Perimeter Expansion strip?

[junglejim]

I’m a bit confused by what seems to be standard practice so must be missing something….

 

The advice is to fix the expansion strip to the perimeter prior to floor insulation. (See attached) Surely as this is much less insulating than pir it provides a potential cold route to the slab around the edge of every room?

 

 Would it not be better to fit the insulation tight to the edges and then add the expansion strip above this. Taping the ‘skirt’ to the top of the insulation / visqueen layer will ensure a good seal for the screed without the need to compromise on floor insulation? 
 

i also thought it was good practice to install a vertical strip of pir around the edges but does the expansion strip negate the need for this?

 

 I’m likely missing something so any advice greatly appreciated. Thank you

[Nickfromwales]

@junglejim

 

Does the picture depict your situation? Masonry build with a cavity?

[Nestor]

I have enclosed  a shot, it is inside a hall cupboard!

I fitted 150mm celotex tight to the walls, taped all the joints, fitted plastic sheet, taped then UFH.

The perimeter edge insulation has an adhesive strip on one side which fixes to the upstand / wall and polythene apron which lies on the floor.

Just need to make sure the screed cannot escape.

Hope that helps.

 

[saveasteading]

1 hour ago, junglejim said:

does the expansion strip negate the need for this?

I think you have a good point. If built as your first picture, then the heat is travelling from the screed horizontally through that thin isolation strip, and into a masonry wall. That block wall presumably sits directly on a footing in the ground.

So increase the heat resistance of the strip by thickness and choice of material. Eg 25mm of PIR instead of the thin strip of something as shown.

 

On the other hand, the surface area of screed to wall is minor as compared to the lower face of the screed.

And heat loss through to earth is much less than with exposed surfaces.

 

 

[Spinny]

In our build the architect included a continuous row of celcon blocks around the inner leaf at insulation and concrete slab level (your screed level) to provide some insulation in the inner leaf between foundation/footing level and wall above. I think some people also build the inner leaf entirely from celcon blocks.

 

I think our builder put the 25mm PIR around the perimeter and then used the underfloor insulation butted up to the edge of it to hold it in place - simply a matter of practicality to hold the edge insulation in place for our concrete pour.

 

Think it may help if you can say whether you are doing new build or retrofit, and to describe the makeup of your wall structure - concrete block, celcon block, stone, sipps etc.

[junglejim]

Thanks for the advice. The image I first posted was from Google and not exactly illustrative of my case (timber frame). I realise the external wall will need additional insulation such as 25mm perimeter pir up to ffl I think. 
 

My main question is around the use of the expansion strip. I’ve attached an image that our screed company has provided as an example of what to do. However my instinct is to run the pir right to the stud then install the expansion strip above. Unless it’s serving another purpose?

[JohnMo]

I just used PIR (70mm) didn't bother with the expansion strip.

 

It purpose is to make up the expansion and contractions of the screed, as the UFH temperature cycles. A second (minimal) benefit is a small thermal break, to decouple the floor. I get the impression it's for retrofit applications where flow temps can be high.

[saveasteading]

59 minutes ago, junglejim said:

Unless it’s serving another purpose?

It is an edge to the screed , otherwise it runs into your wall spaces. That could as easily be hardboard or osb or dpc.

Screed shrinks so it isn't for expansion, but does provide a clean edge and some freedom to move and limit cracking.

If it is against block then an insulating material is advisable. But in the pic above, heat moves into the wall void then through the timber so it would be prudent to stuff the space with fibreglass or similar.

 

In that picture they have not (yet?) taped the pir to the perimeter so grout will get under and it may float.

 

I don't know what the polythene is for, or why it is being taped down.  Anybody?

[junglejim]

9 minutes ago, saveasteading said:

It is an edge to the screed , otherwise it runs into your wall spaces. That could as easily be hardboard or osb or dpc.

Screed shrinks so it isn't for expansion, but does provide a clean edge and some freedom to move and limit cracking.

If it is against block then an insulating material is advisable. But in the pic above, heat moves into the wall void then through the timber so it would be prudent to stuff the space with fibreglass or similar.

 

In that picture they have not (yet?) taped the pir to the perimeter so grout will get under and it may float.

 

I don't know what the polythene is for, or why it is being taped down.  Anybody?

Thanks. The polythene is a skirt on the expansion strip to be taped to visqueen layer laid ontop of insulation. I’m really questioning the logic of running the expansion strip full depth rather than above the insulation… seems like standard practice to go full depth so I’m wondering if there’s something that I’m not understanding.

[JohnMo]

2 minutes ago, junglejim said:

questioning the logic of running the expansion strip full depth rather than above the insulation

Stops it floating away when you screed?

[junglejim]

3 minutes ago, JohnMo said:

Stops it floating away when you screed?

Thanks… that’s an interesting point… was planning on taping it to the insulation and also to the stud

[saveasteading]

Being wider (deeper?) Using a board cut to size would make it easy to cut and fit to the finished level as  your screeding line. 

2 hours ago, junglejim said:

polythene is a skirt on the expansion strip to be taped to visqueen layer

OK so that stops the floating. So I'm guessing the product is supplied as a standard width, then you slice off any that's above the finished screed.

Seems quite complex. Is it expensive?

Edited 7 hours ago by Nickfromwales
Typo

[Nickfromwales]

Ok. Let’s stop. 
 

The perimeter around external

walls is a different detail to the internal ones; one has heat loss outwards, and the other is part of the heated envelope. 
 

The external walls need the 23-30mm PIR up-stand (or a bit more if your flooring can cope) for a thermal break.
 

