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U Value for solid wall


saveasteading
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17 hours ago, saveasteading said:

Under osb with 2mm gap per 1200 then no, and that is what the 50 gap is for.

 

Also, sarking has always (?) been standard in Scotland and so there are far fewer accidental leaks. 

 

I couldn't believe that in a modern English house there was just the felt, and in an older one there were only tiles, and the snow could blow in.

BS 5250 recommendation is for 50mm ventilated gap over sloped ceiling insulation if underlay isn’t LR breathable.

 

Yeah timber sarking seem peculiar to Scotland area. No sure about current trend there. Our older stock is all slates on battens (quite leaky lol) Current trend everywhere as far as I can see is slates & battens on breathable underlay.

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15 hours ago, Iceverge said:

 

Sounds nasty. Can you describe the metal roof, was it flat panel or wobbly cladding ventilated at both ends? 

 

Was it a habitable building ?

 

I think @saveasteading has a slate roof which should have more gaps for ventilating the sarking in any case. 

 

 

It was in a few high spec one-off dwellings under the supervision of a particular architect who had his own ideas about things (including his own house) and didn’t accept advice about much. The makeup was standing seam zinc roofing on the isolation fleece on breathable underlay on OSB / ply deck on rafters with full fill PUR insulation. No ventilation to US of deck.

 

Slates are considered impermeable in BS 5250 however we all know that traditional slate is imperfect therefore permeable to some degree

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45 minutes ago, Gordo said:

slates & battens on breathable underlay.

 

Yes primitive isn't it?

 

Sarking is standard in Scotland. 150 or 225mm boards still available in quantity at merchants, £20/m2, pressure treated.. But mostly used for maintenance.  Osb is normally cheaper, and is certainly quicker.

 

Contrary to the understanding of some, not all Scotland has any more severe weather than the rest of the UK, but it is simply a sensible thing to do for UK weather generally.

 

I don't think our steading would still be standing after 120 years if it didn't have it.

 

Did the people who wrote the BS consider the differences? 

48 minutes ago, Gordo said:

Slates are considered impermeable in BS 5250

Nuff said. The slates are impermeable until they break, but at least have double cover unlike modern tiles. The gaps are very permeable.

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Today I will mostly be doing the same again, trying frametherm in the U value calcs, adding in the effect of heat loss through the rafters and studs, and finding a way to get better numbers.

 

1. pass all the tests to prove it to the BCO

2. actually get what is best for the building and bills in the long term.

 

Perhaps these are in reverse order.

 

 

Question. 

 

Please excuse the avoidance of jargon and scientific precision, and  I am thinking here only of keeping heat inside. 

 

We 'know' that a shiny layer helps insulation, by reflecting the radiation, but only when it faces an air gap. Theoretical  U value for the gap jumps from .18 to .44 if I remember correctly.

 

BUT does that not depend on the aspect? 

Heat (radiation) from inside travels  through plasterboard, hits metallic layer on inside face and it is reabsorbed back into the plasterboard and the membrane itself  No help and no hindrance

OR hits the shiny stuff on the other side of the gap and bounces back into the gap. Warms the air and helps the insulation level.

 

Then the reverse occurs at the outside...let's say PIR is left 50 short of the sarking. to provide a ventilated space. Then the reflection is again inwards, to no advantage.

and anyway the air gap is ventilated so the effect of the gap is low or nil..

 

Which could bring us onto multifoil, which is not in our recipe, so I won't go there.

 

Red pens out as necessary.

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1 hour ago, saveasteading said:

Yes primitive isn't it?

 

Did the people who wrote the BS consider the differences? 

Nuff said. The slates are impermeable until they break, but at least have double cover unlike modern tiles. The gaps are very permeable.

Primitive maybe but seem to work lol. N Ireland wind speeds are similar to Scotland other than North. I thought the Scottish were supposed to be thrifty lol
 

impermeable was a very poor choice of words for air tightness sorry for confusing you lol 

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4 hours ago, Gordo said:

The makeup was standing seam zinc roofing on the isolation fleece on breathable underlay on OSB / ply deck on rafters with full fill PUR insulation. No ventilation to US of deck.

 

Oh god. 

 

A plethora of completely impermeable materials trapping water in a minimally ventilated part of the roof that's freezing cold outside the insulation. Breaking every rule. 

 

If the roof was uninsulated he would have gotten away with it. 

 

If the roof was battened to allow airflow above the OSB he'd have gotten away with it.

 

If the humidity in the building was tightly controlled he'd have gotten away with it. 

 

If the PUR was above the OSB he'd have gotten away with it. 

 

I'm guessing there was no vapour barrier either.

 

I don't think most people realise that the water that rots their structures comes out from inside the building as water vapour. 

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3 hours ago, saveasteading said:

Today I will mostly be doing the same again, trying frametherm in the U value calcs, adding in the effect of heat loss through the rafters and studs, and finding a way to get better numbers.

 

1. pass all the tests to prove it to the BCO

2. actually get what is best for the building and bills in the long term.

 

Perhaps these are in reverse order.

 

 

Question. 

 

Please excuse the avoidance of jargon and scientific precision, and  I am thinking here only of keeping heat inside. 

