UFH Insulation

NewBuild2012

Hi,

I have 175mm for UFH and insulation. My UFH installer is recommending a screed depth as close to 100mm to ensure a sufficient thermal mass as I intend to install a heat pump. I am thinking of installing 80mm polyisocyanurate insulation leaving the rest for the screed. I know that Part L states a floor should have a u value of .15 where UFH is used but i dont see any other way around this bar reducing screed which they say will inscrease running costs which I don't reall want to do. Has anybody any opinions on this? How important is it to achieve the u-value in the floor?

BryanF

NewBuild2012 wrote: »

Hi,

I have 175mm for UFH and insulation.

why not tell us why this is all the space you have? seems to me this is very little to allow in a new home..

My UFH installer is recommending a screed depth as close to 100mm to ensure a sufficient thermal mass as I intend to install a heat pump. I am thinking of installing 80mm polyisocyanurate insulation leaving the rest for the screed.

the insulation is more important than the thermal mass, thats for sure. many 00's homes with UFH suffer with high fuel bills due to a lack of insulation.

I know that Part L states a floor should have a u value of .15 where UFH is used

it states that .15w/m2k is 'satisfactory' I generally spec for circa .1w/m2k ..

but i dont see any other way around this bar reducing screed which they say will inscrease running costs which I don't reall want to do.

eh increased running cost? is this house timber frame? what's the air-tightness? whats your architect suggesting?

Has anybody any opinions on this?

sure but you really need on-site instruction, with the heating eng, energy assessor and architect there

How important is it to achieve the u-value in the floor?

more important than the 100mm thermal mass slab IMO (subject to the eng screed spec of course)

Do-more

From NewBuild2012's thread in heating and plumbing.

The house is 2900sq ft with a concrete slab first floor. Plan is to insulate with pumped cavity 100mm, 50mm insulated board on external walls and either spray of boards in the roof. Also floor insualtion was planned to 100mm but this may be reduced to allow for 100mm screed.

The issue of thermal mass is more critical when you are using a ground source heat pump with night rate electricity where you are charging up the floor with heat during the night and then the floor will radiate heat during the day.

In your situation as you are looking to use an air source heat pump thermal mass is not such an issue and you are as well off having a thin screed and hence a fast reacting heating system. You won't have any thermal mass in the walls to make use of anyway as you are insulating them internally.

As a matter of interest does the level of insulation you are going for meet the current regs?

sydthebeat

are we referring to a ground floor here?

the minimum allowed target is the building regulations. In this case its 0.15 w/m2k u value. this generally is approx 140 PIR insulation.

Whats your ground floor to underside of floor slab height?

NewBuild2012

hi,

Yes it is the ground floor. There is a concrete sub floor of 150mm leaving 175mm for UFH, insulation and screed. For the system I am looking at it has been recommended to have a 100mm concrete screed to give a greater thermal store. The reason I have 175mm left is when we started we were not intending to put in this system and so had left the height for 120mm insulation and a 50mm hemi hydraite screed.

Do-more the air source system is going to work the same way by using night rate electricity to heat the floor for the next day.

BryanF

If you don't have adequate insulation, which appears to be the route your considerin, forget the ufh

sydthebeat

NewBuild2012 wrote: »

hi,

Yes it is the ground floor. There is a concrete sub floor of 150mm leaving 175mm for UFH, insulation and screed. For the system I am looking at it has been recommended to have a 100mm concrete screed to give a greater thermal store. The reason I have 175mm left is when we started we were not intending to put in this system and so had left the height for 120mm insulation and a 50mm hemi hydraite screed.

Do-more the air source system is going to work the same way by using night rate electricity to heat the floor for the next day.

new build.. you say you have concrete slabs as first floor... so can i ask again,

whats the height between the existing concrete ground floor slab and the underside of the concrete slabs?

Also, if the system that you are looking at doesnt fit within the parameters you have... then you must go find a system that will fit. Not all UFH systems require a 100mm screed.... but all UFH systems require a minimum u value for the floor.

Do-more

NewBuild2012 wrote: »

Do-more the air source system is going to work the same way by using night rate electricity to heat the floor for the next day.

There are a couple of issues for you with this approach.

Sure night rate electricity is considerably cheaper than standard rate, [What are the current best rates for both?] but air temperature is lowest at night and this will effect your efficiency, particularly in terms of the number of de-frost cycles you will have as you try to run the heat pump for one continuous run during the early morning to charge the floor.

As you are dry lining you have no additional thermal mass to stabilise the interior temperature other than the mass in the floor. For the thermal mass approach the best solution is to have lots of thermal mass in both the floor and the walls so that once that mass has been charged up it will keep your interior temperature stable with nightly recharging of the floor, but you won't have that.

Are you going to have UFH upstairs as well or what will you use?

Running the ASHP through the night combined with less than adequate levels of insulation under your slab is really choosing the worst of all options IMO.