How much insulation.

[Russell griffiths]

Ok so I'm confused as normal, trying to design my wall build up, can anybody point me in the right direction to find out how to calculate u value. 

I am considering two methods of wall construction both timber frame both with external timber cladding. 

Method one. timber studs with a soft fluffy type insulation inbetween and a firmer slab type insulation on the outside. 

Metod two.  Double stud wall with all fluffy type insulation inside. 

 

What at I am struggling to get to grips with is the depth of insulation and what u value it will achieve 

 

please do not suggest any of the solid board type insulation as I have previous experience with poor fitting of this stuff and hate it. 

Cheers russ. 

[ProDave]

Someone more clever than me will be along I am sure to show you how to do the sums.

 

In the mean time what thoughts do you have on the external insulation slab? and what sort of frame thickness are you thinking of?

 

I have a 190mm frame, filled with Earthwool frametherm 35 roll and 100mm Wood fibre board on the outside, achieving a U value of 0.14
 

[Jeremy Harris]

To calculate U value (approximately, without corrections for surface effects) you find out the lambda value for each material layer in the wall and then convert that to the R value (thermal resistance) using the thickness of that layer and then add all the R values and take the inverse (divide them into 1) to determine the U vale of the whole wall.

 

For example, say you have a wall like this:

 

15mm plasterboard and plaster skim:  λ = 0.22 W/m.K

50mm service void (calculated λ using method in BS EN ISO 6946 horizontal heat flow method):  λ = 0.28448 W/m.K

12mm wood board inner airtightness layer:  λ = 0.15 W/m.K

300mm cellulose insulation:  λ = 0.04 W/m.K

12mm Panelvent outer wood skin:  λ = 0.15 W/m.K

25mm partly ventilated void (calculated λ using method in BS EN ISO 6946 horizontal heat flow method):  λ = 0.14653 W/m.K *

20mm outer timber cladding:  λ = 0.14 W/m.K *

 

Converting these to R values (using thickness in metres):

15mm plasterboard and plaster skim:  R = 0.22 W/m.K x 0.015m = 0.068 m²K/W

50mm service void:  R = 0.28448 W/m.K x 0.05m = 0.176 m²K/W

12mm wood board inner airtightness layer:  R = 0.15 W/m.K x 0.012m = 0.08 m²K/W

300mm cellulose insulation:  R = 0.04 W/m.K x 0.3m = 7.5 m²K/W

12mm Panelvent outer wood skin:  R = 0.15 W/m.K x 0.012m = 0.1 m²K/W

 

Sum of all the above R values = 7.924 m²K/W

 

Take the inverse to get the U value:

 

1 / 7.924 m²K/W = 0.1262 W/m².K

 

* Note that the outer void and cladding improve the U value by reducing wind washing heat loss and by changing the surface loss, so although not included in this simplified calculation method, the true U value for the wall will be lower as a consequence of this.

 

 

 

 

[Russell griffiths]

Hi Dave the system with the external insulation will be modelled on the type of build up you did. 

Except with external battens and timber cladding. 

How thick was your internal insulation in your 190 frame. 

[ProDave]

I used 100mm thick wood fibre board. It's available from 20mm to 160mm and you can use multiple layers if you want to.
 

[Russell griffiths]

Thanks js I will get the wife doing some sums 

I am just a hammer wielding Neanderthal so need a bit of help with such things. 

[Russell griffiths]

1 minute ago, ProDave said:

I used 100mm thick wood fibre board. It's available from 20mm to 160mm and you can use multiple layers if you want to.
 

Did you not use frame therm on the inside? 

[Jeremy Harris]

1 minute ago, Russell griffiths said:

Thanks js I will get the wife doing some sums 

I am just a hammer wielding Neanderthal so need a bit of help with such things. 

 

 

I have a spreadsheet that will calculated multiple layers (up to 6) and has λ values for a common materials if you want a copy.

[jfb]

I found this tool quite useful for getting an idea of u values (and condensation risks)

 

http://www.vesma.com/tutorial/uvalue01/uvalue01.htm

[Jeremy Harris]

I've just uploaded the spreadsheet I mentioned earlier to our website, in the "downloads" menu at the top here: http://www.mayfly.eu/

Edited February 4, 2017 by JSHarris

[Russell griffiths]

5 minutes ago, JSHarris said:

 

 

I have a spreadsheet that will calculated multiple layers (up to 6) and has λ values for a common materials if you want a copy.

