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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorsteveleigh
    • CommentTimeMay 28th 2007 edited
    • CommentAuthortony
    • CommentTimeMay 28th 2007
    Anyone who claims 100% air tightness lives in cloud cuckoo land ( sadly ) and to prove it they are not even talking in the correct type of units. ( 100% is meaningless in this context ).

    Your questions are interesting and your analogy with aircraft has not been considered here before.

    A timber structure would need insulation across the studs as they are thermal bridges. You should install as much insulation as you can and 400mm sounds good. There are very few, if any, in the UK that can get any where close to German air tightnesses.

    We discuss thermal mass here and I am in favour of it inside the insulated envelope but to some extent it depends on building use times and life styles. From the point of view of the building I think that the smaller the range of internal temperatures then the happier and healthier the building will be and the longer it will tend to last. For sure you should go through the thermal mass debate before you do build.
    One point with water and vapour proof exterior claddings is that they have to be bonded to insulation to prevent any moisture condensating on the inside of cladding. Then you need a vapour barrier to inside to prevent moisture migrating out so I suggest another layer of rockcrete on inside bonded to insulation.
    A 300mm panels of polystyrene with rockcrete bonded inside and out should get close to passive house spec and similar to the aircraft analogy. The twin skin panels would structural as well, here is a system in NZ which could be adapted
    "Design with Energy" by Littler and Thomas suggests that: -
    The oxygen supply needed for breathing is about 0.009 grams/sec/person (at an energy expenditure rate of 2500 kcal/d) giving a requirement of 0.03l/s/p.

    I’ve run a quick spreadsheet based on the above data and airtightness of 0.6ach/hr@50pa. Using the background infiltration/exfiltration rate of 0.028 ach/hr the Oxygen provision is fine. However, if you drop to an airtightness of <0.035ach/hr@50pa (equates to 0.033m3/m2@50pa) you start to run into problems. In this case if the MVHR breaks down you could die from oxygen starvation. As I understand it the worlds most airtight building apparently achieved 0.15ach/hr@50pa under lab conditions, so all in all unless you have a "100%" airtight building you should be OK.

    CO2 is another issue as an airtightness of 0.6ach/hr@50pa provides a background air change rate that offers just 42% of the required fresh air ( 1.25 l/s/p required to prevent respiratory CO2 exceeding the maximum permissible level ). If the MVHR breaks down could this be a problem? I’m not certain how people behave towards varying/high levels of CO2, but given that at this time the air will get quite foul and the RH may begin to get uncomfortably high (due to limited ach) I suspect that discomfort will cause people to open the window to before CO2 level really becomes an issue......they can then get the MVHR engineer to repair the system.

    Incidentally an airtightness/leakage of 1.5ach/hr@50pa will provide adequate background ventilation and purge the CO2 in a suitable manner.....I wonder whether this is why the Canadian R2000 standard has set this leakage requirement? (Smells/RH/condensation and other pollutants will not be addressed however, hence why MV is required.)

    • CommentAuthorsteveleigh
    • CommentTimeMay 30th 2007 edited
    • CommentAuthorLizM
    • CommentTimeMay 30th 2007
    If you are going down the mechanical ventilation with heat recovery route, install openable windows. There's a develpoment in Manchester where they installed unopenable windows and the homes overheat in winter!MVHR is great in winter but you'll need natural ventilation in summer to stop the house overheating otherwise the occupant will install air conditioning and so any carbon saved by being airtight/superinsulated will be emitted and more by the ac.
    • CommentAuthormoogaloo
    • CommentTimeMay 30th 2007
    If a house where 100% airtight and the ventalation system failed, presumably the occupents could suffercate?
    I find the concept of 100% air-tightness worrying ... breathe fresh air -- good in / bad out ... add to that the risks from Radon, Formaldehyde, VOC's etc. and it gets terrifying.

    Perhaps we should all live in heated / cooled body suits and breath bottled air? :)
    There's no such thing as a 100% airtight house. But mechanical ventilation has been the norm here in Canada for at least a decade (as required by the building codes). As Mark mentioned earlier, the Canadian R2000 standard for airtightness (which pretty much no-one achieves in the UK except for the imported, from Canada of course, "Super E" houses) is such that even if the MHRV fails there is still enough leakage to supply oxygen for breathing. Of course, the occupants would soon notice that the air quality was diminishing and would presumably open windows. In a UK climate, the concept of keeping windows closed all the time is pretty much non-existant. In Canada, when it's in the -20s outside, windows tend to be kept closed much more - and, of course, MHRV is more cost effective when there's such a huge temperature differential between exterior and interior.

