Floor insulation installation questions

SodiumCooled

Hi All, will soon be doing the floor insulation (ourselves) on a new build and just wanted to start a thread to ask some questions. I had somen inital ones but others may arise. Most of the questions are looking for a 2nd/3rd opinion rather than having no idea. I have found the advise very helpful here over the last few months.

• Is polyethylene required under the insulation? The consensus is no but on some tutorials online they do include it though I suspect its not needed on a new build with adequate dpc already in place
• Is taping the insulation joints required? Again mixed information online though it seems once covering with polyethylene then taping is not needed.
• Should the polyethylene over the insulation turn up in front of or behind the 8mm edge expansion strip around the walls?

rossarovers2023

Completed our own new build and we did the following.

• We didn't put any polyethylene underneath.
• We did tape the joints with foil tape. Stopped any movement of the insulation boards.
• Turned up the plastic inside the insulation edge strip to stop the liquid floor screed from leaking through any gaps there. That was my thoughts anyway.

SC024

https://www.boards.ie/discussion/comment/121218864#Comment_121218864

About right, Your radon barrier / DPM should be below the concrete sub floor. However if there is not subfloor with radon barrier/Dpm then yes you need it below insulation also if there is 2 layers Eg 1 layer off 100mm + 1 layer of 50mm It is important to run sheets in opposite directions & to fit in a way that you dont have joints lining up with the joint in lower layer. Would normally be a 25mm thick strip of insulation around all edges Inc (internal walls between rooms) if using underfloor heating

SodiumCooled

https://www.boards.ie/discussion/comment/121218864#Comment_121218864

https://www.boards.ie/discussion/comment/121220097#Comment_121220097

Thanks for the responses. Its a raft foundation so DPM and radon etc are all in place below the slab. We are lying in two layers (downstairs) and as suggested are aware of staggering the joints. I know the 25mm strip is often used but our architect does not want this as doesn't think it's necessary and leaves a gap at the edge of the floor even after plastering which is a messy finish (according to him). Instead he has specified we use the 8mm expansion foam at all wall interfaces - I am happy to go with his advise.

keno-daytrader

https://www.boards.ie/discussion/comment/121222487#Comment_121222487

Does he also negate the use of 25mm board insulation at external walls? If he does then it sounds like your architect is stuck in the past and not in tune with proper building standards of today. Thermal or cold bridging is a source of unnecessary heat loss in the building fabric, its your house, he doesnt have to pay the heating bills.

If he does want 25mm at external walls then surely continuing on internal walls is the same "messy finish" ? Maybe he doesn't want you using skirting boards, not sure what he means by "messy finish"?

SodiumCooled

https://www.boards.ie/discussion/comment/121223681#Comment_121223681

He has specified the 8mm edge on all walls including external walls. He said he has moved away from using edge insulation rather than something he hasn't adopted. I guess with a 250mm pumped cavity and thermal blocks at the raft the cold bridging is not seen as significant.

Messy finish i understand somewhat as you will have a gap at the edge of the floor with no screed whereas the 8mm strip is narrow so when the wall is plastered it meets the finished floor leaving no gap. Should also be better for airtightness and removes the need for any airtightness work on the bottom block which isn't done and would be a big job to sort now so going to 25mm insulation not an option really. He is a well respected architect so I am inclined to trust him.

Biker1

https://www.boards.ie/discussion/comment/121223951#Comment_121223951

While I admire your faith in the architect I cant help but wonder about his knowledge of proper airtightness methods. Regardless of the insulation strip thickness, the block below your screed level must be sealed either with a 10mm smooth plaster coat or airtight paint prior to the insulation and screed being laid. This is even more important in a house with bonded bead in the cavity that is not made wind tight on the outside. Despite popular belief among professionals bonded bead is very porous to air infiltration. Have proven this in multiple airtightness tests.

SodiumCooled

https://www.boards.ie/discussion/comment/121224736#Comment_121224736

My chance to do that passed a long time ago. I had the plasterer told to plaster the bottom block when he was plastering the top block under the hollowcore and the architect happened to come on site and told him it’s not needed the way he has it detailed. I thought about it and it does make sense, if the sand and cement plaster meet the finished floor then this should be airtight unlike if you have the 25mm edge strip which leaves an channel for air to penetrate.

Everything is ready to go, materials on site and I’ll probably start it tomorrow evening so no changing the way it’s being done now.

Water John

This architect is not an engineer. The 25mm is required to prevent heat loss form the floor slab in UFH. Sorry, he's just plain wrong. Aesthetics don't trump engineering.

SodiumCooled

https://www.boards.ie/discussion/comment/121225654#Comment_121225654

He is the engineer for the build and is signing off on everything. I am using the architect title (as that is what he is registered as) as on a previous thread where I used the term engineer it caused confusion as what ever I was talking about it was pointed out that an engineer wouldn’t really be up to that task (I can’t remember exactly what it was).

I don’t think aesthetics are part of the equation at all to be honest.

Water John

Well, whatever he is, he needs to prove to you as to why his take on the matter is correct. Has he done heat loss testing on a house that he has built to that spec? The whole point with UFH is an insulated slab.

