Green Building Forum - Decrement delay

Decrement delay

Peter Clark — Thu, 29 Jan 2009 11:10:23 +0000

Thanks paul,

Posted By: PaulTBUT - and this is the realy important fact - heat is still being lost and will have to be replaced by the heating system!

Of course, but this is true for both homes, but more so for the lightweight one, balancing the initial extra energy needed for the heavy one?

Posted By: PaulTJUST BECAUSE THE TEMPERATURE DROP IS LOWER IN A HEAVYWEIGHT HOME IT DOES NOT MEAN THAT IT WILL REQUIRE LESS ENERGY TO HEAT THE HOME.

How so, because of the initial extra to account for the higher specific heat capacity of the heavier insulation? - balanced by the slower heat loss at the other end?

But it is not worse, and it might be better?

Might it feel more pleasant? Might that mean less fuel is used?

Posted By: PaulTHowever Cellulose materials are not High thermal mass - they are medium thermal mass.

Yes I appreciate this. I was contrasting two approaqches to the design of insulation for a house. The one I called 'lo density' insulation, and the one I called 'hi density', for convenience, sorry if confusing.

The latter ('hi density' insulation) seems to mean, have medium density insulation material with good decrement delay and allow that to have a positive effect on the insulation (including stopping heat get out).

The 'lo density' insulation approach... doesn't do this, simply looks at the U-value and ignores decrement delay?

I'm still left wondering of reasons for doing the 'lo'.

Perhaps if you have a very low occupancy and only need to quickly heat for a few hours?

It will be less costly initially

Peter

Decrement delay

tomsusweb — Thu, 29 Jan 2009 11:11:36 +0000

Posted By: PaulTHowever Cellulose materials are not High thermal mass - they are medium thermal mass. THis means that you have a medium amount of thermal mass available with a medium amount of thermal resistance. This means that the wall can asborb a relatively significant amount of heat at a slow rate. The home will quite repsonsive to heating and have slow release quantity of thermal mass.

That's how I see it too. And I'd say that's what makes it work well for us in the Irish climate - we need a reasonably responsive house to heating because the weather changes so often but also need a bit of slow release to even out the temperature swings. It seems to work from where I'm sitting! As I said above - if you want figures on real buildings in the UK I'd get in touch with Neil May at NBT.

(@Viking House: Hi Seamus: yes it's me - glad to hear you're building some houses this way. Will send some photos sometime).

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Peter Clark — Thu, 29 Jan 2009 16:12:58 +0000

Looks like the only application everyone is agreed on for 'heavy' insulation is room in roof, or roofs generally? To protect against summer over heating.

I which case another area of my general building ignorance kicks in. How likely is it that an existing roof or dormer structure, with no insulation, or rockwool, will be able to bear the extra weight of pavatherm or something similar?

Pavatherm has a density of 160Kg/m3

so 8 Kg for a m2 at a thickness of 50?

Can anyone enlighten me?

Peter

Decrement delay

PaulT — Fri, 30 Jan 2009 11:12:18 +0000

Actual Peter I do not agree that we need to go heavyweight

The important point, lost in a long post, is that Reducing heat loss/gain is the most important factor!

The majority of heat gain is through windows. with actualy very little (relatively, in the UK) coming in through roofs.

The most effective reduction in heat gain in a roof is to use a breathable foil layer under the tiles.

Also please do not confuce temperature stability with reduced heat loss. For "thermal mass" to buffer temperatures you need to put extra heat into the building on every heating cycle. Far better to focus on reducing heat loss.

Decrement delay

dickster — Fri, 30 Jan 2009 11:26:01 +0000

Our new build will be I beam wall and roof, 300mm+ thick filled with recycled newspaper.

Am I right in thinking that our worries about lack of thermal mass in walls are somewhat diminished given the performance difference between the two insulators?

Hope so!

Decrement delay

PaulT — Fri, 30 Jan 2009 11:37:48 +0000

Hi dickster

Have you thought about using recycled concrete blocks with a wet plaster on your internal walls?

- especially on walls opposite south facing windows where they can capture solar gain (The most important benefit of "thermal mass" in my view).

Decrement delay

dickster — Fri, 30 Jan 2009 11:50:02 +0000

Hi Paul,

3rd set of plans in with planners, last chance before our current pp runs out.