    A properly designed MHRV system provides superb interior air quality and recovers more energy than is used to run it - and saves vast amounts of energy compared to the alternative of having a leaky house to begin with.

    Paul in Montreal (with 1st hand experience of the benefits of MHRV)
    I've read a Canadian report that found that the indoor air quality was of houses with MVHR was better than that of the standard Canadian home. Radon, Formaldehyde, VOC's etc. were within safe levels, where the standard build was found to have problems far more frequently. Houses with MVHR are healthier homes than those without (the survey included many homes i.e. >100 no. so the data should be reliable). For MV to be cost effective (can you put a price on health?) and energy efficient good standards of airtightness are required.

    Radon: One thing that people fail to consider is that increased airtightness can help to keep the Radon out rather than keep it in i.e. only when you get exfiltration do you get air infiltration, which can in turn bring the Radon in. Again this statement is based upon Canadian research. By having MV(HR) you ensure that the air is continually purged of pollutants rather than just when you happen to open a window/trickle vent (what happens on a still summers day??). MV vents can be located away from sources of pollution i.e. ground level radon.

    I've also read BRE reports noting that UK housing....shoddy as it is....can exceed WHO recomendations for indoor pollutants......

    Completely agree, Mark, which is why MHRV is enshrined in the Canadian building codes. Ontario has finally mandated it as of this year; Quebec has mandated it for a few years now by stating that "any residence within range of a permanent electrical supply shall have MHRV that operates at least during the heating season". This sounds reasonable - in Summer indoor air quality issues are less likely due to more open windows, doors etc.

    Radon is a particular problem in Canada because most homes have basements. Radon, being much heavier than air, tends to accumulate there. Some places which are especially prone usually have special treatments on the concrete foundations to prevent radon entry.

    Trickle ventilation, IMHO, is simply a waste of time and energy. It's impossible to control and so cannot be designed correctly to ensure sufficient air changes are provided to remove interior pollution. As you correctly note, it's more than just providing fresh air - it's removing all the human generated nasties as well as the outgassing that many modern materials have. Of course, humidity control is also important, particularly in winter - though many Canadian homes have a problem with excessive dryness in winter rather than damp - this being caused by the infiltration of the extremely dry air that characterizes the cold winter climate in much of the non-maritime regions of the country.

    Hopefully the UK will eventually catch up.

    Paul in Montreal.
    A quick clarifification:
    Radon barriers are required when ground conditions are problematic. Airtightness is not a substitute for this technology, thought is does compliment it.

    • CommentAuthortony
    • CommentTimeMay 31st 2007
    What will stop radon? It passes slowly through granite, concrete, polythene, pretty much everything. The general approach is to have a labyrinth of ventilated mini passageways under the home and exhaust the radon out with air usually with the help of a small fan. This is bad in terms of heat loss.

    radon is produced by granite - that's the main source. As it is an alpha emitter, the alpha particles themselves are relatively easily stopped. Radon decays to Polonium, which is also an alpha emitter. These Polonium particles lodge in the lungs where they cause a lot of damage due to the alpha particles being emitted inside the body.

    The following link shows some construction techniques for mitigating radon through plastic sheeting and proper venting:


    Paul in Montreal
    • CommentAuthortony
    • CommentTimeMay 31st 2007
    The plastic sheet will not stop radon, however if the area below the polythene is ventilated then the radon will tend to get out into the atmosphere rather than following the more difficult route through the polythene. The diagram shown in your link seems not so sensible as the radon has to find its way up hill and out through the roof! You suggest that radon tends to pool in the bottom of basements so won't it tend to sit among the stones under the house awaiting the opportunity to get inside?
    The plastic sheet will easily stop the alpha particles and will provide a severe impediement to the diffusion of the radon (though, as you point out, not 100% effective). It should also stop any polonium particles that are produced as part of the decay of the radon - it is the polonium that's the problem, not the radon itself.

    Even though radon is heavier than air, it is still subject to diffusion. If this wasn't the cases, there would be no way for CFCs to have made it to the upper stratosphere to cause ozone damage - they'd all have stayed pooled on the ground. That said, the concentration of radon in a closed basement is likely to be higher than that in the upper levels of a house. My comment earlier was probably too simplistic. Basements are a problem because they provide a source for the radon to enter a house (ground water tends to have a higher concentration than the air at ground level).

    I presume the construction method shown is reasonable given that it is coming from the US Environmental Protection Agency.


