Minimum Reasonable wall width for good U values?

[puntloos]

I've been searching but can't seem to find a decided answer. (so I guess 'it depends') but here goes:

 

What's the minimum wall width one can achieve with 'modern' (fancy?) materials, yet still have sufficient room for services as usual, and achieve PassivHaus-level U values? (happy to be just on the border thereof for now..)

 

Are materials the key element or does one simply need some width due to pesky physics that are all obviously fake news?

 

Not made my mind up on material types yet, so for now let's leave any other consideration (aesthetics, or perhaps renewablocity) out of it?

[Temp]

There is a physical limit. How crazy do you want to get? I think Aerogel has the lowest thermal conductivity known to man so  wall made only of that stuff would be the thinnest possible. However its horribly expensive not readily available and wouldn't carry much load. Its also translucent.

 

The next best material is PIR (polyisocyanurate) insulation which is a lot cheaper and more widely available then Aerogel. Some form of timber frame infilled and covered with PIR is probably going to give the best U value for minimum thickness. The outside could be rendered, clad with brick slips or battens and some form of rain screen/siding. 

 

One issue is the difficult of cutting and fitting PIR tightly between a timber frame. Poor workmanship can ruin a good theory. Some form of compressible rockwool batt between the frame might work better because its easier to get right even though the U-value is theoretically worse than PIR. A combination of Rockwool between and PIR outside would be reasonable.  A layer of plywood or OSB would probably be needed to prevent racking.  You might also look at a SIP panel construction.

 

 

 

[A_L]

52 minutes ago, puntloos said:

What's the minimum wall width one can achieve with 'modern' (fancy?) materials, yet still have sufficient room for services as usual, and achieve PassivHaus-level U values?

 

With PIR in a timber frame, a 50mm service cavity and a thin render on carrier board U<= 0.15 (PH max) can be achieved at 350-375mm

[ProDave]

U value 0.14 overall thickness 360mm

 

Inside to out:

 

plaster skim

12.5mm plasterboard

25mm battened service void

Air tight membrane

2 layers 12mm OSB (racking layer)

195mm timber frame filled with Frametherm 35 insulation

100mm Pavatex wood fibre board

Baumit thin coat render system.

 

[puntloos]

Nice, I was assuming 440 everywhere, this is def an improvement. 360 I can work with

[Simplysimon]

350 i beams with blown cellulose 50mm service void,  u value 0.11

[Oz07]

9 minutes ago, Simplysimon said:

350 i beams with blown cellulose 50mm service void,  u value 0.11

With what on the outside and inside?

[Simplysimon]

from outside in, timber cladding or render, 50mm cavity, pro200 breather paper, medite vent, i beam, medite propassive, 50mm service void and p/board

[Oz07]

Can render board go straight on I beam? With timber cladding and plasterboard your talking at least 60mm on that original figure of 400?

[Oz07]

Another consideration is mortgageability. What is considered timber frame? When do lenders and insurers start calling it mmc 

[Simplysimon]

17 minutes ago, Oz07 said:

Can render board go straight on I beam? With timber cladding and plasterboard your talking at least 60mm on that original figure of 400?

see no reason why not

[Mr Punter]

23 minutes ago, Simplysimon said:

see no reason why not

 

Most warranty providers want sheathing board, breather membrane and a min 25mm drained and vented cavity.

[Dan F]

The MBC 0.14 system (other companies have similair systems) is about 225mm if I remember correctly (OSB, 140mm frame with mineral wool infill,  40mm PIR + service cavity)
      +15mm internal plastboard
      + 45mm for external cavity, renderboard + render

      = 285mm total  (or 325mm for 0.11)

 

Passivhaus doesn't have any specific u-level requirements, if you need 0.1, or if 0.15 is fine to meet "passive standard", will depend on climate, u-value of floor and windows, thermal briges and airtightness etc.

 

Our walls will be 425mm inlcuding extenal render system and internal PB, but thats because it's a twin-wall system with celuose (as a number of other people on this forum have used). You need more depth of celulose, because its less insulating that mineral wool or PIR.  Celuose is 0.038 W//(mK) vs mineral wool 0.032 and PIR 0.022.  It does have other advantages though; i) better eco-credentials ii) better decrement delay. 

 

Edited January 11, 2020 by Dan Feist

[Markblox]

51 minutes ago, Dan Feist said:

 

Our walls will be 425mm inlcuding extenal render system and internal PB, but thats because it's a twin-wall system with celuose (as a number of other people on this forum have used). You need more depth of celulose, because its less insulating that mineral wool or PIR.  Celuose is 0.038 W//(mK) vs mineral wool 0.032 and PIR 0.022.  It does have other advantages though; i) better eco-credentials ii) better decrement delay. 

 

Anyone have info regarding the decrement delay on the MBC 0.11 U value single wall  as opposed to their cellulose filled twin wall.

 I know the twin wall with cellulose fill has a long delay but I would like to know what their best single wall system is like in comparison.

[ADLIan]

Do not render on insulation on timber frame. Render should be on carrier board with vent cavity behind. See Bldg Regs and any good document on TF construction (see STA or TRADA guidelines). Wood fibre manufacturers guidance should not be relied on. Some did have BBA certs covering direct render but these all appear to have been withdrawn.

[puntloos]

So would MBC's 0.14 conform to all the reqs, and be mortagable and insurable and all the rest of it? ? 

(since obviously 225mm is a very slim wall right there... nice..) I might go for a slightly larger services area since I'm a bit of a nerd (cablescablescables) but if I can stay under 300 I'd be quite happy...

[Dan F]

13 minutes ago, puntloos said:

So would MBC's 0.14 conform to all the reqs, and be mortagable and insurable and all the rest of it? ? 

