Screed and Perimeter edge foam - or not

[UKenGB]

This is for a new workshop floor. To make it smooth and level, I'm using a flow screed onto the concrete slab (with adhesion promoter/seal on first). However, do I need perimeter edge foam?

 

The floor is entirely unheated so why would the concrete slab and bonded screed expand at different rates? I've also seen stated that with unheated floors, edge insulation is all about preventing heat transfer between floor and walls, but in this case they'll basically be at the same temperature so that would not be an issue.

 

Apart from the (not huge) cost of 100m of perimeter edge foam, seems to me that allowing the screed to bond fully to not only the concrete base, but also into the roughness of the concrete block wall has to be a good thing. Preventing such bonding surely results in a weaker structure - as long as thermal expansion is not going to create a problem.

 

I will need to seal up any gaps to prevent the screed from flowing away where it should not, but is perimeter edge foam required, or would it be better to let the screed fully bond not only to the concrete base but also the block walls?

[nod]

You will need the 10 mil  roll foam around the solid walls to allow som expansion 

[UKenGB]

What expansion?

 

The blocks walls sit on the concrete base. The screed will be bonded direct to the concrete so all will expand together. I just don't see any expansion causing any movement between the 3 parts of this equation, concrete base, screed and walls.

 

An 'unbonded' or 'floating' screed on top of insulation and heating pipes etc, yes I get it. Must allow room for different expansions. But this is just concrete blocks on a concrete base with a concrete screed and no differential heating involved. They'll simply expand all together. Hence why I mostly read that for unheated floors, expansion is not the issue. Edge foam in that case is purely for thermal insulation, which in my case is also irrelevant.

 

So I'm questioning the need for edge foam as seems to me the structure will be stronger if all elements are bonded together. If the screed is moving independently of the concrete base, somethings gone wrong with the bond between the 2.

[nod]

Just now, UKenGB said:

What expansion?

 

The blocks walls sit on the concrete base. The screed will be bonded direct to the concrete so all will expand together. I just don't see any expansion causing any movement between the 3 parts of this equation, concrete base, screed and walls.

 

An 'unbonded' or 'floating' screed on top of insulation and heating pipes etc, yes I get it. Must allow room for different expansions. But this is just concrete blocks on a concrete base with a concrete screed and no differential heating involved. They'll simply expand all together. Hence why I mostly read that for unheated floors, expansion is not the issue. Edge foam in that case is purely for thermal insulation, which in my case is also irrelevant.

 

So I'm questioning the need for edge foam as seems to me the structure will be stronger if all elements are bonded together. If the screed is moving independently of the concrete base, somethings gone wrong with the bond between the 2.

Why ask the question 

[UKenGB]

Because I was hoping for confirmation of my analysis, or a reason why I'd got it wrong.

 

I can't help but doubt that all new workshops have edge foam installed when flow screed is laid to provide the smooth and level floor they want. Also, most suppliers of this sort of thing only quote its necessity for heated floors, without mention of the unheated alternative.

 

My feeling is that without the foam will be stronger, but maybe I'm missing something vital and hoped for confirmation either way.

[Alan Ambrose]

I think in practice you may get cracking around the edges, probably not a problem. There's a chance the cracking will venture away from the wall, again probably not a problem. I think you normally put foam e.g. in doorways, to control where the cracking is. Maybe down to thermal, maybe just slight differential movement. Precision machinists have a saying along the lines "everything behaves like rubber in the end" - meaning however stiff you think the part is, it'll bend a bit when you machine it, even if only by microns.

 

If that doesn't bother you, fire ahead.

[UKenGB]

Thanks for the replies. I'm going with edge foam. Better be safe than sorry. 😀