    • CommentAuthorsteveleigh
    • CommentTimeJun 3rd 2007 edited
    • CommentAuthorbiffvernon
    • CommentTimeJun 3rd 2007 edited
    Posted By: Paul in Montreal Radon decays to Polonium, which is also an alpha emitter. These Polonium particles lodge in the lungs where they cause a lot of damage due to the alpha particles being emitted inside the body.
    Hmm, not quite I think. According to Wikipedia the full decay series of 238U which produces natural radon is as follows (with half-lives):

    238U (4.5 x 109 yr), 234Th (24.1 days), 234Pa (1.18 min), 234U (250,000 yr), 230Th (75,000 yr), 226Ra (1,600 yr), 222Rn (3.82 days), 218Po (3.1 min), 214Pb (26.8 min), 214Bi (19.7 min), 214Po (164 µs), 210Pb (22.3 yr), 210Bi (5.01 days), 210Po (138 days), 206Pb (stable).

    But more importantly radon is a gas. When it decays it doesn't spontaniously clump together to form polonium particles.

    But anyway, humans have not evolved to live in air-tight houses. Leave your windows open. Enjoy the fresh air. Draughts are good :)
    • CommentAuthorTerry
    • CommentTimeJun 3rd 2007
    But anyway, humans have not evolved to live in air-tight houses.</blockquote>

    Agreed Biff, however an airtight house does not mean you cant open a window or have fresh air(via MVHR), it just means you have more control over heat loss.
    In NZ, Kiln dried timber for framing is between 10-14% moisture content and air dried timber must have below 18% before you can line the interior (this is tested at prelining building inspection). I am not sure what RH the testers are calabrated too?
    Posted By: steveleighTo put it another way what would be the average moisture content of a piece of softwood in an atmosphere of 40-50%RH?

    Don't forget you're comparing apples and oranges here. Relative humidity as a percentage is air is quite different than the percentage humidity of wood as the saturation figures are completely different as one is a gas and one is a solid. I don't know how to calculate what the moisture content of such a piece of wood would be in an atmosphere of that humidity. Also, don't forget, relative humidity percentages change radically with temperature as the saturation vapour pressure increases significantly the warmer the air - leading to lower RH%. Here in Montreal, air at 50% RH in winter is pretty dry whereas 50% RH at 35C is very uncomfortable.

    Posted By: biffvernonBut more importantly radon is a gas. When it decays it doesn't spontaniously clump together to form polonium particles.

    Erm, yes it does. A quick google search on polonium and radon brings up a lot of articles which talk about the damage polonium particles do to the lungs they get embedded in - though a polonium appears to be quite concentrated, relatively, in tobacco ... and of course exacerbates the damage that nicotine does.

    Paul in Montreal
    • CommentAuthorsteveleigh
    • CommentTimeJul 17th 2007 edited
    • CommentAuthortony
    • CommentTimeJul 20th 2007
    No I don't think so, Steve All sounds too good to be true and I am sure it is. Problems apart.
    • CommentAuthorsteveleigh
    • CommentTimeJul 20th 2007

    RoofKrete are fully involved with the Building Control Departments of three local authorities because of the LANTAC certification to building regulations. Building Control are extremely cautious about LANTAC certification because of the Multifoil debacle. They know the technical capabilities of the product over many years. Therefore no technical problems are known. It is a simple cladding system which is applied over insulation. There is nothing complicated about it. It does the business. The building officers involved have agreed that the figures can be achieved and Passive House standard is achievable with this system. In fact they were the ones who asked for this cladding system before ASMET (which is on-going) 'because it is needed'. My goal as a RoofKrete contractor is to get the first council house to passive house standard with a MVHR fitted. Please can you ellaborate on you reasons for not accepting the figures? Thanks
    • CommentAuthortony
    • CommentTimeJul 20th 2007
    1 Workmanship

    2 100% air tight is not possible for houses.

    3 £70 per square meter is high price to pay.

    4 breathability, condensation risks and the analogy with cold stores is dodgy as these reverse problems to houses.

    5 aesthetics.
    • CommentAuthorsteveleigh
    • CommentTimeJul 21st 2007
    Thanks Tony. Here are my answers. Let me know what you think.

    (1) Workmanship

    The system can be taught to unskilled people in two days.

    (2) 100% airtight is not possible for houses

    Maybe not but we can aim for this target. We are very good at making roofs permanently watertight with maintainable seals. So we will apply this experience to provide airtight houses via sealed cladding on insulation.

    (3) £70 per square metre is high price to pay.

    This price includes insulation and a MVHR system. The savings in building maintenance would be very substantial and would very quickly become cost positive for councils and housing associations. But of course the price would reduce with quantity.