So that’s where that’s needed. Size it to be at least an inch higher than top of screed, so your screed doesn’t spill over the top during the pour.

 

Then, the perimeter insulation (foam “skirting”) is applied over that at external

walls, and is simply stapled to the studs for all other (internal) walls. Some 30mm screws will be suffice to pin the skirting to the PIR, literally pushed in by thumb.

 

Some have the self adhesive strip, that is also suffice (I mention screws for the ones that don’t have this, as not all do). 

 

2 hours ago, JohnMo said:

Stops it floating away when you screed?

The screed rolls out over the taped-down apron (the affixed membrane of the foam skirting) and that stops any movement, and stops screed getting under and out.  
 

Nothing ‘floats’ as you’re not pouring screed under it. Friction from the weight of the screed pushing out against the foam skirting holds it very firmly in place.

 

Do away with this perimeter insulation skirting at your own risk, but some screed companies will insist upon it so check before they arrive; you don’t want them refusing to pour because of some cheap strips have been omitted.

 

The skirting sits on the floor insulation, tight down against it, and should also finish at least an inch higher than the screed.

 

Once cured you just lay a multitool or old handsaw against the screed and cut away the inch excess of both PIR and skirt strip.
 

Lay flooring, move to next problem. 

PS, I’d use a cementitious screed vs gypsum. Much better / easies eg less time, mess, fuss as you’ll be mechanically scrubbing off laitence etc so you can stick stuff to it. 

[Nickfromwales]

13 hours ago, Nestor said:

I have enclosed  a shot, it is inside a hall cupboard!

I fitted 150mm celotex tight to the walls, taped all the joints, fitted plastic sheet, taped then UFH.

The perimeter edge insulation has an adhesive strip on one side which fixes to the upstand / wall and polythene apron which lies on the floor.

Just need to make sure the screed cannot escape.

Hope that helps.

 

Don’t take pipes so close to the perimeters. These should observe around a 200mm gap to allow heat to diminish before a cold bridge is encountered; this reduces heat loss to the fabric of the build as is generally observed as ‘good practice’. 

[junglejim]

12 minutes ago, Nickfromwales said:

Ok. Let’s stop. 
 

The perimeter around external

walls is a different detail to the internal ones; one has heat loss outwards, and the other is part of the heated envelope. 
 

The external walls need the 23-30mm PIR up-stand (or a bit more if your flooring can cope) for a thermal break.
 

So that’s where that’s needed. Size it to be at least an inch higher than top of screed, so your screed doesn’t spill over the top during the pour.

 

Then, the perimeter insulation (foam “skirting”) is applied over that at external

walls, and is simply stapled to the studs for all other (internal) walls. Some 30mm screws will be suffice to pin the skirting to the PIR, literally pushed in by thumb.

 

Some have the self adhesive strip, that is also suffice (I mention screws for the ones that don’t have this, as not all do). 

 

The screed rolls out over the taped-down apron (the affixed membrane of the foam skirting) and that stops any movement, and stops screed getting under and out.  
 

Nothing ‘floats’ as you’re not pouring screed under it. Friction from the weight of the screed pushing out against the foam skirting holds it very firmly in place.

 

Do away with this perimeter insulation skirting at your own risk, but some screed companies will insist upon it so check before they arrive; you don’t want them refusing to pour because of some cheap strips have been omitted.

 

The skirting sits on the floor insulation, tight down against it, and should also finish at least an inch higher than the screed.

 

Once cured you just lay a multitool or old handsaw against the screed and cut away the inch excess of both PIR and skirt strip.
 

Lay flooring, move to next problem. 

PS, I’d use a cementitious screed vs gypsum. Much better / easies eg less time, mess, fuss as you’ll be mechanically scrubbing off laitence etc so you can stick stuff to it. 

Thank you Nick for such a thorough response. That’s as I imagined. I’ll definitely be using the foam strip as advised by the screed company but think it’ll be better above the insulation rather than all the way to slab. These also suggested osb strips at the bottom of the studs to give the strip a solid backing to adhere to… makes sense although possibly belt and braces. Any further advice welcome 👍

[Nestor]

45 minutes ago, Nickfromwales said:

Don’t take pipes so close to the perimeters. These should observe around a 200mm gap to allow heat to diminish before a cold bridge is encountered; this reduces heat loss to the fabric of the build as is generally observed as ‘good practice’. 

Thanks Nick, this was just an internal cupboard but do understand the principle.

Thought that these perimeter strips were more to allow for expansion and stopping the screed escaping, the external walls are 420mm wide!

Have a great Christmas.

 

[Nickfromwales]

4 hours ago, junglejim said:

Thank you Nick for such a thorough response. That’s as I imagined. I’ll definitely be using the foam strip as advised by the screed company but think it’ll be better above the insulation rather than all the way to slab. These also suggested osb strips at the bottom of the studs to give the strip a solid backing to adhere to… makes sense although possibly belt and braces. Any further advice welcome 👍

Ok, so you’re installing a constructional slab over insulation then?

 

If so, as your pics keep suggesting a thinner screed , then yes, just install it for the depth of the concrete for belt and braces.

[junglejim]

1 hour ago, Nickfromwales said:

Ok, so you’re installing a constructional slab over insulation then?

 

If so, as your pics keep suggesting a thinner screed , then yes, just install it for the depth of the concrete for belt and braces.

Difficult to explain but here’s what I’m thinking. The difference from what seems to be the norm is not taking the perimeter strip all the way to the concrete slab. It feels better to make sure the insulation is tight/snug to the edges of the room. Unless the perimeter strip (green) is supposed to also provide some compression/expansion around the pir.