Is insulation needed on 1st floor concrete slab?

Cks5 · 24-01-2015 12:30am #1

Just wondering if fitting wet ufh to a 1st floor concrete slab, covering with sand/cement screed, is insulation needed?
If not, it's going to save me approx 2k
Is insulated plasterboard on the ground floor ceiling an option, then a 4-6" service void followed by suspended ceiling

galwaytt · 25-01-2015 1:55am

Cks5 wrote: »

Just wondering if fitting wet ufh to a 1st floor concrete slab, covering with sand/cement screed, is insulation needed?
If not, it's going to save me approx 2k
Is insulated plasterboard on the ground floor ceiling an option, then a 4-6" service void followed by suspended ceiling

Absolutely it is: otherwise you're going to be heating the r/c slabs and walls before you heat the rooms............which may be never.

Cks5 · 25-01-2015 10:44am

I have read conflicting advice on this.
Ground floor obviously is a must or you are heating the ground, but 1st floor is different as the heat has no where to escape to only into the slab or into the service void , but the heat isn't actually leaving the house

galwaytt · 25-01-2015 10:58pm

Cks5 wrote: »

I have read conflicting advice on this.
Ground floor obviously is a must or you are heating the ground, but 1st floor is different as the heat has no where to escape to only into the slab or into the service void , but the heat isn't actually leaving the house

Conflicting - lol. It's just physics.

But remember why the ff slabs are called 'hollow core' -it's because they are hollow (sic) and are open to the cavity (think heat path/airtightness - or the lack thereof). Remember they're on the walls (think thermal bridging). And finally, even if they were none of those things, why would you want to bury €€€€ in heating a structural part of the building, when you only need to heat the screed.

At the end of the day though, it's up to you, you'll be paying for the fuel.

D_D · 26-01-2015 4:21pm

galwaytt wrote: »

Conflicting - lol. It's just physics.

But remember why the ff slabs are called 'hollow core' -it's because they are hollow (sic) and are open to the cavity (think heat path/airtightness - or the lack thereof). Remember they're on the walls (think thermal bridging). And finally, even if they were none of those things, why would you want to bury €€€€ in heating a structural part of the building, when you only need to heat the screed.

At the end of the day though, it's up to you, you'll be paying for the fuel.

But by insulating the underside of the first floor slab, are you not removing potential thermal mass?

BarneyMc · 26-01-2015 4:47pm

I think the important thing here is without FF insulation it would just be too hard to control the heating of the first floor bedrooms. For example if the hollow core slab is heated then even if the heating is off at night, it will be releasing heat into the bedroom at night whether needed or not.

Are there any recommendations to the thickness of insulation needed here - 1 to 2 inches?

galwaytt · 26-01-2015 10:43pm

D_D wrote: »

But by insulating the underside of the first floor slab, are you not removing potential thermal mass?

No, you're heating it from above.

galwaytt · 26-01-2015 10:45pm

BarneyMc wrote: »

I think the important thing here is without FF insulation it would just be too hard to control the heating of the first floor bedrooms. For example if the hollow core slab is heated then even if the heating is off at night, it will be releasing heat into the bedroom at night whether needed or not.

Are there any recommendations to the thickness of insulation needed here - 1 to 2 inches?

I'll defer to anyone else with better info, but I'd have thought even 50 mm is skimping it.

Was this not worked out on your BER OP ?

BarneyMc · 26-01-2015 10:49pm

galwaytt wrote: »

I'll defer to anyone else with better info, but I'd have thought even 50 mm is skimping it.

Was this not worked out on your BER OP ?

No, it's not in my BER. Anyone else out there like to share what they used?

The_Bullman · 27-01-2015 12:58am

galwaytt wrote: »

Conflicting - lol. It's just physics.

But remember why the ff slabs are called 'hollow core' -it's because they are hollow (sic) and are open to the cavity (think heat path/airtightness - or the lack thereof). Remember they're on the walls (think thermal bridging). And finally, even if they were none of those things, why would you want to bury €€€€ in heating a structural part of the building, when you only need to heat the screed.

At the end of the day though, it's up to you, you'll be paying for the fuel.

Wouldn't wrapping the slab deal with the airtightness issue?
If the slab sits on the walls, and is externally or cavity insulated - would there be a thermal bridge?

It also wasn't mentioned on my BER. Not sure where on the BER its even an option to mention it.

I don't want to be paying for extra fuel either!

Calahonda52 · 27-01-2015 1:27am

If insulation is used, does the screed not have to be thicker to be more rigid?

Not insulating it will make temp control in bedrooms much harder as noted.

the screed should be insulated from the perimeter like downstairs.