 

We 'know' that a shiny layer helps insulation, by reflecting the radiation, but only when it faces an air gap. Theoretical  U value for the gap jumps from .18 to .44 if I remember correctly.

 

BUT does that not depend on the aspect? 

Heat (radiation) from inside travels  through plasterboard, hits metallic layer on inside face and it is reabsorbed back into the plasterboard and the membrane itself  No help and no hindrance

OR hits the shiny stuff on the other side of the gap and bounces back into the gap. Warms the air and helps the insulation level.

 

Then the reverse occurs at the outside...let's say PIR is left 50 short of the sarking. to provide a ventilated space. Then the reflection is again inwards, to no advantage.

and anyway the air gap is ventilated so the effect of the gap is low or nil..

 

Which could bring us onto multifoil, which is not in our recipe, so I won't go there.

 

Red pens out as necessary.

 

 

@saveasteading

 

Have you done a basic look at heat loss from the structure yet?

 

If you post your, wall area, roof area, window area, and floor area I'll stick it in PHPP and show how little fighting over the last decimal place matters with U-values. 

 

Airtightness, MVHR, triple glazing, and thermal bridging are much more important. 

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9 minutes ago, Iceverge said:

 

 

@saveasteading

 

Have you done a basic look at heat loss from the structure yet?

 

If you post your, wall area, roof area, window area, and floor area I'll stick it in PHPP and show how little fighting over the last decimal place matters with U-values. 

 

Airtightness, MVHR, triple glazing, and thermal bridging are much more important. 

It’s all about the laws of diminishing returns. I think we have well exceeded the reasonable financial payback for increasing insulation now with U values. Detailing and quality of install are more important now than the mm of insulation in middle

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1 hour ago, Iceverge said:

Shiny insulation makes almost an unquantifiable difference. 

and yet BRE say use 0.18 for a 22mm air gap in a wall and 0.44m if it has a shiny layer.

Instinctively I think this feels about right, because aluminium absolutely does reflect a lot.

 

Build quality et al are much further down the line and are not in the warrant application.

 

For now I want to put in a reasonably accurate heat loss proposal, that is fairly mainstream and uncontroversial.

But I really don't want to change it later, so am deciding on fundamentals like PIR or mineral wool. shiny membrane or not.

 

It isn't easy as the standards for a change of use are rather high, not that we wish to cut corners anyway, and there are lots of constraints due to the existing geometry and structure.

 

I'll see if I can summarise the points you mention. The surface area is huge.

 

 

back to shiny:   we are fitting out a roof area as site office, with PIR between the rafters and lined with a shiny membrane.  With Infrared heating I think we will get quite a lot of that bouncing back in, and it will be a bit like being the turkey in the foil. Will report on this when done.

 

 

 

 

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36 minutes ago, saveasteading said:

and it will be a bit like being the turkey in the foil.

Moist and falls apart easy.

 

You can work out the energy levels, Fermi did it years ago. for any given temperature, then you will see how little difference there will be.

It gets worse after time as well.

1 hour ago, Gordo said:

Detailing and quality of install are more important now than the mm of insulation in middle

Too right. A watt is a joule per second. And a joule of energy is the force needed to move 1 kg, 1 metre. So 0.1 on your insulation us like moving an apple  0.01 is just the repairs if the core.

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Shiny layer is ok straight off the cooking foil roll but fades over time?

Is it the shinyness or the aluminiumness?

 

It appears to me that a silvery barrier will add about £300 to the project cost. It all adds up but that seems ok.

 

Up for suggestions of where better to invest the money.

 

7 minutes ago, SteamyTea said:

Moist and falls apart easy.

I'm more likely to be a bit overdone and tough.

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45 minutes ago, saveasteading said:

Is it the shinyness or the aluminiumness?

Bit of both.

 

46 minutes ago, saveasteading said:

Up for suggestions of where better to invest the money.

£300 buys 2 or 3 PV modules.

There is waste water heat recovery.

A fair bit of sticky tape for airtightness detailing.

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2 minutes ago, SteamyTea said:

waste water heat recovery.

 I need convincing on that! anything I have looked at would never pay back the capital.

Even running the cold intake past the waste involves a lot of rerouting.

 

es sticky tape and mastic. Comes cheap by the box.

 

With a new 'stick' construction ( now 1/5th of the job) there are lots of potential gaps, at the base especially. Do people place the bottom string onto mastic before fixing to the footings?

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4 minutes ago, saveasteading said:

With a new 'stick' construction ( now 1/5th of the job) there are lots of potential gaps, at the base especially. Do people place the bottom string onto mastic before fixing to the footings?

I have often wondered that.  I think it certainly needs something to make it airtight, but not trap any water, and last, forever.

We used to silicone steam rooms to the tilled floor, but not the saunas.

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I am familiar with the substantial air gap at the interface of a bottom cleat for metal cladding (disregarding the old detail of not facing it at all. Our detail had a flexible mastic under it, but there were still noticeable gaps. Best found by looking out from dark to light. Tape was the answer if it was too wide for mastic. ( a long timber or steel onto concrete will have gaps even if in tolerance), and you hope it stays there permanently.

We will be doing this properly! No air test required so all the more reason to look for the gaps as we go along.

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