Understanding R

[Garald]

I am trying to compare two insulation products. One of them (https://www.actis-isolation.com/documents/hybris-125mm-fdes/) gives the following data:

 

 

There are are some things one could comment here in terms of assumptions (17.5 C would do for a bedroom, but not, in my opinion, for a study or living room), but let us leave those aside. What really interests me is:

a) does this estimate of the effects of an air layer appear to be reasonable?

b) are the effects of an air layer on other insulating materials similar? If not, how should find out the R value of (insulation material + air layer), given the R value of the material (and whatever other data is needed)?

 

The point is that there are other materials where an air layer is strongly advised, for various reasons (soundproofing, moisture, fire protection). Take, for instance, the other product under comparison, BIOFIB Trio (https://www.biofib.com/biofib-trio/ )  :

 

 

 

If this is what the manufacturers of the other product called the "intrinsic R" (how to check? write to the manufacturers?) and the effect of the air layer is similar, then 120mm of material (R=3.15), together with the two air layers involved in a proper installation as above, should be enough to give R=3.8. (The first product claims the same performance with 105mm of material.)

 

Am I understanding things correctly?

 

[SteamyTea]

18 minutes ago, Garald said:

Am I understanding things correctly

I think you are understanding it correctly.

 

There is an optimum width for an air gap to reduce convection currents, for Windows it is about 16mm. It varies with height, more than width, but is affected by the dissimilar properties of the materials (the turbulence, not the materials).

 

"All models are wrong: some models are useful"

 

[JohnMo]

Basically there are a number of different air gaps, ventilated, unventilated, and partially ventilated.  I would read the partially and ventilated as pretty much the same.  Air movement .means they don't perform as well compared to unventilated, where there is minutes air movement.  To this you also add if you have a low emmisivity air space, i.e. reflective surface.  The reflective part adds to keep heat in or out, depending on the direction it faces.

 

Actis and others rely on the low emmisivity unventilated air space to get the product performance.

[Garald]

7 hours ago, JohnMo said:

 The reflective part adds to keep heat in or out, depending on the direction it faces.

 

Hm, but one has the same set of walls for summer and winter! Does having reflective surfaces (what kinds?) on both sides of the gap help?

 

7 hours ago, JohnMo said:

 

Actis and others rely on the low emmisivity unventilated air space to get the product performance.

 

Surely one can pull that same trick with the same effectiveness with other products, e.g., Biofib? Or are there issues with having an unventilated airspace, e.g., humidity? Do they depend on the product one uses?

[Temp]

10 hours ago, Garald said:

If this is what the manufacturers of the other product called the "intrinsic R" (how to check? write to the manufacturers?)

 

One way to check is to see if the resistance is linear with thickness. If 100mm has twice the resistance of 50mm (or half the U-value) then chances are the figures are based on properties of the raw material. If its not linear and 100mm isnt double 50mm then they are taking into account some surface properties, for example foil covering or an air gap.

[Temp]

10 hours ago, Garald said:

 

 

If you plot a graph of R vs thickness you can also see it passes through 0,0 (if extrapolated) which it wouldn't do if foil or a fixed thickness air gap was included..

 

Not sure if I can post this spreadsheet..

 

spreadsheet.xlsx

Edited July 16, 2022 by Temp

[Temp]

Duplicate post deleted.

Edited July 16, 2022 by Temp

[Temp]

10 hours ago, Garald said:

 

 

And for this one..

 

spreadsheet 1.xlsx

[Ferdinand]

I'm trying very hard not to post the "Talk like a Pirate Day" song, as a group of people who really understand "R".

 

Sorry - as you were 🙃.

Ferdinand

[Garald]

Yes, it's obvious that the thickness->R relation is linear for Biofib. Good call.

 

Now I wonder about my other questions:

1 hour ago, Garald said:

 

1. Does having reflective surfaces (what kinds?) on both sides of the gap help?

2. Surely one can pull that same trick with the same effectiveness with other products, e.g., Biofib? Or are there issues with having an unventilated airspace, e.g., humidity? Do they depend on the product one uses?

on

[Gus Potter]

11 hours ago, Garald said:

I am trying to compare two insulation products

You are not alone.

 

My practical suggestion is this. The R value is linear and based on an infinite surface area. That is your starting point. You use this to get the U value of your build up.

 

You can get into perimeter effects, try and delve into the manufactures' test data.. but they don't give away the family jewels and the underlying research data. If they do folk will post it on BH and it will get ripped by the likes of @SteamyTeaetc.

 

You have two choices..

 

1/ Torture yourself doing calcs, trying to piece together obscure data.

