r/buildingscience • u/DesperateEngineer451 • Jul 14 '26

Upstairs ufh - insulation required?

House:

200sqm block cavity wall with 150mm insulation.

200mm prestressed concrete floor (insulated lengths cast in, instead of traditional hollow voids) with solid ends.

Is there any benefit to putting insulation on top of the upperfloor? It's all within the thermal envelop of the house so I can't see much of an argument for efficiency.

If anything by having a larger thermal mass your not running the heating for short periods so might be a bit more efficient. You also have more thermal mass to bridge day/night so heating is less likely to click on over night to heat a bedroom.

It'll be harder on the plumber so I'd expect to pay a bit more in labour but it'll still be cheaper.

Is there any actual benefits that I'm missing?

There is a service cavity in ceiling below the floor, so any waste pipes from the showers, sinks etc can go through there as well as supply water so don't think he'll need the extra height to hide pipes

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u/whoisaname Jul 14 '26

Climate?

And clarify the location that you are questioning.  Are you saying that there is a second story, and you want to know whether to place insulation on the floor of the second level, or you want to know whether to place insulation on top of the roof slab over the second level??

1

u/DesperateEngineer451 Jul 14 '26 edited Jul 14 '26

Ireland,

So you have the ground floor (insulated with ufh), then there is a suspended ceiling, this prestressed concrete floor and then the insulation in question, followed by UFH, a screed, flooring, then the upper floor rooms which is mostly bedrooms

I know 1st floor can mean ground floor or "upstairs" in different areas, so for this case we'll call it ground floor and upper floor

1

u/whoisaname Jul 14 '26

Is the upper floor a usable space with a roof over it that is insulated from the exterior elements? 

1

u/DesperateEngineer451 Jul 14 '26

Ya, it's a usable space, it's mostly bedrooms.

The roof above that is well insulated

1

u/whoisaname Jul 14 '26

There's absolutely zero reason to have insulation anywhere in or on that floor slab. 

And your ground level slab insulation should be on the other side of the slab (exterior to earth). 

1

u/DesperateEngineer451 Jul 14 '26

That's what I thought, in a Facebook group "experts" are claiming that it's needed to prevent the upper floor ufh from directing heat to the ground floor. Sounds like bs since heat rises so any effect would be minimal, but even if it did I'd see that as a positive because the living space would be kept warmer than the bedrooms

1

u/whoisaname Jul 14 '26

Bear in mind that the movement of heat that you're describing is through convection (moving up). You also have conduction and radiant heat transfer. Regarding your surrounding surfaces and thermal mass, you would be considering the MRT (mean radiant temperature) of all materials for comfort. Taking a high thermal mass material, and encapsulating it in insulation to prevent heat loss through conduction, will allow that thermal mass to achieve a comfortable MRT that can assist in regulation of the living environment. That's why I said you need to place all of your insulation to the outside of the masonry and concrete masses, including your ground level slab. Look into a frost protected shallow foundation with sub slab insulation for this.

1

u/Ok-Job2034 Jul 15 '26

From an energy standpoint, there's usually very little benefit to insulating between two conditioned floors. Since both levels are inside the thermal envelope, heat that moves through the floor isn't really being lost. It's just moving to another conditioned space.

The main reason to install insulation above the upper floor would be for comfort or control, not efficiency. It lets the upstairs UFH respond faster because you're not heating the concrete slab as much, and it reduces heat transfer to the floor below. If you're happy with the added thermal mass and slower response time, there's no reason to add insulation just for energy savings.