U Value for solid wall

[ADLIan]

The test method for thermal conductivity accounts for thermal drift and worsening of lambda over time often found with PUR, Phenolic and XPS foams. Declared values should reflect long term lambda (20-30 years)

[Iceverge]

 

22 minutes ago, ADLIan said:

DS(TH)4 and DS(-20,-)2 are short term (48hr) dimensional stability tests at elevated temp and RH or at low temp (-20) as spec. Has nothing to do with long term shrinkage often associated with pur products, newer chemical formulations may have cured this problem now (???)

Many Thanks. I'm more than happy to be proven wrong by any longterm independant study of PIR and shrinkage. However with @Marvins contribution the anecdotal evidence seems to be pointing the other way.  

 

1 hour ago, saveasteading said:

OR some better idea, that some genius is about to come up with

 

1. While you're at it can you offer a robust way of keeping rodents out of the PIR also if you have a free cavity? All fine and well in a new blockhouse  with no gaps in the mortar. No so for old stone walls. 

https://singletrackworld.com/forum/topic/stw-helprats-in-house-wall-and-ceiling/

 

More swaying away from PIR .

 

Two examples of the roof with PIR and then Rockwool full fill. Maybe It'd be a good idea to push a membrane above the rockwool like a suspended floor insulation. Someone else will know. 

 

 

125mm PIR £21.53/M2 

 

150mm ROCKWOOL £13.87/M2

 

 

 

 

 

 

FIRE and nasty gasses. I once saw an old fridge burning. PUR insulation I assume. It was horrible.  

Some extracts from the below. 

https://core.ac.uk/download/pdf/339952.pdf

 

 

1 hour ago, saveasteading said:

All buildings move. The studs will prob move less than the rest of the building. Character anyway. They are wrapped in membrane.

My understanding of thermal looping is that it needs a route, and a sizeable gap.  for example, double glazing has a small gap and it isn't an issue, but secondary glazing has a big gap and there is circulation. 

 

 

https://www.sciencedirect.com/science/article/pii/S1876610217348622

It's been tested and checked. Here's an extract from the study published on the Kore website. ( I know everyone has an axe to grind but that's where i found it) 

The bottom line, 3mm gap reduced U value from 0.34 to 0.54. 

 

Edited October 31, 2021 by Iceverge

[Gordo]

Yeah not a fan of PUR insulation. Toxic chemical soup that I understand of gasses of concern to hyper allergenic people (sick building syndrome) and definitely shrinks over time, performance drips over time. In a fire it gives of very toxic smoke. Anyone in authority talks about it gets bought off or sued. Glenfell tells you all you need to know about how much you can rely on their declarations.

Edited October 31, 2021 by Gordo

[Marvin]

22 minutes ago, Iceverge said:

While you're at it can you offer a robust way of keeping rodents out of the PIR also if you have a free cavity? All fine and well in a new blockhouse  with no gaps in the mortar. No so for old stone walls. 

The only way I know to stop rodents entering a building is to have it completely covered in something that they cannot chew through and keep holes no bigger than about 10mm.

A lot of pipes through walls are not properly sealed around.

 

[Marvin]

22 minutes ago, Gordo said:

Yeah not a fan of PUR insulation. Toxic chemical soup that I understand of gasses of concern to hyper allergenic people (sick building syndrome) and definitely shrinks over time, performance drips over time. In a fire it gives of very toxic smoke. Anyone in authority talks about it gets bought off or sued. Glenfell tells you all you need to know about how much you can rely on their declarations.

I understood that most people who die in fires die from smoke inhalation- thank goodness.  

[Gordo]

1 minute ago, Marvin said:

I understood that most people who die in fires die from smoke inhalation- thank goodness.  

That is a undisputed fact. 

[Cpd]

2 hours ago, saveasteading said:

Cpd, did you apply an impervious membrane to the inside, bottom of the walls? I think this is sensible but it does not seem to be standard.

The  building is just used as a workshop and storage at the moment. 
the walls have no DPM and because they are sealed on the outside but vented into the rubble centre they are very dry. 

My floor build up is 

ground - volcanic rotten rock joining shale ……, the sub floor angles in to the centre of the building where there is a central drainage channel / french drain to the outside. 

150mm of aprox 20-70mm clean (no fines)compacted rock that is fully permeable. 

