them with anything.

 

7 minutes ago, Shaun McD said:

2) The introduced layer of insulation, while thin at 20mm, provides sufficient assistance to heat up times AND thermal store of the primary slab, i.e. allows to heat up quicker, and cool down quicker.

 

In a well-insulated house, you don't need a fast reaction time from underfloor heating. When it's cold, you want to use the slab as a heat reservoir. If you use a low-and-slow source like and ASHP, then you don't want/need fast reaction. If you use a higher temperature source like a gas boiler, a slower response will help smooth out peaks and troughs.

 

Personally, I think you're adding considerably cost and complexity by having a further layer of insulation and a separate screed, without really gaining much benefit.

[Iceverge]

Strip foundations can be made to work too. 

 

About 5k cheaper for us but we're lucky with our ground conditions and blocklayers were cheaper 4 years ago. 

 

Although UFH has been proven to be robust there's nothing wrong with radiators in a well insulated house if you're suspicious of pipes burried in concrete. 

 

Otherwise I wouldn't go reinventing the wheel.   

 

 

 

 

 

 

[Shaun McD]

Thanks Jack, I think we are on the same page, but is new to me that it was the "old" way, at least it indicates it is not a new thing which is unproven, instead just more complex than needs be!

 

I would have assumed that other plumbing service pipes to sinks, showers etc. would run through the slab? And also any electrical conduit to island etc?

 

Re loops near walls, I would expect to have loops 100-150mm from wall based on loop, but I think the fear is more that large stays needing to be anchored at distances greater then a meter is where the concern would be from. I would also question anchor depth and if that would really trouble ufh pipes in a 150mm slab, but only relaying the info I was given.

[Shaun McD]

3 minutes ago, Iceverge said:

Strip foundations can be made to work too. 

 

About 5k cheaper for us but we're lucky with our ground conditions and blocklayers were cheaper 4 years ago. 

 

Although UFH has been proven to be robust there's nothing wrong with radiators in a well insulated house if you're suspicious of pipes burried in concrete. 

 

Otherwise I wouldn't go reinventing the wheel.   

 

 

 

 

 

 

Thanks Iceverge, 

 

Not hugely concerned tbh, was more interested in the validity of what I was told.

Our TF company has told us they typically carry the internal insulation board, 70mm, down below the sole plate to subfloor level, which in itself should do a lot, if not solve, the cold bridging issue. Once insulation is then added on top of the subfloor, it is basically encased within the extended internal wall insulation. Most likely will be our route, but I think insulated slabs are for sure an improvement.

 

We also have the cost consideration, our large single story footprint (270m2) will call for much more than average linear meter of dead work strips, and cost a bomb to lay two sheets of floor insulation on the sub floor. I have a suspicion that an insulated method my be as cost effective, if not actually cheaper, based on how its made. Would love if anyone had any figures of costings that we could gauge this on... 

[JohnMo]

25 minutes ago, Shaun McD said:

dead work strips

What are they?

[Iceverge]

What is the spec of your TF wall? 

 

It can be relatively easy with TF to make pretty much thermal bridge free foundations. 

 

Depending on the layout of the building it may be possible to eliminate all load bearing internal walls. This means that all internal walls won't bridge to the ground. 

 

2020 prices but our 114m2 base cost €16226+VAT vs an insulated slab at €20300. 

 

 

[JohnMo]

Strip foundation are easy to do, simple blockwork, just need to be careful you design out cold bridges.

 

Single storey big houses are expensive floors to put together there is no getting away from it.

 

Our internal structural walls are sitting on thermolite blocks topped of with 65mm engineered bricks.  Simple to install low cost.  200mm of PIR, 100mm concrete screed, perimeter of wall has 70mm PIR upstand.

 

 

 

[Shaun McD]

TF external wall make up is proposed;

Breathable racking board (forget thickness)

140 stud, pumped with cellulose

70mm polyurethane insulation layer which doubles as airtight layer.

 

Proposed U value = 0.15 W/m2K

 

Agreed that there are options to remove cold bridging. Heard horror stories around thermal blocks cracking/breaking when sole fixings are installed but no idea if thats a real concern. 

I am guessing we can use them for both external and internal rising block for TF

[JohnMo]

5 minutes ago, Shaun McD said:

horror stories around thermal blocks cracking/breaking when sole fixings

Hense putting them below engineered bricks.  The engineered bricks take the fixings.