    (4) Breathability, condensation risks and the analogy with cold stores is dodgy as these reverse problems to houses.

    Indoor moisture is fully controlled with a MVHR system and this should eliminate any problems with condensation. The insulation people think that a sealed airtight system on the exterior of buildings would be more in line with Cold Store performance standards because the absence of thermal bridging and outside air infiltration makes insulation super-efficient. Present use of insulation in house wall buildup is a tremendous waste of up to 70% of the insulations performance capabilities and has spawned new industries manufacturing air and vapour membranes and thus increasing the use of petrochemicals and energy.

    If saving resources in combination with reducing carbon emmisions can be achieved then that is the ultimate goal if it can be made cost effective.

    I think this system can save resources by reducing building maintenance and replacement and also the councils like the idea of homes without the burden of maintaining central heating systems, gutters, fascia boards, soffits, and roofs. Also the improvement of indoor air quality is important for long term health.

    (5) Aesthetics

    Any covering can be fixed onto the system because fixing lugs can be formed into the surface without compromising the airtight integrity. Wooden cladding can be fixed to walls etc and slate battens can be fixed to roofs. All the nice bits can be added after the airtight system is installed. Or just simply leave it as a rendered finish.

    cheers Steve
    Hi Steveleigh, I am interested to hear how you can create an airtight seal between external cladding and adjacent insulation. What cladding material would you be proposing?
    • CommentAuthorsteveleigh
    • CommentTimeJul 22nd 2007
    The material we are using for the cladding is the airtight membrane. It is simply RoofKrete Waterproofing Membrane which we presently install on top of insulation as a trafficable ‘warm roof’. A typical buildup for a flat/terrace roof would be, starting from bottom, plaster board, joists, 18mm timber decking, vapour barrier, 120mm eps insulation with 6mm cover-board and finally RoofKrete Waterproofing Membrane. RoofKrete has been installed on individual walls for many years as a waterproof protection but it was never used for totally encapsulating a building because of the sealing affect that an impervious airtight skin would have on indoor moisture levels . Times have changed and the management of moisture is now very effective through a MVHR system.

    The main advance on existing thermal cladding systems is the fact that it can replace PVC gutters fascia boards and soffits and can be formed into a slate or tile roof which is indistinguishable from the real thing. It is very tough, flexible and passes the BRE impact resistance test (http://www.roofkrete.com/html/impact_and_heat_.html). If damaged it can be easily repaired back to its original state.

    Mike I would appreciate any help on types of insulation. RoofKrete are not getting involved with the insulation specification they are leaving that to individual contractors and clients to source. I am meeting with Knauf insulation on Tuesday morning and I'm trying to pin them down on a thickness for this sealed system - their computers will continue to say no! when confronted with anything outside the expected thermal bridge and air filtration allowance which is normal for building walls and roofs. That is why a cold store spec is suggested as being more in line with this system because they are sealed airtight and have no thermal bridges.

    Cold store insulation has to cope with a large temperature differential between internal (-40 degrees) and external (approx 20 degrees). Cold store insulation has a maximum thickness for PIR, EPS and PU of 200 mm. Therefore, is the passive house standard of 400mm wasting 50% of insulation on airtight structures with no thermal bridging? If 200mm is the optimum thickness for such a large temperature differential. How do we translate this to an airtight building envelope where the temperature differential is far lower? Any ideas please?

    There is a pdf draft document (http://www.sustainconstruction.com/uploads/pdfs/RoofkreteTC.pdf). Have a look and would appreciate your thoughts, ideas etc.

    Cheers Steve
    • CommentAuthortony
    • CommentTimeJul 22nd 2007
    The stainless steel ties are thermal bridges and seem to go throug the vapour barrier. The whole idea of sealing the insulation internally and externally does not seem sensible or workable in the real world. One tiny hole or miss or non joined up edge or damage (all of which will occur) will allow moisture to 'pump' into the insulation causing untold problems, rot, damp less effective insulation to mention a few. The idea that MHVR will stop this is wrong it wont stop it. Most insulation systems rely on one side being breathable hence the common use of modern white roofing sarkings and breather layers. To leave the choice of insulation to someone else is unbelievable. If you have been involved in flat roofs you will know that their ultimate fate is always to leek and have no doubt seen plenty of sodden insulation. Cold store owners pay for the energy used to keep the goods frozen and clearly should be insulating better than they are. As I mentioned before they need to keep moisture out and I'm pretty sure that lots of times it will be getting in -- most easily noticed when / if the plant is overhauled and water keeps seeping out of unexpected places.
    Steve Leigh,

    With your complete encapsulation system, what happens around windows?
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