Wall thickness

[AliG]

Architect has set to work on sketch design for my parents' house.

 

We are trying to limit ourselves to 20% of the plot area, which is 153square metres. I thought that this was quite comfortable until I calculated the area taken up by the external walls.

 

We were looking for 140-150sq metres internally including a single garage, single storey.

 

However, when I calculated the wall thickness required for around 0.15 U-Value, I realised that with 370mm thick external walls they will take up over 20square metres. This is a big chunk of our area to give up. This assumes 100mm external blockwork and a 50mm cavity then 140mm SIPs/timberframe with PIR insulation plus a service cavity.

 

So what's the best build up to limit the wall thickness. I am guessing that we will have roughly 150square metres of floor and roof, 110 square metres of external wall and 40square metres of windows/doors. So the external walls are less than 25% of the external envelope. There is pretty much no limit on the insulation we can get in the roof and floor and we would use 3g windows at around 0.7 U-Value.

 

It appears from @JSHarris heat loss calculator that the difference for example between 0.15 and 0.2 U-Value walls will be negligible assuming good insulation elsewhere.

 

I did suggest that we could use render board but the architect wasn't keen. This will massively reduce the thickness of the walls. The other issue I notice is that most timber frame/SIPs systems show a cavity between the panels and the interior, if we keep all switches internal is this really necessary? Or we could use the minimum amount of insulation in the panels as it doesn't affect the overall insulation of the house much.

 

Appreciate any thoughts, and what are the downsides of various solutions, such as external noise?

 

I think we might just go to 22% of the plot to give us a bit of leeway but I would rather have the most efficient use of space possible. I am not too fussed about the cost as again, the external walls won't be a large percentage of the build.

 

 

 

 

 

 

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[AliG]

It's likely that most of it will be rendered as it's the normal local finish, but you're right, if we go for wood cladding anywhere there is a big space saving.

 

Not sure I'd want a SIP with no cavity at all, doubt they'd like that in Scotland.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[Oz07]

Think I've seen timber frame without cavity internally before. Plasterboard straight on frame. Think I'd prefer the flexibility a cavity offers though. What size service cavity are you allowing for? 

[nod]

8 hours ago, AliG said:

Architect has set to work on sketch design for my parents' house.

 

We are trying to limit ourselves to 20% of the plot area, which is 153square metres. I thought that this was quite comfortable until I calculated the area taken up by the external walls.

 

We were looking for 140-150sq metres internally including a single garage, single storey.

 

However, when I calculated the wall thickness required for around 0.15 U-Value, I realised that with 370mm thick external walls they will take up over 20square metres. This is a big chunk of our area to give up. This assumes 100mm external blockwork and a 50mm cavity then 140mm SIPs/timberframe with PIR insulation plus a service cavity.

 

So what's the best build up to limit the wall thickness. I am guessing that we will have roughly 150square metres of floor and roof, 110 square metres of external wall and 40square metres of windows/doors. So the external walls are less than 25% of the external envelope. There is pretty much no limit on the insulation we can get in the roof and floor and we would use 3g windows at around 0.7 U-Value.

 

It appears from @JSHarris heat loss calculator that the difference for example between 0.15 and 0.2 U-Value walls will be negligible assuming good insulation elsewhere.

 

I did suggest that we could use render board but the architect wasn't keen. This will massively reduce the thickness of the walls. The other issue I notice is that most timber frame/SIPs systems show a cavity between the panels and the interior, if we keep all switches internal is this really necessary? Or we could use the minimum amount of insulation in the panels as it doesn't affect the overall insulation of the house much.

 

Appreciate any thoughts, and what are the downsides of various solutions, such as external noise?

 

I think we might just go to 22% of the plot to give us a bit of leeway but I would rather have the most efficient use of space possible. I am not too fussed about the cost as again, the external walls won't be a large percentage of the build.

 

 

 

 

 

 

Render and board is fine If well insulated 

But you need to allow for exspansion joints above and below windows and every six mtrs on unbroken areas 

[Onoff]

1 minute ago, nod said:

Render and board is fine If well insulated 

But you need to allow for exspansion joints above and below windows and every six mtrs on unbroken areas 

 

Do you fill the expansion joint with "mastic"?

[nod]

3 hours ago, Onoff said:

 

Do you fill the expansion joint with "mastic"?

No 

We prefer to use the formed  kind that are joined together

They look better and are much neater 

 

[AliG]

The standard seems to be a 35mm cavity plus 12.5mm plasterboard for services. If all that is in there is switches then the cavity can be reduced to 15mm probably or you can uses a layer of 19mm plasterboard chased out.

 

I think an alternative is that you use a thinner stud then insulated plasterboard straight onto it with sockets then fitting into the depth of the plasterboard. I am not sure though if this has an issue with the VCL being partly behind the insulation.

 

If we have brick outer skin I think that 320mm ish is about as thin as we can go.

 

I note that most timber frame companies seem to suggest that U-Values are worse with a clad outer skin than a brick outer skin so that you lose some of the gain in having to make the insulation thicker.

