Jump to content

U Value for solid wall


saveasteading
 Share

Recommended Posts

I am playing around with the U values before finalising the construction details for internal lining and insulation, inside a 600mm granite wall.

Thanks to those who have provided info previously on this: the reality of solid walls.

 

I thought I would check out once last time  for any official view  on actual U value of thick walls, as it may help with the presentation for warrant application.

 

Some people on here say solid stone walls offer little benefit, others that it is undervalued.

(Another document suggests that both may be right depending on circumstances)

 

Therefore I now share this. I came across it in the official Scottish documents (for SAP rating) and based on in-situ testing. My red underscoring for our own situation.

 

560059053_Uforgranite.png.a75f0d7e5baba4ef47a9fde89d82f47a.png

Link to comment
Share on other sites

In real life, I dispute that dry lined walls in old stone buildings are better.  They might be if detailed properly.  But almost without exception my findings are the cavity created with the dry lining is open to the loft space and just allows cold air into that cavity,  bypassing most of the insulation properties of the wall.

 

This shows itself in winter when you remove a switch or a socket and an icy blast of cold air comes out of the vacated hole.

  • Like 2
Link to comment
Share on other sites

8 minutes ago, ProDave said:

an icy blast of cold air comes out of the vacated hole.

 

 

This is a very good point. Shows non-joined up thinking in the regs.

Some posts have shown that BCO requires vents to outside to increase the draught!

The Historic Buildings guide says both that the insulation value of stone  is much better than thought AND to put vents in if lining.

 

Idea, for support or shooting down:

 

We detail it without the link to the the roof ventilation gap, ie physically closed at the top of the wall. There will still be a draught but much reduced without the chimney effect.

Also if the areas are defined, as partly by fire barriers anyway, this will keep all planes individual and so little draught from wind, just natural movement.

 

 

Link to comment
Share on other sites

2 hours ago, saveasteading said:

The Historic Buildings guide says both that the insulation value of stone  is much better than thought AND to put vents in if lining.

 

It really depends on the internal moisture content within the walls. My last house was ~300 down to 150 year-old Cotswold-like stone around 600 mm in depth, with the internal face rendered then hard plastered.  We had an injected DPC that worked really well: when we moved in the infill was like damp soil; within 10 years it had entirely dried out to a mix of grain husk and clay powder.  Based on heat losses I reckoned the U-value was around 1.0 once dried out.  Still a bloody cold house though to heat!

 

You also be better off with MVHR than vents.  That would also mitigate / remove any dampness.

Link to comment
Share on other sites

Here's a report of walls that were in the 400mm to 600mm range of solid historic construction. 

image.thumb.png.c121e4adfc64f486414932a40871d452.png

 

 

I'm trying to find another Scottish document that I read that had a more comprehensive report. 

 

Specifically it echoed the comments above about the moisture content of the walls being all important.  

  • Like 1
Link to comment
Share on other sites

14 minutes ago, TerryE said:

internal moisture content within the walls.

 

we are on very good draining sand: at least 10m of it. Once there is a complete roof and a drainage system it will dry out dramatically.

 

I think that the footing stones are solid stone, 600-800 wide, with core rubble/mortar starting at ground level,  so that will reduce rising damp up the core too.

So I am optimistic that the walls will be quite dry in the longer term.

 

 

Link to comment
Share on other sites

5 minutes ago, Iceverge said:

another Scottish document

 

file:///C:/Users/Jenny/Desktop/Lower%20Remore/hstp102011-u-values-and-traditional-buildings.pdf

 

2.4 W/m2 K for 70% stone / 30% mortar compared to 1.5 W/m2 K for a dry wall.

 

sandstones: the average values are 1.9 W/mK for dry material and 3.0 W/mK for wet sandstone. 

 

Link to comment
Share on other sites

9 minutes ago, Iceverge said:

the moisture content of the walls being all important.  

Traditionally these walls were rendered so that the rain ran off.

 

Without the render they look very pretty.......of course we are going for pretty.

But with good new pointing most of the rain will run away.

 

I am  not trying to fiddle any numbers, but am hoping for a bit more certainty in the future performance, tweaking of any details to improve the performance, and to submit a proposal that BCO will accept without any further justification.

Link to comment
Share on other sites

I'll eat my words, that is indeed the correct document.  

 

Have you considered a breathable internal insulant and paint like lime hemp insulation? You would get the full benefit of the insulative value of the wall by doing this and it would not be detrimental to the moisture drying capability of the wall, as well as maximising room space. 