[Shaun McD]

2 hours ago, JohnMo said:

Hense putting them below engineered bricks.  The engineered bricks take the fixings.

Good to know!! More than a few things to be remembering ha

[dpmiller]

Our TF guys had no issues putting the sole plate onto quinnlites.

[Iceverge]

13 hours ago, Shaun McD said:

TF external wall make up is proposed;

Breathable racking board (forget thickness)

140 stud, pumped with cellulose

70mm polyurethane insulation layer which doubles as airtight layer.

 

Proposed U value = 0.15 W/m2K

 

Agreed that there are options to remove cold bridging. Heard horror stories around thermal blocks cracking/breaking when sole fixings are installed but no idea if thats a real concern. 

I am guessing we can use them for both external and internal rising block for TF

 

I'm not a massive fan of internally insulating with PIR. (I guess this is what you meant rather than polyurethane). It keeps the timber too cold IMO and the end of your first floor joists will be cold. You could put the PIR outside the frame like they do in the state's. 

 

 

Have you considered just using a 220mm stud full of cellulose and a battened insulated service cavity? 

 

Putting PIR inside studs is a PITA, ask @Thorfun

 

 

 

 

 

[jack]

14 hours ago, Shaun McD said:

Thanks Jack, I think we are on the same page, but is new to me that it was the "old" way, at least it indicates it is not a new thing which is unproven, instead just more complex than needs be!

 

I was more referring to the general concept of an insulated screed over a structural slab being known. In that situation, all of the insulation is between the slab and the screed. I've never heard of starting with an insulated structural slab, then adding only 20 mm of insulation before adding a screed. 20 mm will have very little practical effect. Your screed (assuming concrete) will need to be something like 75 mm thick, so assuming a structural slab averaging 150 mm, you're adding at least 50% more concrete to the build for no real reason.

 

14 hours ago, Shaun McD said:

I would have assumed that other plumbing service pipes to sinks, showers etc. would run through the slab? And also any electrical conduit to island etc?

 

I'm curious about why you're assuming this is how it would be done? Has whoever you're talking to said this is how they'd do it?

 

There's a huge difference between running the odd conduit for a particular reason (we have one to get power to the hob on our island) and running all of your services through the slab. I've never heard of anyone doing the latter, and I can think of lots of reasons why you wouldn't, starting with accessibility for maintenance. @Nickfromwales, you've worked on loads of houses with insulated slabs. Care to chip in?

 

14 hours ago, Shaun McD said:

Re loops near walls, I would expect to have loops 100-150mm from wall based on loop, but I think the fear is more that large stays needing to be anchored at distances greater then a meter is where the concern would be from. I would also question anchor depth and if that would really trouble ufh pipes in a 150mm slab, but only relaying the info I was given.

You don't run UFH under any walls, including internal, so assuming any internal stays go through the base of internal walls, I don't see what the issue is.

 

Again, a large number of people on BuildHub have insulated raft foundations, and the issues you've raised either don't exist or are easily dealt with.

[Thorfun]

14 minutes ago, Iceverge said:

 

I'm not a massive fan of internally insulating with PIR. (I guess this is what you meant rather than polyurethane). It keeps the timber too cold IMO and the end of your first floor joists will be cold. You could put the PIR outside the frame like they do in the state's. 

 

 

Have you considered just using a 220mm stud full of cellulose and a battened insulated service cavity? 

 

Putting PIR inside studs is a PITA, ask @Thorfun

 

 

 

 

 

we didn't put PIR inside studs. our studs are filled with 140mm glass wool (Isover 32) with 80mm/100mm PIR internal to the studs. What IS a PITA is cutting that internal PIR around the roof attic trusses. no issues when insulating internally on cut roofs as it's simply a large sheet screwed through to the rafters/studs.

 

but, if the OP is going pumped cellulose in a 140mm TF then I would suggest they consider other companies that offer twin wall 300mm thick walls fully pumped with cellulose and get rid of the petroleum based insulation. Although, if the TF company is doing the PIR install work for you then it's not such a ball ache! I'm sure I've saved a load of money doing it myself but it is so very time consuming I do kind of regret it. 