Yes please js 

she is looking really thrilled at having to use her brain on the weekend. 

[Jeremy Harris]

It's the "Simple U Value Calculator"  download.  There's also a simple heat loss calculator for a whole house on that same menu.

[ProDave]

36 minutes ago, Russell griffiths said:

Did you not use frame therm on the inside?

the Frametherm is a glass wool based roll insulation (you can also get it as batts) that's a lot less nasty to use than most glass wool type insulation. It's used to fill the frame.  It's stiff enough that when you push it into the frame it stays there.  As a test, I fitted a bit to the frame of the 45 degree pitched roof very early on as a test to see if it fell out or slumped before it got boxed in, and it just sat there for months without moving.

 

Here is some going into the roof before I fitted the OSB to the inside

 

 

[Russell griffiths]

Thanks for that Dave. 

But how thick was it? 

[Crofter]

I know you said you're not considering using rigid board insulation of any kind between the studs, but just in case you find you need to, it's not so bad to get a good result if you leave a gap all round and foam it in place. Or if you're really clever you can insulate as you build, laying down stud, board, stud, board, pressing everything together nice and tight.

 

On my own build I did glasswool between the studs and then rigid Kingspan on the inside of the frame. Partly down to detailing of my build and also I believe this follows the principle of each layer becoming more vapour permeable as you go from inside to outside.

[ADLIan]

Have a chat with the insulation manufacturer on Monday morning. Their technical services will do the number crunching for you and send a nice pdf print out that BCO will accept.

 

Ian

 

[Jeremy Harris]

And, most importantly, remember that the U value doesn't indicate how comfortable the house will be.  Using a lot of low decrement delay insulation may get a decent U value, but the house thermal time constant could end up being short, and this leads to higher swings in temperature inside the house.

 

Decrement delay is at least as important (perhaps more important) in terms of comfort, although the regs are only concerned with heat loss, not the thermal time constant.  There's a good article on decrement delay here, that's worth a read: http://www.greenspec.co.uk/building-design/decrement-delay/

 

I can vouch for the fact that our all-timber house (no masonry) has reasonably high decrement delay insulation and a very long thermal time constant, and it makes it slow to heat up or cool down, so it tends to stay at a steady, comfortable, temperature all year around.  Most importantly, there is virtually no diurnal variation.  For example, the high decrement delay means that the heat from the sun takes two or three times longer to get through the insulation that it would if it were something like foam with the same U value, and in practice this means it never has long enough to get through the walls or roof during the duration of exposure to the sun even in mid-summer.

[ProDave]

1 hour ago, Russell griffiths said:

Thanks for that Dave.

But how thick was it?

2 layers of 90mm thick
 

To add to Jeremy's point above, take a look at the analysis of the wall make up done for our house and see how little the internal temperature varies with a big swing in outside temperature.

 

u-wert-berechnung (10).pdf

[Jeremy Harris]

Not far off ours Dave.

 

A 13 hour phase shift (or decrement delay) is not at all bad.  For comparison, our old brick and block house (brick outer skin, 50mm EPS bead foam filled cavity, 100mm block inner skin, plus 12 to 15mm plaster) had a decrement delay of around 4 hours, and that meant that by lunchtime the inside face of the walls was getting warm, and would often get to well over 25 deg C on the South end by the end of the day.  At night, these walls would then carry on radiating heat into house making the house uncomfortable to sleep in at times.

 

Our new house is over 20 hours, IIRC, I'd need to go and dig out the analysis I did.  I do know that there is no measurable temperature rise on the inside face of the external walls even on the hottest of days; the heat just doesn't get through the wall/roof before the outside temperature drops at night and heat starts flowing from somewhere in the middle of the wall/roof back out again.

 

Edited February 4, 2017 by JSHarris

[ADLIan]

@ProDave 

 

Is this wall 'standard' timber frame? - the calculation does not appear to account for the thermal bridging of the timber studs. The default is 15% bridging and this would increase U-Val nearer to 0.15 W/m2K. Also there is a layer of '25mm spruce' - seems strange to have an inch of solid timber at this point! Should it be 25mm air space & batten.