Sorry to say minimal solar gain as too many trees, so solar gain design was an option that went in the bin earlydors.

Thanks for suggestion.

Decrement delay

Peter Clark — Fri, 30 Jan 2009 11:59:27 +0000

Hi Paul,

Posted By: PaulTThe important point, lost in a long post, is that Reducing heat loss/gain is the most important factor!

Of course, that is what insulation is for.

Posted By: PaulTThe majority of heat gain is through windows. with actualy very little (relatively, in the UK) coming in through roofs.

I find this impossible to believe. I have two rooms with identical windows, same elevation, one is a room in the roof, one on the storey below. The room in roof becomes impossibly hot during a summer day, the one below much less so. Sometimes the difference is 10 degrees.

Posted By: PaulTThe most effective reduction in heat gain in a roof is to use a breathable foil layer under the tiles.

Yes foils are interesting, especially for room in roof where there is often a lack of height. Are you aware of any data from trials supporting this position - that foil is most effective?

Posted By: PaulTAlso please do not confuce temperature stability with reduced heat loss. For "thermal mass" to buffer temperatures you need to put extra heat into the building on every heating cycle. Far better to focus on reducing heat loss.

Have I been confusing these two? sorry if I have. I am responding really to the two graphs Viking House referred to on his website.

â€˜dense insulation kept the house 4 degrees warmer.â€™

And something similar for keeping cool in summer?

Are you are suggesting that the house with denser insulation, that is cooler in summer and warmer in winter, will use more fuel for heating and cooling overall?
In which case, I will have to think some more, very interesting tho, thanks again.

Peter

Decrement delay

PaulT — Fri, 30 Jan 2009 12:34:35 +0000

Hi Peter

Heat gain through roof - I am assuming a well insualted/low thermal bridging / air tight structure...

Trails (BRE) have shown the majority of room in roof constructions are perfromning very poorly - often having effective U values sereral times worse than the design value. - Hence, this may be the most likely problem you have.
- This is why I am a huge fan of SIP roofs.

- as a balance though consider earth buildings in deserts - they stay cool by 1) reducing solar gain and 2) using massive heat loss at night from a flat roof (radiation straight to space) to cool the mass prior to the soalr gains of the day....

My problem with high thermal mass roof is the structure required to hold them up there and the redcution in head hight - often a deciding factor.

Foils - Florida state has trialed this and tells people to buy foil from the supermarket to line their loft spaces! (Not that I would advise this as there are breathable low E membranes, such as tyveck reflex that do work)

But, most importantly, sealed cavity foils do work very well for downward heat flow! This CAN be confimred in hot box tests and using ISO calculations. - A 25mm cavity with foil backed plasterboard has a thermal resistance of R = 0.93 downwards and R = 0.41 upwards.

When I have performed equivalent hox box tests (see paint thread) I have ensured that the test method was isothermal and repeatable with clear identification of all variables (ALL forms of heat loss) between the two tests. For the Vicking house I simply do not have enough information, as such I can not accept the results.
-------------------------
What I can say is that high "thermal mass" homes are less responsive to heating input and require more heat energy to achive thermal comfort (combined air and surface temperatures).
"Thermal mass" does not reduce heat gain in homes in summer -it merely buffers up the resulting temperature gains. However in a long heat wave it is quite posssible for this thermal mass to stay warmer for longer.

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Paul in Montreal — Fri, 30 Jan 2009 12:54:09 +0000

Posted By: PaulTHowever in a long heat wave it is quite posssible for this thermal mass to stay warmer for longer.

No doubt about that. I used to live in a small stone cottage in Glossop. In the hot summer of 1990 the internal temperatures made it into the 30s centigrade and no amount of open windows made any difference as there was so much heat absorbed into the two foot thick stone walls. I believe that high thermal mass is useful in climates that high large diurnal temperature swings (Colorado and such like are places that come to mind) but in a damp climate like the UK where there isn't much diurnal range, or a climate like mine where the temperature may not dip below 24C overnight, it's actually a hindrance rather than a benefit. The further north one is located, the less usable solar gains are anyway and triple glazing reduces these modest gains even further.