[Thorfun]

13 minutes ago, jack said:

I've never heard of starting with an insulated structural slab, then adding only 20 mm of insulation before adding a screed. 20 mm will have very little practical effect. Your screed (assuming concrete) will need to be something like 75 mm thick, so assuming a structural slab averaging 150 mm, you're adding at least 50% more concrete to the build for no real reason.

our basement has 200mm EPS under the slab as an insulated slab. at the time I wanted to run the UFH pipes within the slab but it didn't work out that way and so we eventually put down 25mm PIR on top of the slab (mostly to aid in the stapling down of the UFH pipes rather than anything else) and then UFH pipes followed by a 50mm cemfloor liquid screed.

 

I don't see a problem doing it this way except for the extra cost.

[Nickfromwales]

1 hour ago, Thorfun said:

our basement has 200mm EPS under the slab as an insulated slab. at the time I wanted to run the UFH pipes within the slab but it didn't work out that way and so we eventually put down 25mm PIR on top of the slab (mostly to aid in the stapling down of the UFH pipes rather than anything else) and then UFH pipes followed by a 50mm Cemfloor liquid screed.

That's for a basement though  

 

1 hour ago, Thorfun said:

I don't see a problem doing it this way except for the extra cost.

I don't like installing a skinny heated layer over a decent constructional slab as it simply turns on and off ( as a heat emitter ). Not desirable AFAIC vs the huge thermal time constant(?) you achieve from heating an entire slab. That allows you to strategically 'inject' pockets of low cost energy into the floor as a battery / heat bank and for that to slowly release heat into the home with near zero difference in temp shift ( hysteresis ). Simply night and day better IMHO.

Heating a constructional slab / passive aka insulated raft is absolutely the better option, and installing a 300mm wall a-la MBC and fully pumping with cellulose is just an absolute and inarguable no-brainer ( budget allowing ).

If you are time-constrained ( or impatient ) then removing all the extra labour / cost / detailing / airtight measures of a multi-discipline wall system will pay massive dividends in 'getting a wiggle on', as one MBC PH TF I was on took 31 days from a hole in the ground > passive raft > frame up and complete ( YES, 31 DAYS ) including a couple of weeks on stop for the slab to cure before the frame went on.

When the stars align, I will build my own out of MBC PH TF, and develop with that or Nudura. Just waiting for a ship to come in, and then it's off developing I go .

 

Anyone who wants quick, stress free and to have a huge amount of liability / responsibility removed should look long and hard at MBC's offering with a minimum guaranteed 

airtightness score of 0.6ACH to boot! Folk, some are / were on here, have had issues with MBC and them farming stuff out to sub-contractors, I'm on one that suffered this atm, but thankfully Joe took direct action and this has now been resolved. No company can say they've been 100% issue-free 100% of the time, I don't care who they are, ( or if they do then they've just been very good / clever in covering it up the trail of breadcrumbs ).

 

DIY'ers won't want to pay the premium for a turnkey package, some will actively desire acquiring their self-build wings, so, as with anything, it's horses for courses. The choice of company and service will be down to the clients own remit. 

 

@Dreadnaughtran services through the slab for his single storey abode, but underneath in the insulation with ducts / sleeves / conduits all planned out well, in advance.

For clarity, you should be running zero IN the heated slab afaic, but under, in the lower ( lowest ) layer of insulation is prime real estate IMO ( providing that wet services are still wrapped in insulation and any pipes do NOT have ANY joints WHATSOEVER.

[Nickfromwales]

Beware running electrical cables directly in insulation though! You need much larger ducts / conduits to run cables, or you will not evade regs requirements for de-rating a circuit or massively increasing the conductor size to allow you to run through insulation. You don't need to worry about short rises / drops though the 300mm thickness, but after 300mm horizontal for eg the regs kick in. 

[Shaun McD]

2 hours ago, Iceverge said:

 

I'm not a massive fan of internally insulating with PIR. (I guess this is what you meant rather than polyurethane). It keeps the timber too cold IMO and the end of your first floor joists will be cold. You could put the PIR outside the frame like they do in the state's. 

 

 

Have you considered just using a 220mm stud full of cellulose and a battened insulated service cavity? 

 

Putting PIR inside studs is a PITA, ask @Thorfun

 

 

 

 

 

 

It actually calls out PUR (polyurethane) rather than PIR in the quotes. I had not noticed this detail so thanks for calling out. Not sure if its an issue but my understanding was PIR is better for fire so prob would be the one to go with? 