 

Here is another study I came across on this. 

https://tinyurl.com/ek3kfw72

 

Link to comment
Share on other sites

Hearts and trophies to be distributed later this evening. Good work so far team.

 

This is the draft drawing, some changes to make, eg increase service void to 50mm.

 

As a solution to ProDave point above, I think we add the closer filler at the top of each wall, and also vertical the same at about 8m cc.

This can be a strip of rockwool bat compressed in the 25mm gap. Then the wall can breathe but not create a flow as such.

829818577_Screenshot2021-10-31162152.png.87f5dee7b2cc257498bc8fd76aec971f.png

Link to comment
Share on other sites

1 hour ago, SteamyTea said:

this document 

This will take some time to digest.

 

Worthy of note everyone interested in big buildings (over about 13m width) is page 35....the insulation suppliers used to publish this but now don't....hence insulation is often over-specified.

This is for any ground, so will be better on dry granular.

 

 

Link to comment
Share on other sites

1 minute ago, Iceverge said:

is that there dem PIR boards between dem studs

 

tis.

 

them twice better dan mineral wool.

 

Twice the price too and not nice to work with but we only have 4.5m between the walls,  so don't want to lose another 200mm.

 

Vapour and moisture layers encapsulating it within timber studs, effectively a cassette, so we don't expect any issues apart from, grumpy installers.

Link to comment
Share on other sites

Do you know the actual make up of the walls.

I doubt if it is actually 2 foot of solid stone.

 

This may be helpful if you have access.

https://www.sciencedirect.com/science/article/abs/pii/S0013795209001434

 

Air, atmosphere (gas)    0.0262 W.m-1.K-1

Ground or soil, dry area    0.5 W.m-1.K-1

Granite1.7 - 4.0 W.m-1.K-1 

 

Just need to take a stab at what the wall is made from.

 

 

Edited by SteamyTea
Link to comment
Share on other sites

29 minutes ago, SteamyTea said:

it is actually 2 foot of solid stone.

There are 3 layers.

Outside is solid stone, mostly granite. ashlar at features and roughly flattened faces elsewhere.

At openings it is ashlar to all faces, and usually sandstone.

Inner is granite , smaller stones but still about 300 x 300

In the middle is rubble, and by all accounts it is normally 40% mortar.

 

From what I have been pointed to and read, the mortar being in the middle contributes to the decent U values, and the outer stone keeps the wall pretty dry.

 

this photo is not it, but was in someone's website and is a decent example and a literal cross-section.

 

 

.1115624206_sawcutrubblewall.thumb.jpg.41696959c2baf128cb37a55b40e9bf26.jpg

Link to comment
Share on other sites

I am doing very similar to you, I have allowed the walls some ventilation to the rubble middle from the outside and have left enough ventilation between the inside stone and the stud work to keep the air moving but more restricted than the middle of the wall. I painted the outside and then sealed it with masonry paint as the stone was very fractured and would let a lot of water in if not sealed. It’s bone dry in the building and no problems in 10 years. 
 

83DCA3AF-6E5A-44C6-8865-482C2894693B.jpeg

BF5FF84E-9FC9-4CF1-A4C9-C862C8E3068E.jpeg

Edited by Cpd
  • Like 1
Link to comment
Share on other sites

1 hour ago, saveasteading said:

Hearts and trophies to be distributed later this evening. Good work so far team.

 

This is the draft drawing, some changes to make, eg increase service void to 50mm.

 

As a solution to ProDave point above, I think we add the closer filler at the top of each wall, and also vertical the same at about 8m cc.

This can be a strip of rockwool bat compressed in the 25mm gap. Then the wall can breathe but not create a flow as such.

829818577_Screenshot2021-10-31162152.png.87f5dee7b2cc257498bc8fd76aec971f.png

Make sure service void is sealed at top to avoid thermal bypass. No reason not to fill this void with mineral wool before plaster board maybe. It will be a challenge to fit the breather paper. I’d still go with more breathable, easy worked with, toxic & cheap  drytherm but your choice. 

Link to comment
Share on other sites

A couple of probing questions if you don't mind.

 

1. How do you plan to install the breather membrane while maintaining the ventilation cavity?  

 

2. How do you maintain that PIR is twice as good as mineral wool? I get a U value of 0.304 for PIR and 0.398 for full fill rockwool (lambda = 0.035w/mk) with 100x45 studs @400cc. 