Personally, I wouldn't use UFH upstairs.
Neither would I use HC slab, but there you be

I would spend the mula on air-tightness and insulation

galwaytt · 27-01-2015 1:29am

BarneyMc wrote: »

No, it's not in my BER. Anyone else out there like to share what they used?

.. then there's something remiss with your BER then. ..

gooseygander · 27-01-2015 1:41am

BarneyMc wrote: »

No, it's not in my BER. Anyone else out there like to share what they used?

I went with just 1 inch under screed, also butted up the insulation around the perimeter of all upstairs walls to give screed a bit of room to contract and prevent cracking

sydthebeat · 27-01-2015 8:46am

galwaytt wrote: »

.. then there's something remiss with your BER then. ..

No theres not.

The BER assumes no heat loss here therefore It doesn't come into the equation.

BarneyMc · 27-01-2015 9:09am

sydthebeat wrote: »

No theres not.

The BER assumes no heat loss here therefore It doesn't come into the equation.

Exactly my thoughts! The slab sits within the insulation layer so should have no impact on the overall heating demand. CONTROL of how upstairs is heated is another issue and that's what we're talking about here.

ferryman35 · 27-01-2015 3:41pm

FWIW we're planning on a 75mm sand cement screed over 50mm on insulation on the first floor, (with upstands around the perimeters)

rampantbunny · 27-01-2015 4:21pm

BarneyMc wrote: »

No, it's not in my BER. Anyone else out there like to share what they used?

Mine is called out as 30mm insulation/70mm screed.
I'm actually going 40mm insulation and 60 screed. I was buying 40mm thick board for elsewhere so it made sense to try and get more of the one thickness instead of having lots of differing thicknesses to work with.
Will use 40mm for edge insulation also.

mfceiling · 27-01-2015 5:43pm

It's an interesting topic for me.

I'm often asked if the underside of the hollowcore should be insulated before I install the ceiling? Or should rockwool be rolled out above the ceiling grid? Or should an insulated board go on the underside of the ceiling?

My answer - I don't know!!

If someone can give me a good answer i'll be able to look them confidently in the eye and think "boardsies have my back here"

galwaytt · 27-01-2015 5:58pm

sydthebeat wrote: »

No theres not.

The BER assumes no heat loss here therefore It doesn't come into the equation.

...so a hollowcore slab on a cold exterior wall, or on a interior wall not insulated from the ground is not considered in the BER calculation ?? That doesn't make sense.

galwaytt · 27-01-2015 5:59pm

mfceiling wrote: »

It's an interesting topic for me.

I'm often asked if the underside of the hollowcore should be insulated before I install the ceiling? Or should rockwool be rolled out above the ceiling grid? Or should an insulated board go on the underside of the ceiling?

My answer - I don't know!!

If someone can give me a good answer i'll be able to look them confidently in the eye and think "boardsies have my back here"

...ignoring any issues of bridging or airtightness, the question is why waste energy into the slab above you ?

sydthebeat · 27-01-2015 6:01pm

galwaytt wrote: »

...so a hollowcore slab on a cold exterior wall, or on a interior wall not insulated from the ground is not considered in the BER calculation ?? That doesn't make sense.

if it breaks the insulation envelope, then yes technically the acceptable details thermal bridge factor of 0.08 shouldnt be applied.

But that would just be bad design to allow it to do that, and i have never seen a case where that happened.

BarneyMc · 27-01-2015 6:19pm

galwaytt wrote: »

...ignoring any issues of bridging or airtightness, the question is why waste energy into the slab above you ?

Is it 'wasted' or 'thermally stored'?

Strolling Bones · 27-01-2015 7:34pm

wasted

ferryman35 · 27-01-2015 8:28pm

Would you not think its 'thermally stored'?

The hollow core slab is resting on the inner leaf of the external wall....so apart from the insulation factor provided by the airtightness membrane which is probably negligible in this case, the hollowcore is already thermally connected to the wall, and is there not going to be some thermal balancing going to be happening there anyway.

On the other hand - if you insulate the slab underneath and above are you not creating an interstitial situation again.

Remember that the ff screed is insulated from the walls by the upstands (in addition to them providing an expansion zone) as well as from the hollow core below by the insulation beneath.

And I guess detail is further improved by using quinnlites to break the thermal bridges between the internal wall leaf and external elements.

BryanF · 27-01-2015 10:08pm

What is all the talk about thermal bridging? Is the thickness of cavity insulation not the same at the floor slab as it is elsewhere in the cavity ? is the question not just about heat delivery only?

BarneyMc · 27-01-2015 10:12pm

BryanF wrote: »

What is all the talk about thermal bridging? Is the thickness of cavity insulation not the same at the floor slab as it is elsewhere in the cavity ? is the question not just about heat delivery only?