50mm concrete cap on top of this rock terminating 70mm away from the wall edge 

Doubled up DPM for reassurance…. That goes up the side of the stone walls on the inside to just above the final floor level and links into the breather membrane. 
100mm Concrete slab terminating 70mm from the edge of the walls 

the 70mm gap around the slab is filled with pee gravel so that any water that makes it’s way in from the bace of the wall will filter down to the drainage layer below the slab. 
Insulation (not done yet )

22mm chipboard flooring (not done yet) 

final floor (not done yet ) 

Unlike you with your Sandy soils i have two different sub bases meeting each other and in extreme wet weather  water springs up below the building, this is why the whole floor area acts as a massive french drain.  The design works extremely well and the water never actually wells up as the french drains take it all away but I wanted to be absolutely sure. 
The perimeter drainage around the slab was to catch any water coming in from the outside as the ground level is higher than the inside sub floor level, again I over build this as in reality any outside water coming in is filtering straight down into the permeable layer very quickly as the bace of the walls in even extreme wet weather are bone dry. 
 

all in all it’s all working very well, if not a little bespoke for very specific ground conditions. 

 

 

 

[saveasteading]

4 hours ago, Iceverge said:

3mm gap reduced U value from 0.34 to 0.54. 

ouch. can we even fit it without 1mm gaps?

[Gordo]

9 hours ago, saveasteading said:

ouch. can we even fit it without 1mm gaps?

Gaps and thermal loops are a huge concern that few give adequate consideration to. Sealed gaps are your U value friend.

[saveasteading]

14 hours ago, Marvin said:

The only way I know to stop rodents entering a building is to have it completely covered in something that they cannot chew through and keep holes no bigger than about 10mm.

A lot of pipes through walls are not properly sealed around.

 

I'm good on rodents! Had buildings approved 'as built' by Mars , M and S, Waitrose among others. 

It was interesting observing their inspections as they headed straight for where they expected to find gaps....and were surprised to find it was dealt with.

 

6mm for a mouse I think, but insects can be much smaller of course.

Once in mice, rats and wasps will chew away PIR if necessary to make a nest.

 

On site so far we have also seen a slow worm, so we will create piles of recovered stones and timber for refuges.

 

There is already  a midge dormitory, so we need to encourage swallows and bats, and will.

[SteamyTea]

1 minute ago, saveasteading said:

so we need to encourage swallows and bats, and will

You have no intention of ever modifying it then.

[saveasteading]

Just now, SteamyTea said:

You have no intention of ever modifying it then.

Good point.  No they will have to stay politely outside, which is selfish I know. but we can put boxes up either on walls or on trees.

Bat nests are just made of ends of scaffold plank and some batten. Cost nil if from offcuts, and half an hour to make?

[saveasteading]

Ok, so it didn't take much persuasion to get me to reconsider mineral wool instead of PIR.

 

My original plan was to use cavity wall batts, especially as they are waterproof, would completely fill the gaps between rafters and would be easy to cut and fit.

 

Against it? not such a good U value, although it doesn't shrink.

wastage will be bad with cutting to fit 390-410mm gaps.

 

Also a lot of batts have developed form dense to rather loose, and almost like loft insulation, and I somewhat doubt the performance.

(No names but I have found that the cheapest fibreglass needs a lot of puffing to get to theoretical thickness, and just doesn't look 'right'.

 

Considering filling between rafters in the existing roof, with sarking boards 200 wide and penny gaps, slates above but no vapour barrier.........does this really need an air gap?

My logic is that any drips getting in the gap will as easily evaporate again, whether running down a 25mm space or sitting on the tightly placed  surface.

[ProDave]

37 minutes ago, saveasteading said:

Against it? not such a good U value, although it doesn't shrink.

wastage will be bad with cutting to fit 390-410mm gaps.

Consider what I used, Frametherm 35 on a 1200mm roll, should do your cuts with little waste.  Stiff enough to remain in place without slumping, I had one test piece left there for 6 months before getting covered and it did not budge.

 

Mineral wool has the advantage of longer decrement delay, which basically means it takes longer for the building to warm up or cool down in response to a temperature change.

[saveasteading]

9 minutes ago, ProDave said:

longer decrement delay

Which I do appreciate will be good in the traditional hot week in August, especially where we plan to have metal cladding instead of slates.

 

If I was to mention decrement do you think the BCO will understand?

 

We have bought samples of PIR and Knauf batt and tried them for practicality. both ok, and will maybe get a bit of Frametherm to try too.

 

I'm a bit confused as I think I have seen some of your photos showing PIR.

What is the construction here?  frametherm tight up to osb?  

You will have a VB above the osb, which we won't., as the rafters, sarking and slates are already there.

[ProDave]

8 minutes ago, saveasteading said:

I'm a bit confused as I think I have seen some of your photos showing PIR.

What is the construction here?  frametherm tight up to osb?  

You will have a VB above the osb, which we won't., as the rafters, sarking and slates are already there.

Very little PIR in this build, the only place is under the bathroom UFH.