(since obviously 225mm is a very slim wall right there... nice..) I might go for a slightly larger services area since I'm a bit of a nerd (cablescablescables) but if I can stay under 300 I'd be quite happy...

 

Not everyone will mortgage it, but it's definitly mortgageable.  Agree with @ADLIan that using a suitable/approved render/cladding system is important for mortgage/warranty/insurance if you aren't using a block skin.

 

This link has a lot of interesting information on insulation options, seems to suggest decrement delay of of mineral wool & PIR is around 7-8hrs: https://www.ecomerchant.co.uk/news/insulation-materials-compared/

 

(In our experience mortgage providers willing to mortage MBC system was more limited because they do not have BBA or BOPAS accreditation.)

Edited January 11, 2020 by Dan Feist

[puntloos]

1 hour ago, ADLIan said:

Do not render on insulation on timber frame. Render should be on carrier board with vent cavity behind. See Bldg Regs and any good document on TF construction (see STA or TRADA guidelines). Wood fibre manufacturers guidance should not be relied on. Some did have BBA certs covering direct render but these all appear to have been withdrawn.

1 hour ago, Dan Feist said:

 

Not everyone will mortgage it, but it's definitly mortgageable.  Agree with @ADLIan that using a suitable/approved render/cladding system is important for mortgage/warranty/insurance if you aren't using a block skin.

 

This link has a lot of interesting information on insulation options, seems to suggest decrement delay of of mineral wool & PIR is around 7-8hrs: https://www.ecomerchant.co.uk/news/insulation-materials-compared/

 

(In our experience mortgage providers willing to mortage MBC system was more limited because they do not have BBA or BOPAS accreditation.)

 

Sorry, I appreciate I'm a newbie here, but combining the above 2 statements, can I say safely-ish that:

 

- With MBC's 0.14,

- making sure the render is on carrier board, vent cavity behind (I assume this is not part of the default MBC necessarily, so needs to be separately checked)

-> and therefore staying under 300mm

 

I would be 'okay' - mortgageable (perhaps not the industry-best rates but nothing crazy), warranty, insurable etc?

 

[Oz07]

@ProDave what's your detail? Is your woofibre board vented to rear?

[Oz07]

@joe90 what's your u value? Are you at around 425mm thickness total?

[ProDave]

5 minutes ago, Oz07 said:

@ProDave what's your detail? Is your woofibre board vented to rear?

No. 100mm thick wood fibre direct onto timber frame and render onto that.

 

It did have a BBA certificate for that system when we built it, I have not checked of that is still valid or not.

[Oz07]

Is it a case if it had bba when you built then still valid? Surely can't retrospectively withdraw certificates 

[ProDave]

15 minutes ago, Oz07 said:

Is it a case if it had bba when you built then still valid? Surely can't retrospectively withdraw certificates 

I kept a copy in case it disappeared.  It's just that someone mentioned above that some BBA certificates have been withdrawn,

[Jeremy Harris]

21 hours ago, puntloos said:

I've been searching but can't seem to find a decided answer. (so I guess 'it depends') but here goes:

 

What's the minimum wall width one can achieve with 'modern' (fancy?) materials, yet still have sufficient room for services as usual, and achieve PassivHaus-level U values? (happy to be just on the border thereof for now..)

 

Are materials the key element or does one simply need some width due to pesky physics that are all obviously fake news?

 

Not made my mind up on material types yet, so for now let's leave any other consideration (aesthetics, or perhaps renewablocity) out of it?

 

 

There are two considerations, really, as U value is only part of it, decrement delay is at least, if not more, important than U value, as that's largely what determines how comfortable the house will be, in terms of not suffering from rapid temperature fluctuations. 

 

Decrement delay is described here pretty well: http://www.greenspec.co.uk/building-design/decrement-delay/ and, although related to insulation performance, it's equally closely related to the heat capacity of the fabric. 

 

One problem with making very thin walls, apart from decreasing the U value to a sensible level, is in increasing the decrement delay to a sensible level, too.  In general, lightweight foam materials tend to have a pretty short decrement delay, as they have a very low heat capacity, whereas some other insulation materials, like some fibre materials, wood fibre or blown cellulose tend to have a reasonably long decrement delay.  It's quite possible to combine material layers so as to achieve both a low U value and high decrement delay, as another option. 

 

Alternatively, you can accept having a short decrement delay and just fit a heating and cooling system with a very rapid response time, so it can cool the house as fast as it heats up, and vice versa.

 

Location and orientation also play a big part in this decision.  If the fabric of the house isn't exposed to bright sunshine for long periods during the day, then decrement delay becomes less of a concern.  It's primarily an over-heating issue, as in bright sunshine it's pretty easy for an external surface to get to well over 40°C (I've seen outside surface temperatures of around 50°C at times here).  That creates a pretty high temperature differential across any wall or roof, and will tend to drive heat more rapidly through the fabric of the house unless the heat capacity of that fabric can absorb some of that heat and so slow down the rate of heat transfer.  If it can slow it down enough for the heat not to reach the inside face, before the sun has moved away, then generally that's good enough, as as soon as the outer temperature drops heat will start to flow back out.

[scottishjohn]

why is wall thickness the factor you seem to be concentrating on ?

 small plot and room sizes ?

 you can get passiv levels on any type of build

Is this a self build project ?

how much of it do you intend to do yourself

 do you have plans ? 

all the above will guide you intype of construction you choose probably

If you have plans --then send  toa TF company 

an ICF company  .SIPS company 

your exterior finish  you desire will also guide you possibly

type of ground you are on 

can it be slab ,or strip founds ,or piles --

the choice is very complicated until you tie down some real site conditions and choices  and your desires first  

and get some quotes -?

Edited January 12, 2020 by scottishjohn