SodiumCooled

I haven't asked and I am sure he hasn't done heat loss testing. To be honest I have enough to stay on top off with self-building (including doing some of the work myself) that I can't be going into deep questioning on everything detailed - I have to trust the expert on it. I know he was adamant on the use of mannok thermal blocks as first block on the raft and wouldn't accept roadstone thermal blocks for example due to their performance and some of the detailing around windows to avoid thermal bridging was "overkill" according to the blocklayer and you would never find it done by a building contractor so in my opinion he has a good understanding on these things. He has overseen houses for a number of people I know and all have been happy so there is that aspect also.

SodiumCooled

Another follow on question. I have a number of ducts of varying size (multiple 2 inch and multiple 4 inch sewer pipes) running from inside the house to underground outside through the raft. Being used for running water and heat pipes from house to garage, power and internet to garage, pipe work to air to water unit etc.

Do they need to be air-tightened since they are under the floor and if so any suggestions? Some are very full and others have only have one or two things running through. I was thinking of expanding foam and then paint that with a layer of airtight paint but on one or two they are packed with insulated pipes so it looks quite tricky to deal with.

Water John

Furry friends access would be the main concern.

Mellor

https://www.boards.ie/discussion/comment/121223951#Comment_121223951

This sounds like a poor detail tbh. Was the the same architect that failed to properly detail the upstairs window. In the process creating another thermal bridge.

Pumped cavity has no bearing in cold bridging. The pathway into the raft. 8mm is not adequate unless it is a super-insulating aerogel. Thermal blocks do not stop cold bridging. 8mm has no bearing on airtightness, the seal is the same. As for messy detail, seems irrelevant as the floor finish would cover no matter the thickness. But even with no finishes floor, plaster and skirting is ~25mm.

I'm not sure that well respected means much here. Being respected by local clients just means he is personable and know. Has no bearing on his technical ability tbh.

https://www.boards.ie/discussion/comment/121225654#Comment_121225654

Not sure what being an engineer has to do with anything. The detail in question is architectural, not engineering. It needs to be designed correctly.

Curious what sort of engineer/branch of engineering you think deals with this stuff.

https://www.boards.ie/discussion/comment/121227283#Comment_121227283

Mannok have lower thermal conductivity (they are also weaker). Not accepting a lower performing product is pretty basic stuff. Building contractors don't detail construction details, the building designer does. They rarely want to do the best possible and in the absence of a designer insisting on a high spec they will do the bare minimum.

Your friends may be happy with the results, but they are likely comparing to house build years ago to a lower standard.

Calahonda52

OP

re this

He is the engineer for the build and is signing off on everything. I am using the architect title (as that is what he is registered as) as on a previous thread where I used the term engineer it caused confusion as what ever I was talking about it was pointed out that an engineer wouldn’t really be up to that task (I can’t remember exactly what it was). 

So he is registered as an architect and is the engineer for the job and is signing off on everything.

Does this include any structural design sign off: eg steel beams/lintels foundation etc.

In passing how much insulation?

What heating system?

What's the airtightness target?

rossarovers2023

https://www.boards.ie/discussion/comment/121232465#Comment_121232465

Ours came up to floor level but the way I did it was I shoved wool insulation down into the ducts around the pipes, expanding foam where needed and air tight taped around them then. The airtight paint might be handier as you say it was a bit tricky taping around some parts.

SodiumCooled

https://www.boards.ie/discussion/comment/121244108#Comment_121244108

Signing off on everything except corner window steel - all other steel, foundations, structure, roof, insulation etc was specified and was/will be signed off by him.

How much insulation - which insulation in particular?

what heating system - A2W with UFH throughout

Air tightness target - 2 m3/hr/m2 according to the plans

SodiumCooled

https://www.boards.ie/discussion/comment/121236419#Comment_121236419

Just on the 8mm has no baring on air tightness. Won’t 25mm leave a path for air to enter the building past the insulation at the edge whereas with 8mm there is no path as the sand and cement plaster on the wall will be greater than 8mm thus meeting the finished floor and sealing the path for air.

Mellor

https://www.boards.ie/discussion/comment/121247723#Comment_121247723

The 25mm is not a gap, its filled with insulation. While not total impermeable, its an expanded plastic foam. Similar to the expanded foam used to seal gaps in post #17. The floor is cast up to this insulation so that is a tight connection. The rough contact is going to be along the wall, . The plaster finish on the wall doesn't finish air tight to the floor you leave a gap for movement, which is covered by skirting. If any air was to leak in/out. It would be along the wall face, which exists for the 8mm foam too. which is why I said no difference.

But there should be a gas tight membrane up and over the adjacent block. There is not air coming through this junction. If you really want to seal, you'd seal the membrane wall juction before the wall and floor are poured and plastered.

Calahonda52

https://www.boards.ie/discussion/comment/121247706#Comment_121247706

thickness of floor insulation

so who signed off on window corner steel

SodiumCooled

https://www.boards.ie/discussion/comment/121248593#Comment_121248593

150mm downstairs, 50mm upstairs.

The crowd who fabricated the corner steel also had an engineer to sign off on it.