 

2/ Recognise that that constuction process is pretty rough and go for a thicker / higher performing off the shelf insulation, cover your bases and march on with the build.

 

If in doubt up the thickness, go for standard thickness off the shelf insulation, easy buildable details and that way you can save money for the things you will see on a daily basis.

 

I say this as have been involved in development and research in a commercial environment so will bet you'll struggle to get to the bottom of it. We don't spend loads of money on research and then give away the secrets!

 

One of my jobs was to make sure you publish enough but never enough to let you competitors see in the black box!

 

 

 

 

 

 

 

 

 

 

 

 

Edited July 16, 2022 by Gus Potter

[Temp]

22 minutes ago, Garald said:

1. Does having reflective surfaces (what kinds?) on both sides of the gap help?

2. Surely one can pull that same trick with the same effectiveness with other products, e.g., Biofib? Or are there issues with having an unventilated airspace, e.g., humidity? Do they depend on the product one uses?

 

1. Possibly but I'm not sure how effective they are. If my memory serves me correctly radiated power is proportional to T^4 so radaition is much more effective at higher temperatures.  Eg thermos flask or spacecraft.

 

2. The key is not allowing warm humid through insulation to the cold side. Perhaps by using a vapour barrier on the warm side and breathable membrane on the outside.

 

Edited July 16, 2022 by Temp

[Gus Potter]

A few thoughts.

 

A few years back on say timber frame the insulation regs changed. We used to use as standard a 95 or 89mm stud, worked fine structurally and still does, the basic generic loadings have not changed significantly.

 

When the regs changed the TF folk were complaining.. we need to go to a 138 of 145 deep stud (before a 89 or 95 stud was the norm) and this will make us less competetive. So they came up with the wheeze on bubble wrap, reflective membranes. I am being harsh but the consequence of this was that the folk on site had to spend a lot more time with attention to detail than before and this comes at a cost. The buck was passed to some extent.

 

Oh.. and if you want to realise the full reflective effect then the surface has to be clean and free of dust and debris.. as per the manufacture's test conditions. I have yet to see a builder who hoovers and wipes down the surfarce of the insulation.. then a brickie that does not cover the lot in dust from the Sthil saw.

 

In reality on a domestic level this is not realised.  I see it regularly on the some other sites I visit. I don't on my own sites as I specify stuff and design for what I know can be practically build, what is off the shelf.. sounds boring but this leaves extra cash for the finishes and Architectural flair. It also leaves folk with a warm house as I know I'm not relying on the Builder understanding all of this, even if they do I know they won't really price for it.. they just hope to get away with it. This is good pragmatic design for self builders in my view. If your builder does get the hoover out then you get a bonus.

 

I think the best way is to go back to basics. Look at what you can buy cheeply. Standard off the shelf insulation thickness. What services you need to run in the wall. Get your services runs sorted then check your overall U value. Make sure you can get the fixings you need for the thickness of insulated plaster board. Once you get this basic arrangement sorted go back and check you dew point/  condensation profile for due dillegence.

 

In summary the best advice I can give is to stick to the basics. The big money gets lost in the founds and the risk lies in the ground..spend most time on these aspects and this will allow you to increase the insulation thickness etc and not have to worry too much about quality control on "reflectivity". In terms of detail and workmanship you need to concentrate on the junctions / interfaces.. that is where the moist air will get into your build up. and cause trouble.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Garald]

So, in summary, the advice would be to

- see Actis Hybris's total R results as being inflated, but not grotesquely so

- assume one can get as much gain from air layers in the case of BioFib trio as for Actis Hybris

- *not* bother to use reflective surfaces on both sides of the air gap for BioFib trio? Or bother (what kind of reflective surface should we use?) and just be realistic about its helping only a little?

 

An off-the-shelf thickness of 120mm of BioFib Trio gives us an intrinsic R=3.15, which, when taken together with an air layer, would give us R=3.8 (a threshold level in French regulations), assuming the air layer works as for Actis Hybris and assuming also that an air layer helps as much as Actis Hybris's documentation implies.... something that is not necessarily very likely. Of course BioFib Trio is meant to be installed with *two* layers of air, as the diagram above shows.

[Marvin]

1 hour ago, Temp said:

2. The key is not allowing warm humid through insulation to the cold side. Perhaps by using a vapour barrier on the warm side and breathable membrane on the outside.

 

That sounds just like my bungalow's timber frame....

[Gus Potter]

My thoughts in  line with your text and in italic.