It should be.

sydthebeat · 27-01-2015 10:21pm

it is, its simply a responsiveness issue.

i would spec around 50mm, depending on depths to play with.

theres really no point going any thicker because the delta T will be quite small between the two, so heat flux will be slow.

Rabbo · 27-01-2015 10:29pm

sydthebeat wrote: »

No theres not.

The BER assumes no heat loss here therefore It doesn't come into the equation.

It dosen't effect the heat loss but it will effect the responsiveness category and therefore effect the BER.

There are plenty of horror stories out there of almost unusable underfloor heating systems because there were installed in thick concrete slabs. Installing in a screed on a concrete first floor with no insulation in between amounts to the same thing. It would take hours for the heat to penetrate all that concrete.

sydthebeat · 27-01-2015 10:39pm

Rabbo wrote: »

It dosen't effect the heat loss but it will effect the responsiveness category and therefore effect the BER.

There are plenty of horror stories out there of almost unusable underfloor heating systems because there were installed in thick concrete slabs. Installing in a screed on a concrete first floor with no insulation in between amounts to the same thing. It would take hours for the heat to penetrate all that concrete.

it wont effect the BER.

none of the responsiveness catagories are applicable to this situation.

if you have the typical "UFH in screed above insulation" in the ground floor, you would be incorrect to input any other responsiveness category, in my opinion of course

"underfloor heating, pipes in concrete slab" is not a correct input in this situation.

Rabbo · 27-01-2015 10:45pm

True, you would definitely have a reasonable argument in the case of an SEAI audit, especially if the main living room was on the ground floor.

However from the homeowners point of view, I definitely wouldn't recommend that detail.

galwaytt · 27-01-2015 11:51pm

sydthebeat wrote: »

it wont effect the BER.

none of the responsiveness catagories are applicable to this situation.

if you have the typical "UFH in screed above insulation" in the ground floor, you would be incorrect to input any other responsiveness category, in my opinion of course

"underfloor heating, pipes in concrete slab" is not a correct input in this situation.

...going back to OP's original query though, it most certainly would. The screed needs to be insulated from the slab & walls.

Even my own house, an engineered timber joist and deck to FF, I insulated the screed and in a timber build it's in a 'better' environment to begin with.

sydthebeat · 28-01-2015 12:38am

im not questioning that at all, i completely agree.....

just pointing out it wouldnt affect the BER

Calahonda52 · 28-01-2015 12:06pm

galwaytt wrote: »

...going back to OP's original query though, it most certainly would. The screed needs to be insulated from the slab & walls.

Even my own house, an engineered timber joist and deck to FF, I insulated the screed and in a timber build it's in a 'better' environment to begin with.

Nice choice of FF flooring G-tt

Can u share the following details please:
Did u use wall hangers, or as they say in Dublin Wha!
How did u deal with A/T issues at the ends and along the walls where joists are // to walls?
Thanks

Superdaddy · 28-01-2015 3:40pm

galwaytt wrote: »

...going back to OP's original query though, it most certainly would. The screed needs to be insulated from the slab & walls.

Even my own house, an engineered timber joist and deck to FF, I insulated the screed and in a timber build it's in a 'better' environment to begin with.

Is it really necessary especially if you are going to insulate between the joists for sound proofing? I am using posi joists with 18mm WPB and 40-50mm easy screed.

Calahonda52 · 28-01-2015 4:41pm

Superdaddy wrote: »

Is it really necessary especially if you are going to insulate between the joists for sound proofing? I am using posi joists with 18mm WPB and 40-50mm easy screed.

why would u sound proof if the slab is thick enough.
any sound insulation in posi joists with all the services therein would be more miss than hit

Could you use cement board rather than WPB, DONT KNOW!! just asking

delfagio · 28-01-2015 5:02pm

Just my own opinion, but I would be with GalwayTT on this....

If you rested the first floor hollow-core precast concrete slabs onto your ground floor block walls and the floor slabs did in fact extend out to the cavity along the edge of the external block walls, which does happen on a lot of builds up until recently, then common sense would tell you that if you did use UFH on the first floor, with or without insulation, surely heat will leave the house through the precast concrete slabs into the cold bridge at the cavity. What did happen up until recently in a lot of new builds, was that the outer edge of the precast slabs would align up and rest on the outer edge of the inner leaf blockwork of the external walls, and then the first floor blockwork would start on top of the precast slabs. Hopefully this would not be the case for the Original Poster??? :rolleyes:

Even with using airtight membranes on the first floor the cold air in the cavity is still getting to the precast slab and hence keeping it cool/cold. Even with full fill cavity insulation, the cavity is still a cool sometimes moist space between the two blockwork leafs. You would in the long run spend a lot of money on oil/gas or whatever type fuel been used on wasting heat out through the slab at these locations.