As PaulT points out, it's the temperature of internal surfaces that is very important for comfort. As a case in point, my home office is at the NorthEast corner of the house and previously had no insulation in the walls at all (12" brick + 8" stone on the North wall and 8" brick on the east wall). Even though I could maintain the air temperature at 21.6C, it didn't feel comfortable as the wall surface temperatures were around 14C with an outside temperature of -11C (as revealed during an energy audit with an IR thermometer). This past year, I insulated internally with just 12.5mm of foil-faced polyisocyanurate board (and took great care with airtightness details). Now this year with even colder temperatures outside, the internal surfaces are much warmer (I haven't had the chance to measure but I'd estimate that they're between 19 and 20C) and, with the same internal air temperature, I feel much warmer as there isn't a cold radiator surrounding me. By the way, that tiny amount of insulation effectively doubles the R-value of the wall.

Paul in Montreal.

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djh — Fri, 30 Jan 2009 13:20:02 +0000

Posted By: Peter ClarkI am responding really to the two graphs Viking House referred to on his website. â€˜dense insulation kept the house 4 degrees warmer.â€™

I don't see this on the page that was linked to and Google can't find it on the site or indeed anywhere on the web. Where did you find this quote, please?

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Peter Clark — Fri, 30 Jan 2009 13:53:11 +0000

Posted By: djh
I am responding really to the two graphs Viking House referred to on his website. â€˜dense insulation kept the house 4 degrees warmer.â€™

I don't see this on the page that was linked to and Google can't find it on the site or indeed anywhere on the web. Where did you find this quote, please?

Hi djh,

its in this thread, in the previous page, first post by Viking House, not in his link

'Check out how cold it got outside (10 degrees), in the Fibreglass house the temp dropped to 14 degrees and in the Cellulose house the temp dropped to 18 degrees. So the dense insulation kept the house 4 degrees warmer.'

Sorry for any confusion,

Peter

Decrement delay

Peter Clark — Fri, 30 Jan 2009 14:07:02 +0000

Hi PaulT,

I don't think we are really communicating well, probably my fault

Posted By: PaulTTrails (BRE) have shown the majority of room in roof constructions are perfromning very poorly - often having effective U values sereral times worse than the design value. - Hence, this may be the most likely problem you have.
- This is why I am a huge fan of SIP roofs.

- as a balance though consider earth buildings in deserts - they stay cool by 1) reducing solar gain and 2) using massive heat loss at night from a flat roof (radiation straight to space) to cool the mass prior to the soalr gains of the day....

My problem with high thermal mass roof is the structure required to hold them up there and the redcution in head hight - often a deciding factor.

Foils - Florida state has trialed this and tells people to buy foil from the supermarket to line their loft spaces! (Not that I would advise this as there are breathable low E membranes, such as tyveck reflex that do work)

But, most importantly, sealed cavity foils do work very well for downward heat flow! This CAN be confimred in hot box tests and using ISO calculations. - A 25mm cavity with foil backed plasterboard has a thermal resistance of R = 0.93 downwards and R = 0.41 upwards.

I understand all this, I think, and agree. What I am wondering is how the performance of any insulation compares with the kind of 'dense insulation' approach raised by the Viking House site page. Especially in regard to stopping downward heat in a roof, but also, since it has been raised here, in regard to keeping heat in. I fully agree that the 'experiment' described on that web page is not conclusive, I'm not sure Viking House is saying that.

Paul in Melbourne, thanks but you have described more anecdotal evidence, without even the benefit of a comparison, however crude, as in VH's web page.

Posted By: Paul in MontrealAs PaulT points out, it's the temperature of internal surfaces that is very important for comfort.

I thought I had made this point earlier in the thread?

What VH describes interests me, and as fostertom implies at the start of the thread, I think it may be important and seems not to be widely appreciated. So that is always interesting. I certainly need to do some more reading and thinking.

Thanks to fostertom for starting this very interesting thread, and others for contributing, hope I havn't waylaid it.

Peter

Decrement delay

djh — Fri, 30 Jan 2009 22:48:34 +0000

Posted By: Peter ClarkHave I been confusing these two? sorry if I have. I am responding really to the two graphs Viking House referred to on his website.

â€˜dense insulation kept the house 4 degrees warmer.â€™

And something similar for keeping cool in summer?

Are you are suggesting that the house with denser insulation, that is cooler in summer and warmer in winter, will use more fuel for heating and cooling overall?

and

its in this thread, in the previous page, first post by Viking House, not in his link

'Check out how cold it got outside (10 degrees), in the Fibreglass house the temp dropped to 14 degrees and in the Cellulose house the temp dropped to 18 degrees. So the dense insulation kept the house 4 degrees warmer.'