 

Roof, from outside to in,

tiles

battens

counter battens

breathable membrane

100mm wood fibre board

195mm thick rafters full filled with Frametherm 35

13mm OSB

Air tight membrane taped

25mm battened service void

Plasterboard.

 

[ADLIan]

3 hours ago, saveasteading said:

Ok, so it didn't take much persuasion to get me to reconsider mineral wool instead of PIR.

 

My original plan was to use cavity wall batts, especially as they are waterproof, would completely fill the gaps between rafters and would be easy to cut and fit.

 

Against it? not such a good U value, although it doesn't shrink.

wastage will be bad with cutting to fit 390-410mm gaps.

 

Also a lot of batts have developed form dense to rather loose, and almost like loft insulation, and I somewhat doubt the performance.

(No names but I have found that the cheapest fibreglass needs a lot of puffing to get to theoretical thickness, and just doesn't look 'right'.

 

Considering filling between rafters in the existing roof, with sarking boards 200 wide and penny gaps, slates above but no vapour barrier.........does this really need an air gap?

My logic is that any drips getting in the gap will as easily evaporate again, whether running down a 25mm space or sitting on the tightly placed  surface.

 

 

Mineral wool roll or batts with a conductivity o 0.035 W/mK or better are normally rigid enough to be self supporting as ProDave says (to do with density). I think we all know of the fibreglass you refer to - probably made not a millions miles from you!!

[Gordo]

I have seen problems with unventilated cold decks (sarking board) and metal cladding. Where the US of impermeable cladding sweat on clod days soaking the timber deck. There were fungus mushrooms and all. If the US of the deck is properly through ventilated this shouldn’t have been an issue. 
 

Do they accept 25mm ventilation gap to sloping ceilings in your parts? as they insist on 50mm here as per BS 5250. Guess your relying on condensation risk analysis maybe

Edited November 1, 2021 by Gordo

[jfb]

I'll throw a curveball out there!

 

Have you considered woodfibre board?

What is the state of the inner walls? Original render and reasonably flat? Or bare stone?

 

I've done a few buildings with it and it has been very successful - render behind boards provides very good continuous airtight layer.  Haven't gone beyond 80mm thickness of woodfibre board for worries of interstital condensation. You would need a plasterer happy to work with lime render though you can get easy to use bagged mixes for the finish coats if you want to spend more than using sand/lime.

No need for a ventilation gap that is hard to achieve yet a breathable buildup.

 

 

Edited November 1, 2021 by jfb

[saveasteading]

55 minutes ago, Gordo said:

unventilated cold decks (sarking board) and metal cladding.

 

Correct and won't be doing that. Metal cladding will be ventilated .

Also would not happen if every joint was sealed but that is usually not done in  domestic.

56 minutes ago, Gordo said:

25mm ventilation gap to sloping ceilings in your parts

 

If presented as a holistic design  then yes , or I should hope so, as the traditional sarking is 200 wide then 2mm gap, so lots of ventilation.

Under osb with 2mm gap per 1200 then no, and that is what the 50 gap is for. But my question is about the logic of zero gap when the material is full-fill cavity batt.

BCO's like proven details, fair enough, but things only change when questioned.

 

There is a view on this forum that the gap is only there over PIR to boost the U value with a shiny face onto an air gap. As if they would do that to us.

 

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.

 

On our project the sarking is about 50 years old we think. It is rotten only where water pours in due to a complete hole in the roof. Where there are tiles they are sound, (and they were not treated)

 

 

[saveasteading]

2 minutes ago, jfb said:

Original render and reasonably flat

 

yes but.

 

There is quite a variation on it so a 25mm gap is handy for lots of reasons.

Also some areas of loose render that will have to come off.

 

Lime plaster....never again, having been ripped off once. 

 

I have been looking at fibre board today...maybe on some of the roof, although the eaves have zero over-sail so is a tricky  detail. 

Is it an alternative to plasterboard on the ceiling?

 

The 600 granite walls will have a U value of 1.8 according to the documentation IF dry and IF not severely ventilated.

 

But the good news is that the ground is superbly free draining so rising damp should not be a problem, and I am proposing to close the voids off to allow local ventilation but no draughts. 

 

More curveballs please. The analogy fails though if I say I may come back to that one.