 

2 hours ago, jack said:

 

I was more referring to the general concept of an insulated screed over a structural slab being known. In that situation, all of the insulation is between the slab and the screed. I've never heard of starting with an insulated structural slab, then adding only 20 mm of insulation before adding a screed. 20 mm will have very little practical effect. Your screed (assuming concrete) will need to be something like 75 mm thick, so assuming a structural slab averaging 150 mm, you're adding at least 50% more concrete to the build for no real reason.

 

 

I'm curious about why you're assuming this is how it would be done? Has whoever you're talking to said this is how they'd do it?

 

There's a huge difference between running the odd conduit for a particular reason (we have one to get power to the hob on our island) and running all of your services through the slab. I've never heard of anyone doing the latter, and I can think of lots of reasons why you wouldn't, starting with accessibility for maintenance. @Nickfromwales, you've worked on loads of houses with insulated slabs. Care to chip in?

 

You don't run UFH under any walls, including internal, so assuming any internal stays go through the base of internal walls, I don't see what the issue is.

 

Again, a large number of people on BuildHub have insulated raft foundations, and the issues you've raised either don't exist or are easily dealt with.

 

Likewise, I have not seen or known of this done before and sounds like my hunch was right that it would not be the way to go about it.

Agreed, the only services I was assuming within the slab was UFH, first fix plumbing (deepest part of slab) and odd conduits for mid floor electrical, again likely in deepest part of slab.

Sounds like we are on same page here, I was curious if what I was told by this particular installer was valid, but sounds like it is not a real risk.

 

1 hour ago, Thorfun said:

we didn't put PIR inside studs. our studs are filled with 140mm glass wool (Isover 32) with 80mm/100mm PIR internal to the studs. What IS a PITA is cutting that internal PIR around the roof attic trusses. no issues when insulating internally on cut roofs as it's simply a large sheet screwed through to the rafters/studs.

 

but, if the OP is going pumped cellulose in a 140mm TF then I would suggest they consider other companies that offer twin wall 300mm thick walls fully pumped with cellulose and get rid of the petroleum based insulation. Although, if the TF company is doing the PIR install work for you then it's not such a ball ache! I'm sure I've saved a load of money doing it myself but it is so very time consuming I do kind of regret it. 

 

Yeah TF company would be installing both cellulose and the 70mm PUR/PIR onto studs. Ceilings will also have blown cellulose, so dont think there is any area to cut PUR/PIR. Twin wall system would be nice to have, but have we are very limited with installers in our region, and anyone who is willing to travel has been out of budget.

 

1 hour ago, Nickfromwales said:

That's for a basement though  

 

I don't like installing a skinny heated layer over a decent constructional slab as it simply turns on and off ( as a heat emitter ). Not desirable AFAIC vs the huge thermal time constant(?) you achieve from heating an entire slab. That allows you to strategically 'inject' pockets of low cost energy into the floor as a battery / heat bank and for that to slowly release heat into the home with near zero difference in temp shift ( hysteresis ). Simply night and day better IMHO.

Heating a constructional slab / passive aka insulated raft is absolutely the better option, and installing a 300mm wall a-la MBC and fully pumping with cellulose is just an absolute and inarguable no-brainer ( budget allowing ).

If you are time-constrained ( or impatient ) then removing all the extra labour / cost / detailing / airtight measures of a multi-discipline wall system will pay massive dividends in 'getting a wiggle on', as one MBC PH TF I was on took 31 days from a hole in the ground > passive raft > frame up and complete ( YES, 31 DAYS ) including a couple of weeks on stop for the slab to cure before the frame went on.

When the stars align, I will build my own out of MBC PH TF, and develop with that or Nudura. Just waiting for a ship to come in, and then it's off developing I go .

 

Anyone who wants quick, stress free and to have a huge amount of liability / responsibility removed should look long and hard at MBC's offering with a minimum guaranteed 

airtightness score of 0.6ACH to boot! Folk, some are / were on here, have had issues with MBC and them farming stuff out to sub-contractors, I'm on one that suffered this atm, but thankfully Joe took direct action and this has now been resolved. No company can say they've been 100% issue-free 100% of the time, I don't care who they are, ( or if they do then they've just been very good / clever in covering it up the trail of breadcrumbs ).

 

DIY'ers won't want to pay the premium for a turnkey package, some will actively desire acquiring their self-build wings, so, as with anything, it's horses for courses. The choice of company and service will be down to the clients own remit. 

