# Thermal mass and insulation



## Woody2Shoes (14 May 2019)

My new workshop is inching towards reality. The walls will be made of masonry with a cavity. I'm mulling over the options:
a) Outer skin = dense bocks, then insulation (with an air gap on the outside), then inner skin = thermalite blocks.
b) Skins the other way round!

Presumably - on paper - both a) and b) have the same U-value, but given the seasonal variations, and the diurnal cycle of warming/cooling, I suspect they have different performance characteristics over time....

Thoughts please - esp. from Mike..

Thanks in advance, W2S


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## MikeG. (14 May 2019)

How often are you expecting to use the building in the winter, and what is your heat source?


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## Woody2Shoes (15 May 2019)

Hi Mike - in winter I plan to use it most days and to have electric heating just to keep it above about 10C, with additional wood-fired heating to take it above that as required. Cheers, W2S


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## MikeG. (15 May 2019)

The thing is, thermal mass is great for continuously occupied buildings where you can drip feed low amounts of heat in constantly, and in those circumstances it is a lot more energy efficient than lightweight super-insulated structures. Unless you are planning to heat this building every day, the thermal mass will actually work against an intermittent use pattern of occupation, in that whilst it holds warmth well, it also holds "coolth" well. So come to your high thermal mass workshop after leaving it for a day or two, and it will take much longer to heat up than if it were of lightweight construction. Therefore if your plans were for 5 days a week use, with a timed heating system to get it warm enough for a Monday morning, then I would say go with the high thermal mass approach.

If you do decide to go for the lightweight approach, there isn't any point in having heavyweight dense blocks in the outer skin. That outer skin acts as not much more than a rainscreen. Use the cheapest blocks you can find that will accept the finish you desire.

It's your choice to go for a masonry construction, but with walls 250 to 300mm thick and no better thermally (in an intermittently occupied building) than a timber frame construction 150mm thick, it wouldn't be my approach unless I needed a brick outer skin for aesthetic or Planning reasons.


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## samhay (15 May 2019)

As someone who's shed (and house) has fairly high thermal mass - stone and brick construction with no cavity. I can confirm that it isn't ideal.
Also, I believe thermalite is a pain in the behind to fix to, so not ideal for an inner skin as it makes hanging shelves, etc difficult.


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## MikeG. (15 May 2019)

samhay":30ly47aw said:


> As someone who's ......house... has fairly high thermal mass - stone and brick construction with no cavity. I can confirm that it isn't ideal...........



Careful. It's the lack of insulation that's the issue with yours, not the high thermal mass. High thermal mass externally insulated construction can produce the warmest and most energy efficient buildings imaginable. Mine costs £50 to £70 a year to heat, for instance, and at the moment has very little solar gain.


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## samhay (15 May 2019)

That I don't dispute, and I could have stated this more clearly. 
However, high thermal mass, regardless of insulation, takes a long time to heat up (and cool down). This means that if you do not use your heating regularly you spend a long time trying to catch up when you do eventually turn it on - this was your earlier point, so I think we are in agreement. It also means my shed was nice and cool last weekend when it was decidedly warm outside.

"...and at the moment has very little solar gain."
Is your house painted white? What about the roof?


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## Jacob (15 May 2019)

Basically the nearer the insulation is to the object which you are hoping to keep warm (yourself), the more efficient it is. Otherwise you are losing heat warming up other things. So it's thick underwear first! followed by insulation on inner surfaces of rooms - and between rooms if poss so that spaces not in use don't need heating.
"Thermal mass" works as a buffer so may be useful for spaces kept at constant temperatures long term, but wasteful for intermittent occupation.


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## Selwyn (15 May 2019)

MikeG.":1rw8hx0k said:


> samhay":1rw8hx0k said:
> 
> 
> > As someone who's ......house... has fairly high thermal mass - stone and brick construction with no cavity. I can confirm that it isn't ideal...........
> ...



Have you externally insulated your house? I'm seriously considering doing this to mine


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## samhay (15 May 2019)

In my case, external insulation not a good idea as the exterior is stone and this would result in fairly drastic change in character.

Could do internal insulation, but this is messy, complicated (have to re-site power sockets, etc) and eats floor space. I guess at some point in the future, this will become the least worst option. Our we'll move.

Back to the OP. My more major objection would be the hassle of having to do internal fixing to thermalite. Or am I overestimating the downsides to this?


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## Jacob (15 May 2019)

Selwyn":rs6ewery said:


> MikeG.":rs6ewery said:
> 
> 
> > samhay":rs6ewery said:
> ...


Much cheaper (by a huge amount) and more effective to insulate inside surfaces, but you lose space of course


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## Woody2Shoes (15 May 2019)

Thanks Mike. Food for thought.