I don't follow your logic here Peter. What Viking House said about the house staying 4 degrees warmer refers to the graphs on his web page and that is discussing a hot SUMMER performance. I don't think I've seen any implication by anybody but you that the house with denser insulation will stay warmer in the winter!?

Actually, I do agree that exploiting decrement delay in winter will increase the coldest internal temperature but it will also reduce the hottest internal temperature and I believe that the same amount of fuel will be required overall. This effect can be equally well obtained by the use of thermal mass inside any insulation.

Decrement delay

Viking House — Fri, 30 Jan 2009 23:49:18 +0000

I am replying to Paul from Montreal's post above who is talking about Cold Wall surfaces.
The Scandinavians I have spoken to speak of building houses with Cold Materials and Warm Materials and the fact that the house feels Colder by a few degrees when you build with Cold Materials.
If you walk towards a single glazed window in your underware with a Thermometer in your hand in Winter you will feel colder but the temperature will not drop on the thermometer.
There was a graph that I saw that showed that if your walls were 15 degrees you would need to set the temperature of the room at 25 degrees to feel warm. If your wall was 19 degrees you would need to set your room temperature at 20 degrees to feel warm. This is what I call the feeling of warm from warm materials and is why Finnish builders often try to build houses without concrete floors and often just put 3 layers of plasterboard down on 300mm of Polysterene and tile down on that!

Decrement delay

Viking House — Sat, 31 Jan 2009 00:03:34 +0000

Hi djh

I actually believe that house B will use less fuel to heat the house than house A which was one of the reasons for putting up the webpage. Many of my Polish builders believe this instinctivly and have seen it in practice. If they are building a house for a client the heating cost for the house is reduced dramatically when they choose to Externally Insulate with Dense Paroc Rockwool Insulation over Polysterene. The U-value of the wall doesn't change just the density of the insulation. If it is really Cold outside you will choose your heaviest coat which would have the same U-value (thickness) as your lighter coat if tested.

When they started studying heat loss with 3D Thermal Imaging cameras in Sweden they discovered that heavy insulation worked better and that heat moving in cylindrical patterns through dense insulation cannot be detected by a hotbox test.
So the U-value calculations we are using are not accurate!

Decrement delay

Viking House — Sat, 31 Jan 2009 00:27:44 +0000

Hi PaulT

My biggest problem with internal concrete walls is that they normally sit on foundations that are directly on the ground without any Thermal break. Because of the high Thermal Coductivity of concrete any heat taken in by the Concrete is quickly lost to the ground below the foundations.

Concrete blocks always seem to be Cold even when I put one in my living room during the heating season, It never felt warm to touch!

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Viking House — Sat, 31 Jan 2009 00:48:03 +0000

I've tried quoting some people here but I can't make it work, am I doing something wrong?

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marktime — Sat, 31 Jan 2009 10:19:31 +0000

My, ,my, what a nice, snarky thread!

Decrement delay seems to be an artifact of mass and I have seen another a reference to a mass equivalence effect where the the internal temperature reflects a higher external temperature than is actually the case. I have one reference which I am unable to follow up, a formula given by an Italian author, Givoni where:

delta T ext. = 0.01 . M eqiv. (unit mass in Kg/m2)

The effect of this is to make it appear to someone measuring the steady state temperature inside the building that the outside temperature is higher than it really is. It appears to be as a result of thermal inertia but I'm open to any other interpretation (barring radiative transfer in little holes. )

edit. unit mass per area of external surface in Kg/m2,

Decrement delay

PaulT — Sat, 31 Jan 2009 11:36:38 +0000

VH - I would, of course, use insulating block work to reduce thermal bridging on internal walls? (this is a green building forum)

As "internal" ground temperatures are stable and relatively high then the amount of thermal resistance required is actual low.

- This is the method advised by Passiv Haus
----------
The sense of touch - hot an cold - is not as dependent on temperature as you might think. What we are actualy sensing is a rate of temperature change in our nerve cells. So two materials witht he same temperature can fee very different. So it is quite possible for a 'warm' wall (say 20) to feel cold compared to a cold piece of insulation. This is one of the reasons why the actual surface material is the most important layer of all.