 

@Dreadnaughtran services through the slab for his single storey abode, but underneath in the insulation with ducts / sleeves / conduits all planned out well, in advance.

For clarity, you should be running zero IN the heated slab afaic, but under, in the lower ( lowest ) layer of insulation is prime real estate IMO ( providing that wet services are still wrapped in insulation and any pipes do NOT have ANY joints WHATSOEVER.

 

31 days sure sounds like a dream! As you say, maybe the next time..

 

3 minutes ago, Nickfromwales said:

Beware running electrical cables directly in insulation though! You need much larger ducts / conduits to run cables, or you will not evade regs requirements for de-rating a circuit or massively increasing the conductor size to allow you to run through insulation. You don't need to worry about short rises / drops though the 300mm thickness, but after 300mm horizontal for eg the regs kick in. 

Good to know, I had toyed with the idea of running MHRV pipework inlets in the floor but decided against it due to the complexity it would bring in impacting either insulation or concrete depth

 

[Thorfun]

4 minutes ago, Shaun McD said:

Yeah TF company would be installing both cellulose and the 70mm PUR/PIR onto studs. Ceilings will also have blown cellulose, so dont think there is any area to cut PUR/PIR. Twin wall system would be nice to have, but have we are very limited with installers in our region, and anyone who is willing to travel has been out of budget.

sounds good to me. do your TF company offer any sort of guaranteed airtightness level? it'd be a shame to put all that insulation in only to have the house leak air! if they've not mentioned anything maybe ask them what their airtight policies and procedures are.

[Shaun McD]

24 minutes ago, Thorfun said:

sounds good to me. do your TF company offer any sort of guaranteed airtightness level? it'd be a shame to put all that insulation in only to have the house leak air! if they've not mentioned anything maybe ask them what their airtight policies and procedures are.

 

Yeah they provide for a test and its in the contract to be under 2.5, but we have a gentleman's agreement that the target is 1.5! It will be done before floor insulation, finished floor, plaster-boarding, and plaster so will be scope to DIY improve on our result from there too. 

[Nickfromwales]

1 minute ago, Shaun McD said:

It will be done before floor insulation, finished floor, plaster-boarding, and plaster so will be scope to DIY improve on our result from there too. 

I would strongly advise employing an AT testing company who agree to stay on site for an hour or two whilst you locate / cure / re-try. AT is the key to cheap heating bills and maximising the RoI from you MVHR. AT is a far better bang for your buck than insulation, IMO, as ventilation heat loss is a killer vs fabric heat loss.

Go at this with all you have is my advice, and get sub 1.0ACH or better.

 

I'm now going to get myself a DIY rig as suggested by others here, so I can perform my own "day of plugging / testing" on my clients behalf, eg before employing a 4rd party to come and test and issue a cert. Seems the best money my clients could spend, and is already going down well when I suggest it. 

[Shaun McD]

6 minutes ago, Nickfromwales said:

I would strongly advise employing an AT testing company who agree to stay on site for an hour or two whilst you locate / cure / re-try. AT is the key to cheap heating bills and maximising the RoI from you MVHR. AT is a far better bang for your buck than insulation, IMO, as ventilation heat loss is a killer vs fabric heat loss.

Go at this with all you have is my advice, and get sub 1.0ACH or better.

 

I'm now going to get myself a DIY rig as suggested by others here, so I can perform my own "day of plugging / testing" on my clients behalf, eg before employing a 4rd party to come and test and issue a cert. Seems the best money my clients could spend, and is already going down well when I suggest it. 

 

That is a good idea, I am sure I can connect with the AT company the TF company uses, and agree to pay for some time beyond the test paid for by the TF company. Thanks for the tip!

 

What type of DIY rig are you thinking? Your own blower? 

[JohnMo]

1 hour ago, Shaun McD said:

PIR is better for fire

It is - it burns really well

[SteamyTea]

2 hours ago, Shaun McD said:

It actually calls out PUR (polyurethane) rather than PIR in the quotes.

Same base.

PUR is shorthand for polyurethane.

PIR is shorthand for polyisocyanurate.

Polyurethanes often use isocyanurates as the hardening agent.

 

There is a difference, but only really in the manufacturing process of EPS (expanded) and XPS (extruded).

 

It is the thermal and mechanical properties that are important.