In the UK, are spells of hot weather ever long enough to make the thermal mass of concrete blocks a disadvantage for keeping a house cool?
Should we be adding thermal mass to an upstairs bedroom, eg. replacing drywall with real plaster on the blocks, or extra dense plasterboards to reduce overheating?

Also depends where in the UK you are? 21⁰C max in Aberdeen yesterday, but 33⁰C in London -> different design requirements.

For upstairs bedrooms, insulation in the roof and shading on the windows seems to make the most difference.

If the thinking is for thermal mass to absorb heat *from the room* during the day and release it again at night, to even out the temperatures, then it's only the first few cm of thickness that matter, because that's how far heat can travel "in and out" in the same day. So thick/dense plaster could help, ceramic tiles, that kind of thing.

If the thinking is for the thermal mass to absorb hot spells of several days or weeks, or to delay one-directional flow of heat *from the outside world* into the room, then different approach is needed - heavy masonry and wall insulation.

Cut away a small piece of plasterboard and measure the temperature of the plasterboard and the blocks behind it. If the blocks are noticeably cooler then it's worth increasing the coupling probably as you suggest by using real plaster. If the blocks are noticeably warmer then it would be worth while insulating between them and the plasterboard, which might mean stripping off the plasterboard and later reapplying it.

<blockquote><cite>Posted By: WillInAberdeen</cite>But won't that temperature difference change, depending on the time of day you measure it, because the heat flow follows a daily cycle, in and out..

Early afternoon: the masonry surface will be cooler than the plasterboard and absorbing heat
Early hours of morning: the masonry surface will be warmer than the plasterboard and releasing heat
After a hot week followed by a cool evening, the masonry will be a bit warmer than the room.
That's how it evens out the peaks of temperature.

But the contribution of the masonry will be small (as I mentioned above), most of the contribution will be from the plasterboard and the first centimetre or two of masonry closest to the room. The thermal resistance of an air gap is not very much.



Thermal admittance and heat storage capacity figures from<a href="https://www.htflux.com/en/free-calculation-tool-for-thermal-mass-of-building-components-iso-13786/" rel="nofollow">https://www.htflux.com/en/free-calculation-tool-for-thermal-mass-of-building-components-iso-13786/</a>:

Light block, air gap, plasterboard : 2.2 W/m2K 34 kJ/m²K

Light block, no air gap, plasterboard : 3.0 W/m2K 47 kJ/m²K - so about 1.5x better by improving coupling to the block

Light block, 30mm dense plaster : 4.9 W/m2K 66 kJ/m²K - so about 2x better by improving the lining layer</blockquote>

That is so helpful. I am going to download a trial of this software. The inner leaf is what is called a standard block so it isn't a light block. You wouldn't have this data for a middle-of-the-road concrete block would you? Thanks again!