[Jeremy Harris]

30 minutes ago, Sensus said:

 

^^^ Wot he said.

 

If you don't make allowance for the studs, you're conning yourself.

 

Depends.  Our timber frame is cold-bridge free, using the construction method pioneered by John Larsen 30 odd years ago.  So, in my case there's no "kidding" at all going on at all!  A decent timber frame should have designed-in mitigation for stud cold-bridging, not primarily for heat loss reasons, but because solid timber is a pretty poor insulator and solid studs will end up being condensation loci.  We used solid studs in our garage, and with 150mm of insulation between them we had lines of condensation running down the inside panelling in winter - it was so bad in places that I thought we had a leak in the roof somewhere.  There's no way on earth I'd build a house like this.

 

FWIW, I'm certain Dave has largely mitigated the stud cold bridging with the thick layer of wood fibre outside the studs, for those here keen to criticise....................

[ADLIan]

I was being critical of the company doing the calculation. They mention EN ISO (DIN) 6946 which includes accounting for repeating thermal bridges but then totally ignore the effect. A layer of insulation internal or external to the studs mitigates the effect but the UValue will be higher than that shown - not at my PC at the moment so can't check but would still guess nearer 0.15 than 0.13W/m2K.

[Jeremy Harris]

I'm sure Dave will be along soon, but I'm pretty sure when he came up with this construction method there was a fair bit of discussion about it, probably on ebuild before it closed down.  The thickness of wood fibre on the outside does have a pretty big impact on reducing the cold bridging, if my PC wasn't still in the process of being rebuilt from a backup right now (long story - lost the boot HDD four days ago!) I'd probably be able to find the calcs, as I think I did some way back before Dave started building to see how effective the construction scheme he'd come up with was, and concluded that it was reasonably good.  Not as good as a twin wall system, but cheaper to build, and importantly from Dave's perspective in his location, could be relatively easily built on site, rather than use large and expensive to transport prefabricated sections as we did.

[ProDave]

Okay lots of points there.

 

That analysis I linked to was done by the architectural technician who designed the basic structure. The "spruce" layer I think was his best guess at what to use for the service  void.  Later in the design process a structural engineer detailed many of the construction aspects of the frame (including the fact that in many areas we have two layers of OSB inside to give sufficient racking strength).  Also a company subcontracted by the AT did the SAP analysis for building control. they calculated the U values of the walls and roof more accurately allowing for the frame and came up with 0.148 for the walls, so slightly worse. But they only allowed for the one layer of OSB not two so that will very very slightly improve it.

 

If ever we get an "as built" SAP I will make sure the details are corrected.

 

My frame was built by a local building firm. They built the frame in sections in a steading (barn) 2 miles away and brought them here a few panels at a time on a large trailer. Some of the larger gable panels looked very precarious on that trailer and he admitted he drove very slowly!  The roof was cut and framed on site.

 

I really liked the idea of the twin stud frames and tried to get our AT to think of something on those lines but he couldn't or wouldn't come up with anything. I had tried MBC but not got anywhere with them, so unfortunately that option slipped away. So it's a pretty standard timber frame except of course being much thicker than the normal 150mm, and the peculiarity is the OSB racking layer is on the inside. SO many people told me I had put the frame up "inside out"

 

I changed the insulation mid build. It was going to be filled with blown in wood fibre beads, that was until I found the price. I then changed to using the earthwool frametherm 35. The U values are near identical, but I suspect the decrement delay with the earthwool may be a bit poorer.

 

In the end it's in some ways a compromise. Not as good as we could have got if only.... But a lot better than a standard off the peg timber frame.

 

The other "change" I am making is I am installing an air tightness membrane all over the inside walls before the service void battens.  The designer had specified the OSB with taped joints as the air tightness and VCL and is aparently all they do on the buildings they build, but largely on the advice of the builders that actually built it, a separate air tightness /VCL membrane seems better.

[Jeremy Harris]

There's a BS method for calculating the λ for a thin air void, I mentioned it in the post above, that might be worth passing on to your AT.  There's more detail of the method on the BRE website, in fact I think there's even a downloadable document detailing the methods to apply and their limitations.