Regards the BER, yes the BER does not really take into account the first floor heating in precast slabs etc. as the BER views this as been within the thermal envelope of the building. However if the slabs extended out to the cavity resting on the ground floor block walls, then there is a thermal bridge which would have to be calculated on the buildings envelope walls. Which will bring the U-Value in the envelope walls down. Which is not good for over BER.

Back to the original posters query.....YES DEFINITELY, you must use floor insulation on top of the concrete precast slabs. The point of the insulation is to minimise heat loss downwards into the precast slab, so yes use floor insulation on top of the precast concrete slabs to keep heat movement upwards so that you are only heating the sand/cement or liquid floor screed above the insulation to try keep as much heat into the rooms upstairs as you can. As for depth of insulation, Technically you could use 100mm insulation if you wanted, or had the height for it, but that would cost more money, with no real need for it, but in most cases 50mm is used.

In most cases it is the ground floor where you sink the money into using between 150-200mm as this is the baby where you really need to prevent cold coming up from.

For me, I am like GalwayTT on the first floor. I used 50mm liquid screed, on 50mm floor insulation, on 22mm OSB floor decking, on top of Posi open web joists designed to take the loadings. All external walls have 63mm insulated plasterboard which extend from the ceilings down to the floor OSB decking. The edge insulation to allow for expansion of the floor screed, runs around the bottom of all external and internal walls. The joists used on our house, were installed by our timber frame company and internally hung from the ground floor external walls using joist hangers, so this eliminates cold bridging to the cavity. I then insulated the open web joists on the ground floor ceilings with 150mm of wool insulation.

sydthebeat · 28-01-2015 5:08pm

delfagio wrote: »

However if the slabs extended out to the cavity resting on the ground floor block walls, then there is a thermal bridge which would have to be calculated on the buildings envelope walls. Which will bring the U-Value in the envelope walls down. Which is not good for over BER.
.

as long as the slabs do not cause a discontinuation of the cavity insulation (which they dont in the general installation) then they do not affect the u value of the envelope and thus the BER isnt affected.

have you ever seen them being installed actually into a cavity??

if so i hope you ran the installer off site......

Rabbo · 28-01-2015 5:43pm

I've seen them cross the entire cavity and form a balcony at the other side in the good ole' days. Thankfully thing have moved on since then a bit.

BarneyMc · 28-01-2015 7:43pm

Rabbo wrote: »

I've seen them cross the entire cavity and form a balcony at the other side in the good ole' days. Thankfully thing have moved on since then a bit.

Cold bridge porn :eek:

Charlie Charolais · 15-02-2015 4:20pm

Hi guys,
1. is the ground floor ceiling void better to be warm or cold?
2. On the AT, Is it better tape the ceiling plasterboard or the hollowcore, to the walls?
3. Solid ends on the external ends of the HC are an optional extra, would this be better than wrapping the ends with AT membrane?

galwaytt · 15-02-2015 6:41pm

Superdaddy wrote: »

Is it really necessary especially if you are going to insulate between the joists for sound proofing? I am using posi joists with 18mm WPB and 40-50mm easy screed.

Imho, 100% necessary. The screed will expand and contract both in use and post-construction whilst drying out. You need a separating layer that can accommodate that movement.

galwaytt · 15-02-2015 7:05pm

Calahonda52 wrote: »

Nice choice of FF flooring G-tt

Can u share the following details please:
Did u use wall hangers, or as they say in Dublin Wha!
How did u deal with A/T issues at the ends and along the walls where joists are // to walls?
Thanks

All walls full height, joists on hangers between walls is std detail, no membranes.

I'll dig up a sketch.

[edit] sketch attached [/edit]

BryanF · 15-02-2015 7:27pm

Charlie Charolais wrote: »

Hi guys,
1. is the ground floor ceiling void better to be warm or cold?
2. On the AT, Is it better tape the ceiling plasterboard or the hollowcore, to the walls?
3. Solid ends on the external ends of the HC are an optional extra, would this be better than wrapping the ends with AT membrane?

1. It's within the external thermal envelope. warm
2. Wrap the edge of slab with membrane, tape it to the walls
3. Solid ends would be good, but imo you still need to wrap it, or you need ensure the gaps between the slabs are air-tight..

murschauds · 20-02-2015 5:05pm

I haven't read all of this thread, but Knauf do an insulated cement particle board. Their regular cement board (12.5mm thick) is called Aquapanel and I think that the insulated version is called Aquapanel Thermal. To the best of my knowledge it is for exactly the type of application that you are looking at.

Is insulation needed on 1st floor concrete slab?

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