Thermal mass V responsiveness

nailer8

I am at the final stage in specifying our new build. Aiming for A2 rating, passive spec but not certified. I originally favoured highly insulated block but after countless hours of research decided block can't be adiquitley sealed. So building timber frame u=0.12 all round ach 0.6 @50pa

My concern is thermal mass. I was satisfied that while I had lost the walls I still had mass concerete floors down stairs and a 50mm screed upstairs for mass.

My under floor heating provider is now strongly recommending putting a 50mm screed in the ground floor over insulation for responsiveness. My calculations back this up, it will be 4 times faster than 100mm conc. However the house will now have little or no thermal mass to buffer temperature changes.

In people's experience is thermal mass really that important in the irish climate?
Is there any merit in building the ground floor internal walls in block to ad some mass?
Is it worth sacrificing responsiveness and putting the UFH pipes in 100mm of conc?

BryanF

nailer8 wrote: »

1.In people's experience is thermal mass really that important in the irish climate?
2.Is there any merit in building the ground floor internal walls in block to ad some mass?
3.Is it worth sacrificing responsiveness and putting the UFH pipes in 100mm of conc?

1.Yes for several reasons, helping differently depending on the season
2.IMO its worth considering
3.Do you not intend to keep the house at a constant temp? with the help of your heating, Mvhr, and glazing etc

As you say Without thermal mass the house is more responsive, but will prob also cool down quicker.. With 'low energy homes' and with modern 'calcs' we tend to size our heating systems more precisely.
For instance, Without thermal mass etc, if your away for few days in winter the house will cool by say a degree or 2 each day. On return the problem is getting up to temp quick enough.
From experience thermal will have a impact on this, and also once you get used to your UFH system you'll be able to deal with any preceived delay regardless.
Can you not put the insulation below the RC slab altogether?

Is your orentation good, Have you looked at the 'thrombe' wall idea, of your screed can handle it, do consider some block partitions or quinlites, or hemp lime between timber studs, rammed earth, etc or any of the other thermally massive material

Best of luck with the build

Do-more

I'd say a lot depends on what other heat sources you will be relying on apart from the under floor.

Do you intend having a stove? How big are your south facing windows? Large family all coming home together and immediately turning on plasma TVs and the oven?

If you are likely to have a big heat input from any of them then it is a good idea to have responsive ufh to avoid over heating.

In my own case I am building TF and have ufh in a 100mm screed. I employed a passive house certified heating consultant to help me figure out my requirements and he was initially encouraging me to fit either radiant wall heating or ufh pipes in grooved insulation directly under the flooring (no screed).

But when we teased it out, the fact that we won't have a stove and any solar gain in the winter is unlikely to raise the internal temp by more than 2 degrees I stuck with the pipes in 100mm of concrete as it was considerably cheaper for me than the other options. He still would have preferred a fully responsive system but I was prepared to take the compromise.

IMO it will all be down to your other potential heat sources, you won't have major heat loss it's just the risk of overheating that you need to assess.

BTW what heat source will you use for the UFH? Is it fast acting and fully modulating?

nailer8

Do-more wrote: »

BTW what heat source will you use for the UFH? Is it fast acting and fully modulating?

The current plan is:
Primary heat source: 6kw air to water HP
Backup: pellet stove with back boiler in the kitchen/living open plan. All feeding into a tank in tank buffer/dhw storage.

Might put a small wood solid fuel stove room heater in the living room for special occasions.

House not facing due south so have large windows on the sw&se side but nothing exceptional (no glass walls etc).

For the next 5 years or so house will be occupied 24h a day until kids all in school.

brdboard

I was thinking of this during the week when I visited a friends house that is approx 2 years old. It has always been fairly comfortable temperature-wise during summer and winter. However one evening during the week i called in and the house was like a sauna.
The house has a gshp, I think 75mm ufh screed, masonry inner leaf, some of which are block on flat to support pc slabs. So all in all, a lot of thermal mass.
So is thermal mass actually a good thing? In this case, because the external temp jumped by about 10 degrees in a short period of time, it made the house very uncomfortable.

bennybreezer

Hey Nailer,
I'm just finalizing my plans but planning on using a stove with solar for DHW and an Air Source Heat pump for heating. Good luck

Ben

nailer8

bennybreezer wrote: »

Hey Nailer,
I'm just finalizing my plans but planning on using a stove with solar for DHW and an Air Source Heat pump for heating. Good luck

Ben

Good luck

Interesting article on thermal mass.
http://www.constructireland.ie/Articles/Renewable-Energy/The-effect-of-thermal-mass-on-space-heating.html

It seems medium thermal mass almost always wins out. Except in a really well insulayed house heated 24h per day.

THE DON FANUCCI

does anybody know if under floor heating will be ok in 150mm of concrete?
will it conduct through 6 inchs of concrete floor ??
is 150mm too much

(asking coz my engineer is on holidays)

BarneyMc

THE DON FANUCCI wrote: »

does anybody know if under floor heating will be ok in 150mm of concrete?
will it conduct through 6 inchs of concrete floor ??
is 150mm too much

(asking coz my engineer is on holidays)

150mm seems a lot. 75-100mm is much more common.