[saveasteading]

ProDave, I came across this about an energy survey. which is perhaps relevant to your annoyance at your energy rating 

 

The SBEM prediction for post-improvement emissions (8,135 kgCO2) is extremely high in comparison with all other modelling programmes. The reason for this is almost certainly that SBEM assumes a greater use of electricity (mainly for the proposed heat recovery ventilation system) than the other programmes. This is covered in more detail in section 7.4, but the result is that it skews the post-improvement average (an 88% improvement at 2,589 kgCO2); removing SBEM from the equation would change this substantially, giving a 93% improvement to 1,480 kgCO2

 

 

and more on p23 here if still annoyed and interested    https://www.changeworks.org.uk/sites/default/files/Historic_Scotland_Technical_Paper_8.pdf

Edited November 1, 2021 by saveasteading

[Iceverge]

3 hours ago, Gordo said:

I have seen problems with unventilated cold decks (sarking board) and metal cladding. Where the US of impermeable cladding sweat on clod days soaking the timber deck. There were fungus mushrooms and all. If the US of the deck is properly through ventilated this shouldn’t have been an issue. 

 

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. 

 

 

[saveasteading]

5 minutes ago, Iceverge said:

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

Yes that is correct. The wind whips through the slate to the sarking. Currently it then whips through the 2mm gaps and the whole building, and that is where the detailing comes in.

We are now proposing a partial rebuild of about 20% and will use metal to differentiate old and new, but to the same geometry.

 

Metal cladding I know nearly all about. Would normally seal every single joint of panels and flashings with very special mastic tape and fillers, but the whole industry doesn't necessarily do the same.

For the steading we will do what others do, and set it on double battens with the air through it and any dribbles running down and away. NOT secret fix but screws proudly on show. 

 

Metal cladding that is permanently wet will rot in a short time, regardless of coating and galvanising, unless it is utterly undamaged, cut properly and has hole-sealing screws.

The next worst thing is bird mess, just as it is for cars. Hence double thickness plastisol if near the coast..

[Gus Potter]

Great informative thread this as have a particular interest in this type of construction, solid stone walls. It's remarkable how the body / tacit knowledge has built up over the last 30 years and how we are introducing modern materials.

 

2 hours ago, saveasteading said:

If presented as a holistic design  then yes , or I should hope so, as the traditional sarking is 200 wide then 2mm gap, so lots of ventilation.

Under osb with 2mm gap per 1200 then no, and that is what the 50 gap is for. But my question is about the logic of zero gap when the material is full-fill cavity batt.

BCO's like proven details, fair enough, but things only change when questioned.

and " More curveballs please"...

 

"The analogy fails though if I say I may come back to that one..."

 

For say steading conversions in Scotland a common roof build as a starting point up may be 50 - 60mm of insulated plasterboard (with integral vapour barrier) fixed to underside of rafters, another vapour barrier as a second line of defense (my personal preferance which accounts for perhaps reduced standard of workmanship here and there). Then full fill between the rafters which are normally 18" centres on a typical steading. On top of the rafters you have the timber sarking boards, then a breathable membrane, then the slates.

 

You may need to reslate so may well need to stick with a 1/2 in sarking board to plane though with the existing... and this chucks up all sorts of issues, the slate nail penetration being one. If re slating large area then go for a thicker board (19mm) as this takes the bounce out of the sarking. This makes it easier for the slater to get the roof tight but also easy to maintain in the long term... provided you have not double nailed the head of each slate.. slate the way the slaters do in your area, you can void the guarentee on the slates (if buying new) to a certain extent but at least you can make an informed decision considering ongoing maintenance.

 

Practically if you look at an old roof the nails don't pull out as they often have some corrosion on the underside which binds the nail. If you have PIR with shiny foil tight up to the underside of the sarking the foil gets a regular punching.. not sure if this makes much of a difference. The point is that there is a fair bit to mull over.

 

OSB is a bit vapour permeable but not much and not enough to let a full fill roof breathe in the Scottish climate. However timber sarking is much more forgiving. The 2.0mm gap is a figure that crops up. Some of the design codes are based on past experience and observation rather than pure calculation. One way to demonstrate this is to look at the suite of Eurocodes, many of the codes have a national annex.. which reflects the environmental conditions in the UK.

 

If you look at an old steading roof you'll see that the sarking boards have shrunk, gaps can be 5- 10- 14! mm. Also the boards have warped which encourages air flow under the breathable membrane. Normally the sarking boards that get delivered to site in Scotland are pretty wet. I have fitted them tight before.. no gap, give them a month or two and you'll see that even a board 100mm wide has shrunk to give the 2.0mm gap, once you put some heat in the place the gaps widen further.

 

Certain parts of Scotland can get pretty cold -18 to -24 deg for several days. Putting a good thickness of insulated plasterboard on the underside of the rafter can reduce the risk of "ghosting" .. where you get some local surface condensation which shows up as darker lines on the ceiling. If in a kitchen then cooking fats can condense more easily here. Although the ceiling may not be "wet" these marks don't go away.