- Wood cladding is possibly the best surface material you can chose:
It has a low conductivity (warm sup quickly)
It has a lower emissivity than most surfaces (0.75), reflecting more heat back.
--------------------------
Thermal comfort theory - In a typical indoor environment with low air movement (<.1m/s) and moderate humidity (say 50%) then our sense of thermal comfort is defined as being approximately 50/50 balance of air temperature (dry resultant) and mean radiant temperature.

The mean radiant temperature is dependent on the emissivities and surfae temperatures across a room.

So it is a fact that if surface temperatures are raised by 1C then the air temperature can be lowered by 1C to achive the same level of thermal comfort.
(It is for this reason that I recommend secondary radiant heaters in bathrooms.)

--------------------------

Posted By: Viking HouseHi djh

I actually believe that house B will use less fuel to heat the house than house A which was one of the reasons for putting up the webpage.

I think that you will find that belief is not enough on this forum.

(VH - please accept that I am not putting up needlessly critical posts, but it is the case that there is a huge amount of greenwash and misleading advertising out there and this forum is a place to get critical analysis).

Providing information on temperature variations is not enough to establish lower heating input. (Unless I have missed something on web site and postings?)
- I have taught this theory at post graduate level and run simulations on the same building with different thermal mass interiors and have found no evidence to support your theories.
- The only basis I can think of is a reduction intemperature gradients because of the buffering effect on the internal heat capacity, but as I have said before the heat loss through walls and roofs is only a tiny fraction of the total in well insulated homes

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Viking House — Sat, 31 Jan 2009 12:33:00 +0000

Sorry guys if I am the Snarky one, I am new here and have yet to discover the proper etiquette.
I am basically a builder who has tried many methods to build warm houses with a good feeling for what seems to works.

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PaulT — Sat, 31 Jan 2009 14:18:10 +0000

Don't worry!
The important thing is that you are here and engaging with various people interested in green building ranging from University lecturers, Architects, Acusticians, Structural Enginerrs, builders, window makers and many other walks of life.
You will get honest and open opinions.
The key thing to remember is that all building is a compromise and that were the line is in the sand...
(eg I would not touch Polyurethane with a barge pole but am happy to use EPS, others will only use 'natural' materials)

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Viking House — Sat, 31 Jan 2009 14:18:24 +0000

Hi PaulT

You might then be the best guy to explain this or you might like to build a model to check what I am saying is correct;

I was at a trade show in Germany and passed a stand that was selling Natural Insulation.
They had 2 boxes made from plywood with a glass window in the front, the boxes were about 1 metre high and about 50cm x 50cm when you looked down from the top. Halfway down each box they put 10cm of Insulation, one with Woodfibre and the other with Fibreglass.
The Plywood lids had 40W bulbs hanging at a similar height in both boxes about halfway between the lid and the insulation. In the lower section of each box at a similar position there was a hole with a thermostat sticking out.

When the lights were turned on for an hour the temperature below the insulation in the Woodfibre box was 26 degrees
and the temperature below the fibreglass was 40 degrees.
An hour after the lights were turned off the temperature in both boxes below the insulations was 22 degrees.

Both insulation materials have a similar Lambda Value. This shows that lighter insulation materials let heat in and out more quickly.

A friend of mine who works for Ecological Building Supplies tried it out and got similar results. We should probably film it and put it on U-tube.

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Viking House — Sat, 31 Jan 2009 14:37:33 +0000

Hi Paul

I would appreciate your comments on PU in the Renovation section.
Who is the Acoustics guy? He might be able to explain this! I built 2 sound studios for two friends, they are both insulated to similar thickness's with Rockwool and are of similar size. The quieter and warmer one is insulated with 200mm of 100kg density Rockwool and the other is insulated with 200mm of 50kg density Rockwool. The second one needs a heater and the first one doesn't. They both have similar equipment and similar usage.
They explained it to me by saying that the sound is converted into heat going through the insulation or something but it went over my head!

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jamesingram — Sat, 31 Jan 2009 14:52:51 +0000

Hi VH
I use a lot of PU in renovation , I'm aware of its short falls , but I feel restricted by availablity and cost of alternatives
Its hard enough encouraging people to invest that bit extra in things that dont give them instant gratification.
I'd be interested and grateful for info. on suppliers and cost to get these prefered materials for better prices
thanks

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3209&page=1#Item_0
example of the obvious problems here , certain types of insulation might not work so well, be as sustainable in construction , but
on limited budgets , your get a lot more for your money, if reduction in energy use through improved insulation it a priority

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PaulT — Sat, 31 Jan 2009 16:04:09 +0000

Having problems signing in, but can just about get postings up...