 

So, in summary, the advice would be to

- see Actis Hybris's total R results as being inflated, but not grotesquely so

 

Check the fine print. Your starting point here is the European Technical Approval (ETA) Here you should see the test data that complies with the Eurocodes.. pretty much the raw test approval that is signed off. I can see you are putting a good bit of effort into this so hope this helps point you towards what you want to know.

 

- assume one can get as much gain from air layers in the case of BioFib trio as for Actis Hybris

 

An air layer has insulating properties.. yes. If the air layer runs from top to bottom of the the wall then you will get some small convection current within this. But in practice you may have noggings / dwangs and this creates a barrier mid height of the wall. But the main thing is air tightness between the layers. You want to stop convection currents bypassing the insulation. That is where you need tapes if say using PIR insulation.. wool type insulation is more forgiving. The timbers shrink so you can get a gap.. hence air tapes. For starters at early design stage don't place to much (hang your hat) on the air layer. Stick to the basics as later you may need to do a compensatory u value calculation.. if say you have lots of glass. Keep this up your sleeve for now as if you cut it too fine you may have to change the structural design and that won't be fun.

 

- *not* bother to use reflective surfaces on both sides of the air gap for BioFib trio? Or bother (what kind of reflective surface should we use?) and just be realistic about its helping only a little?

 

Again.. keep your powder dry.. cut yourself a bit of slack here as you may have say structural stuff / cold bridging to deal with.. you can always go back to taking advantage of reflectivity later..

 

An off-the-shelf thickness of 120mm of BioFib Trio gives us an intrinsic R=3.15, which, when taken together with an air layer, would give us R=3.8 (a threshold level in French regulations), assuming the air layer works as for Actis Hybris and assuming also that an air layer helps as much as Actis Hybris's documentation implies.... something that is not necessarily very likely. Of course BioFib Trio is meant to be installed with *two* layers of air, as the diagram above shows.

 

Keep it simple stupid for now, see how the rest of the costs build up overall. Don't try and design too tight for now until you get the whole structure nutted out. Then if you want, go back and refine the insulation once you have have got some prices back.

Edited July 16, 2022 by Gus Potter

[saveasteading]

Some random jottings.

 

The effect of thicker insulation diminishes with thickness, but this is often not reflected in calculations.

(The first 25mm does a great deal more than the last 25mm).

 

The effect of a reflective layer has been dropped from the 'advantages' list of the plasterboard manufacturers, but can still be added in a calculation.

How dirty it is is interesting.....it is going to get dull with time even in a cavity. Does a  clean and shiny surface work the same with invisible radiation as with visible?

All paint is black until the lid comes off.

 

A  wide air gap allows air to circulate freely and reduce the benefit.

A ventilated air gap is a cold space.

[SteamyTea]

8 hours ago, saveasteading said:

Does a  clean and shiny surface work the same with invisible radiation as with visible?

Yes.

https://en.wikipedia.org/wiki/Emissivity

 

[saveasteading]

Shiny metal, very good, oxidised, half as good,

dirty.....as good as the material the dust is made of so very poor?

Without an air gap, pointless.

[SteamyTea]

24 minutes ago, saveasteading said:

Shiny metal, very good, oxidised, half as good,

dirty.....as good as the material the dust is made of so very poor

 

"With the exception of bare, polished metals, the appearance of a surface to the eye is not a good guide to emissivities near room temperature. Thus white paint absorbs very little visible light. However, at an infrared wavelength of 10x10−6 metre, paint absorbs light very well, and has a high emissivity. Similarly, pure water absorbs very little visible light, but water is nonetheless a strong infrared absorber and has a correspondingly high emissivity."

 

25 minutes ago, saveasteading said:

pointless

Really not worth the bother in my opinion.

 

An extra €1/m² on curtain material will make a much bigger difference.

[Garald]

5 hours ago, SteamyTea said:

Really not worth the bother in my opinion.

 

An extra €1/m² on curtain material will make a much bigger difference.

 

So, wait, what is not worth the bother? Coating both sides (or one side) of the air gap with a low-emissivity coating (whatever that would be), since that would degrade over time?

 

Shouldn't one recalculate the effect of the air gap as estimated by Actis, then? Very, very roughly, what should I expect? That the air gap should provide half the additional R they claim - whether it's their insulation or Biofib's?

 

Should I expect the two (uncoated) air gaps advised by BioFib (see diagram in my original post) to have roughly what Actis claims for the performance of a single air gap, i.e., an additional R of about 0.6?

[SteamyTea]

31 minutes ago, Garald said:

 

So, wait, what is not worth the bother? Coating both sides (or one side) of the air gap with a low-emissivity coating (whatever that would be), since that would degrade over time?

That.