What is the make-up of the walls in your house?

Cheers, W2S


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## MikeG. (15 May 2019)

It's an inside out timber frame, with the insulated timber framing external to a 100mm cavity fully filled with insulation, and a dense concrete block inner skin.


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## marcros (15 May 2019)

Mike,

Can I ask why you designed it that way, rather than the normal timber frame construction method?

Thanks
Mark


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## MikeG. (15 May 2019)

So that my heating bills would be £50 or £70 per year. As a bonus, you can get the shell watertight in a very few days and then be working inside out of the weather. I don't want to divert this thread away from workshop design though folks. Quite happy to chat away about the subject of low energy housing, but this is W2S' thread about his workshop.


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## Woody2Shoes (16 May 2019)

If it cost me say £40-50 a year to heat my workshop (let alone £60-70 for the whole house) I wouldn't mind! 

The idea behind masonry is both to improve security (thieves have broken into buildings nearby by tearing off timber cladding and climbing through the timber frame underneath - that said, I realise that a determined thief is pretty much unstoppable, if they know what they're trying to get at) and to give some thermal mass (which I realise could be a mixed blessing).

If I have an external rain screen (e.g. horizontal timber weatherboards on vertical battens with a vapour permeable membrane and suitably ventilated batten space), I suppose I can fully fill the cavity with Celotex (no need for an air gap, and a bit more insulation)?

The question then comes, for the outer half of the structural wall, could I use insulation filled-timber frame (say 90mm-ish thick) or 100mm of thermalite - what are the pro's and con's (cost would seem similar; with thermal performance of thermalite maybe a little worse?)? I know how to tie two leaves of masonry together, but I've never tried to tie timber to masonry (with the masonry going up first). Looking at the tie manufacturer's websites there are timber-insulation-masonry ties, but nearly all the examples they give seem to be with masonry on the outside and built second!

Cheers, W2S


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## MikeG. (16 May 2019)

I'm a bit pushed for time.......but where I need to make a timber frame building extra secure I put EML (expanded metal lath or "wire") over the membrane on the outside of the frame, with battens going over that to take the feather-edge. Burglars are overwhelmingly likely to break in doors windows or rooflights first, so protecting those is the priority.


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## DoctorWibble (22 May 2019)

Some readers might be a little confused reading this thread. The question of thermal mass is not whether it is good or bad per se but whether there is too much or too little. Too much makes the building too slow to respond to your heating efforts, but some, even in an intermittently heated shed, is a very positive thing. The right amount of thermal mass will give you a lovely even temperature that doesn't plunge every time you open the door. And equally important when you go to bed the shed will cool only slowly minimising condensation issues. Sheds with too little thermal mass will often feel too cold or too hot and the rapid temperature fluctuations they are prone to will promote damp. 

How much thermal mass is right in a workshop is the question to ask. My 16x12 shed is a lightweight timber affair but it is insulated and lined with two layers of extra dense 12mm plasterboard (sound board). I did that for sound proofing reasons but have found it to offer a near ideal measure of thermal mass. Easy to heat, warm in the winter, cool in the summer and never a hint of damp. A happy accident for me as I never considered plasterboard could provide useful thermal mass but it seems that doubled up in a shed it's just about perfect.


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## Jacob (22 May 2019)

DoctorWibble":2ng7nety said:


> Some readers might be a little confused reading this thread. The question of thermal mass is not whether it is good or bad per se but whether there is too much or too little. Too much makes the building too slow to respond to your heating efforts, but some, even in an intermittently heated shed, is a very positive thing. The right amount of thermal mass will give you a lovely even temperature that doesn't plunge every time you open the door. And equally important when you go to bed the shed will cool only slowly minimising condensation issues. Sheds with too little thermal mass will often feel too cold or too hot and the rapid temperature fluctuations they are prone to will promote damp.
> 
> How much thermal mass is right in a workshop is the question to ask. My 16x12 shed is a lightweight timber affair but it is insulated and lined with two layers of extra dense 12mm plasterboard (sound board). I did that for sound proofing reasons but have found it to offer a near ideal measure of thermal mass. Easy to heat, warm in the winter, cool in the summer and never a hint of damp. A happy accident for me as I never considered plasterboard could provide useful thermal mass but it seems that doubled up in a shed it's just about perfect.


Same here with our chapel conversion. The only thermal mass is the plasterboard and the room contents - low but still functional. The temperature is very steady.
The advantage of low thermal mass is quick heat up. Quickest for us is to light the woodburner with all dampers out and any fuel smallish and dry, even cardboard - it goes off like a blast furnace and heats the main room in minutes. The CH would take a lot longer, hours on a wintery day.


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