I have seen this type of box used o produce some very dubious results...

Firstly the heat flow is downwards and can not be used to demonstrate performance in horizontal or vertical heat flows (because of convection currents)

The box is being used on cyclical heating basis and it would not suprise me if the time lag for the woodfibre was just over one hour.

The important factor is how much heat is being put into the upper chamber and what are the temperatures of these chambers - in reality I only care about the interior environment, air an surface temperaturesa dn not he exterior temperatures (as in this case)

Because of the heat loss from the rest of the box (other than the insualted 'floor') it is actaully impossible to draw conclusions... Heat loss is all about the weakest link.

Glass fibre is very open and it does have big issues with induced convection currents inthe insualtion layers (proven in Canada to increase effective u values in cold lofts by up to 100%)

One complicating factor may be the surface emissivity of the woodfibre(Reflecting heat back up), but of couse in a built environment this would not be exposed, the interior surface being paint, wood, lime render, etc)

- Using the type of box you describe I could even prove that Multi foil insulation "works"

I suspect that you would get the same results with a slab of concrete covered in Kitchen foil!

-------------------
It is an interesting fact that for most open cell insulation the higher the densisty the better the thermal resistance. This is because under a certain size air pockets lose the ability to have convection currents. So very dense mineral wool has a lower thermal conductivity.
This, of course, only goes so far so the best mineral wools have conductivities around 0.032 and trhe worst 0.044.
The layers of inuslation in dense mineral wool are also improtant - under Canadian building codes they must be laid perpendicular to the dierection of heat flow.
(I have noticed that this is not the case in straw bale building and they could be missing a trick!)

Acoustics - User name 'TIMBER'.
Regarding the energy in sound - this is tiny compared to typical heat energy - we are fooled because our ears are very sensitive to minute amounts of kinetic energy

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Peter Clark — Sat, 31 Jan 2009 17:23:34 +0000

Well this is interesting, and confusing.

Viking House, PaulT has suggested that your belief in the superiority of denser insulation is just that, a belief with no objective data to support it.
The report on your web page describes an experiment by a university building physics department. Are you aware of any published description of this experiment, or something similar? Personally I am hampered by a lack of ability in Dutch, or anything other than English.

Is this the place?

http://www.ist-world.org/OrgUnitDetails.aspx?OrgUnitId=6c20b16270014300b6d222ef766c294a&SourceDatabaseId=951ef712ecba4f7ca229527509db4a81

Any linguists find the paper in question?

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Viking House — Sun, 01 Feb 2009 14:30:21 +0000

I met a few Dutch professors at the PLEA conference in Dublin. I will as ask if they can root out the paper.

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Peter Clark — Mon, 02 Feb 2009 11:11:13 +0000

This document is directly relevant to the thread:

http://www.scribd.com/doc/6423343/Thermal-Mass-and-R-Value

And supports the view that the mass effect is not significant in keeping heat in, in a cold climate:

â€˜northern climates, when the temperature during a 24-hour period in winter is always well below the indoor temperature, the mass effect offers almost no benefit...â€™

Is the thermal mass effect described here the same as the use of massive insulation on the external side of a wall or roof?
In other words, is the decrement delay effect of dense insulation just a thermal mass effect?

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bella — Mon, 02 Feb 2009 12:54:23 +0000

This (and the various links) really is important stuff - especially for those needing to upgrade walls in solid walled buildings (in UK) from the inside. But am I getting the right translation into practicalities - or even asking the right questions?

1. Heavy weight insulation probably not advantageous at same u value - but may have other qualities to recommend it (e.g. wood waste, allows movement of water vapour and evaporation)?
2. Still air a v. good insulator so lightish-weight products that hold air within their structure have real advantages and this is so even in thin layers (e.g. polystyrene)? But what about water vapour in our damp climate? Stick to sheeps wool or waste paper unless wall very dry or using MHRV? What about semi-permeable foams?
3. Surface treatment (e.g ceramic microspheres in paint) has real advantages too? What do the spheres do to movement of water vapour?