 

3. How do you plan to deal with the shrinkage and expansion of the studs with varying humidity and temperature? Won't the boards loosen and you'll get thermal looping? 

 

Legend has it that PIR off gasses and degrades in R value over time as well as shrinking like most other foams. I've been searching for over an hour to find a non biased long term study on this but can't.

The below is a screen shot of one manufacturers declaration of dimensional stability.

 

4. Maybe someone more qualified can explain what this means.  

 

image.png.a267d0ba8e0c0bbe84a558f725158a4d.png

There is some anecdotal whisperings of this. 

 

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16490

 

 

By creating a clear ventilated cavity you'll completely negate any insulative benefits of the current stone+ rubble wall.  

 

 

 

 

 

 

 

 

 

  • Like 1
Link to comment
Share on other sites

A couple of probing questions if you don't mind.
I relish the challenge.

 

1. How do you plan to install the breather membrane while maintaining the ventilation cavity?  

Yes, I fell asleep thinking of a solution to this one.   Easy to draw. Has to be done so there will be a solution..

Either:

wrap the membrane round the back of a set of a few studs and staple on. then staple on the face of the last and continue. Theoretically a gap where the 2 faces touch, but de minimus. 

OR after that, warap another piece round the stud to lap both;

OR , build as  a cassette on the ground and do the same

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

 

 

2. How do you maintain that PIR is twice as good as mineral wool? I get a U value of 0.304 for PIR and 0.398 for full fill rockwool (lambda = 0.035w/mk) with 100x45 studs @400cc. 

PIR 0.022 or thereabouts  Mineral batts 0.34 or thereabouts (have not checked). OK then not twice and you are starting to sway me.

 

3. How do you plan to deal with the shrinkage and expansion of the studs with varying humidity and temperature? Won't the boards loosen and you'll get thermal looping?

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. 

 

Legend has it that PIR off gasses and degrades in R value over time as well as shrinking like most other foams. I've been searching for over an hour to find a non biased long term study on this but can't.

The below is a screen shot of one manufacturers declaration of dimensional stability.

 

4. Maybe someone more qualified can explain what this means.  

 

image.png.a267d0ba8e0c0bbe84a558f725158a4d.png

There is some anecdotal whisperings of this. 

 

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16490

 

I have not heard of this before, and would have expected to. However I have an instinctive trust of mineral wool....unless it gets wet. I suspect any such issue will be related to atmospheric change (esp heat) . This will be in the dark behind a very thick wall.

I don't trust PIR, never did even before recent disasters. Several years ago I tried some in a brazier to see what happens.....burns, but needs a lot of heat to start it.

When grenfell was first on tele i said to my wife I could see what the problem was, and was correct.

Also , years a ago, spoke to fire fighters and fire engineers......they said they don't like to be near to PIR filled composite, but can't say that publicly.

 

So show me some better figures for mineral wool please.....it was always my wish but it needs too much thickness.

 

Thankyou for the opportunity. any more questions please?

 

 

 

 

By creating a clear ventilated cavity you'll completely negate any insulative benefits of the current stone+ rubble wall.  

Link to comment
Share on other sites

42 minutes ago, Cpd said:

It’s bone dry in the building and no problems in 10 years. 

Dry is good.

I have a philosophy that nobody wants a building: they just want to keep the weather out.

That doesn't apply to bankers, dictators  and trillionaires who want us to notice their building, using OPM.

 

If I was you I would be showing off your project at any opportunity. Beautiful

 

re the ventilation. Some people say that the walls are porous and no further ventilation is necessary or desirable. Yet it seems that BCO's ask for vents at 1m cc near the base.

 

I don't think we have a issue as there will be no rising damp and the wall is either porous (both ways) or not.

I see a strategy statement coming on.

 

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.

 

 

Link to comment
Share on other sites

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 (???)

Link to comment
Share on other sites

47 minutes ago, saveasteading said:

Legend has it that PIR off gasses and degrades in R value over time as well as shrinking like most other foams

Hmm. I noticed last week that either I cut all of my foam pipe insulation short 4 years ago or it has shrunk....

 

I understand your quote above to be true. Hence sealing all cut edges.... 

Link to comment
Share on other sites

Generally it is the proportion of fillers that reduce shrinkage in PU foams.

We used to add on 3% for shrinkage when moulding cushions and dashboards.  But it was also affected by ambient conditions when moulding, tool temperature, raw material stock temperature.

I have always been concerned about shrinkage with SIPs.  Seen panels that are bowed at a show and when I questioned the salesperson, he just shrugged.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...