 

Interestingly despite timber frame supposedly being more space efficient, a full fill (with PIR) cavity block wall wet plastered would be as thin or thinner.

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[Ian]

12 hours ago, AliG said:

 

However, when I calculated the wall thickness required for around 0.15 U-Value, I realised that with 370mm thick external walls they will take up over 20square metres. This is a big chunk of our area to give up. This assumes 100mm external blockwork and a 50mm cavity then 140mm SIPs/timberframe with PIR insulation plus a service cavity.

 

So what's the best build up to limit the wall thickness. I am guessing that we will have roughly 150square metres of floor and roof, 110 square metres of external wall and 40square metres of windows/doors. So the external walls are less than 25% of the external envelope. There is pretty much no limit on the insulation we can get in the roof and floor and we would use 3g windows at around 0.7 U-Value.

 

It appears from @JSHarris heat loss calculator that the difference for example between 0.15 and 0.2 U-Value walls will be negligible assuming good insulation elsewhere.

 

I did suggest that we could use render board but the architect wasn't keen. This will massively reduce the thickness of the walls. The other issue I notice is that most timber frame/SIPs systems show a cavity between the panels and the interior, if we keep all switches internal is this really necessary? Or we could use the minimum amount of insulation in the panels as it doesn't affect the overall insulation of the house much.

 

I built a 71sq.m holiday home where I had similar issues to some of yours in that I wanted to keep the wall thickness down. I ended up with a 260mm thick wall with a U value of 0.21

I compensated by having better insulation in the roof (0.13), floor (0.1)and windows (0.8 3G)

wall build up from inside to outside was skimmed plasterboard on battens  to form a service cavity - 25mm foil faced Cellotex insulation 140mm timber frame filled with mineral wool insulation OSB with external cedar horizontal weather boarding on battens. 

[Mr Punter]

You could have 15mm plasterboard, 25mm service cavity, vcl, 140mm timber frame filled with 130mm pir (90 + 40), 10mm sheathing, 50mm cavity and either render board system (15mm) or brickwork (100mm).

 

With the render board system you may be able to reduce the 50mm cavity to 25mm, so for render board you will have 230-255mm and with brick it is 340mm.  I think this gives a u-value of about .19 but sometimes it depends who does the calcs and what flavour pir.  I think you can claim the service cavity as a low e gap.

[AliG]

1 hour ago, the_r_sole said:

You can't do full fill cavity construction in the scottish regs unless you are in a sheltered location - and there's not many building control officers that will entertain the idea at all!

 

If you plan all your services right you can not run services on the external walls and then you don't need the service cavity at all.... why are you so concerned with saving on the footprint? have you got a planning condition which dictates 20% site coverage only? It's better to go for the best wall build up you can in terms of insulation, servicing and durability than compromise one of those to limit site coverage.

 

I tried to figure out where you could and couldn’t do full full cavity in Scotland before. I thought that Edinburgh was theoretically ok as it is in the moderate zone but I wasn’t convinced that BC would allow it.

 

Historucally Edinburgh had a guideline that for building in the garden of a “villa” in a conservation area you should limit yourself to 20% site coverage. Theoretically we can include the whole sites bf the original house which allows us to go over 20%. Theg also have a guideline not to build more than 150% of the original footprint but the 20% rule seems to have been more relevant. My current place is more than 2x the size of the previous house but 20% of the plot.

 

There are quite a few examples of this locally  we will be using in our application.

 

Edited October 25, 2018 by AliG

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[AliG]

I think that we will put in a little larger than we need and then offer to reduce it.

 

Also I think there is a good argument that it would be better to have slightly more coverage of the land and better insulation.

 

The plot is newly created so the area should be exact unfortunately.

[AliG]

Good news I found a house nearby in exactly the same situation.

 

B-listed house splitting up garden, new house is 22% of the plot. It is also considerably larger than the listed house and 1.5 storeys. It looks like it will be approved next week.

 

This is the second instance of a listed house splitting its garden and permission being granted for a larger house than the original listed house in the last 4 years.

[ProDave]

Re service cavities. Years ago they didn't bother and you just drilled the studs of the timber frame for cables, and the plasterboard went straight onto the frame.

 

It is only now that most people (still not all) are wanting air tight houses that we fit an air tight layer and make a service void inside that. 25mm battens then 12.5mm plasterboard comfortably fits a 35mm deep back box so copes with all wiring.  Even where I wanted to bring the 15mm hot and cold pipes to the kitchen down the wall, the 25mm gap was plenty (just be careful with cable routing so none have to cross the pipes).  The only place I used 50nmm battens for the service void is the utility room where I wanted 22mm pipes buried comfortably.

[AliG]

Yes @ProDave that's exactly how our old house was built.

 

I had a UFH thermostat on an outside wall in the hall that I had to set 3 or 4 degrees lower as the draught blowing in behind it meant it registered a lower temperature than most of the room.