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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorMike George
    • CommentTimeFeb 2nd 2007 edited
    I went to a very good AECB event yesterday where the focus was the consideration of new silver standard details. One of the ways in which better thermal performabnce is achieved is by having a high level of air tightness. In theory, fully filling a masonry cavity wall will eliminate so called 'thermal bypass' which is the circulation of air around and in between poorly fitted partial-fill rigid boards.

    One concern with this, is the possibility of water ingress into the insulation through the outer leaf of brickwork/blockwork.

    The counter-argument is that as cavity widths increase to say 150mm or more, the proportion of insulation which may get wet, will be less and can be considered sacrificial.

    What do we think?
    • CommentAuthortony
    • CommentTimeFeb 2nd 2007
    What is the outer skin? In areas of severe exposure it is not a good idea to use brick or other pointed masonry. I very much like slate hanging in these situations.
    The idea is that it doesn't matter whether the outer skin is brick, rendered block or slate - the first little bit of insulation is said to be sacrificial. Intuitively, I don't like this, but I have never used full-filll in my life for fear of it getting wet. Easier to change your mind on these things if others show confidence that it is okay.
    • CommentAuthortony
    • CommentTimeFeb 4th 2007
    I have seen inside literally thousands of cavities and never seen any wet in any of them. I live in the south east and have no hesitation about fully filling cavities. The warmth from a house causes moisture to be driven out through the construction and it is eventually blown away from the surface of the wall. The cavity is cosy slightly warm dry place.

    A problem can arise if masonry becomes wet as when the water evaporates it takes with it a lot of heat, cooling the house (the evaporative refrigerator effect). The question can now be asked, "is it green to build with porous materials on outside walls?"
    Yes it can, and yes, I think it is. What you describe makes perfect sense and reflects the Victorian [and earlier] house building method. The only way to render is using lightweight low density mixes. In my area, where we have high rain exposure, the best way to render a solid wall is with porus render. Just use three coats instead of two and never paint it.
    >I have seen inside literally thousands of cavities and never seen any wet in any of them.

    Tony, I am curious to know what it is you do that has enabled this?http://www.greenbuildingforum.co.uk/newforum/extensions/Vanillacons/smilies/standard/smile.gif
    • CommentAuthortony
    • CommentTimeMar 13th 2007
    Mark I have seen lots of walls demolished and 'break throughs' for extensions, windows made larger etc.

    OK, I see where you are coming from now.:tooth:
    • CommentAuthorken davis
    • CommentTimeMar 15th 2007
    the BRE produces a guidance note on cavity fill which will probably explain all (phone me if you can't get one: 01424 752311).
    yes, you certainly do get water in cavities but it does not hang about waiting for holes in the wall to be made but will be there when its raining heavily and especially if accompanied by strong winds. ask anyone who has put on extension without a cavity tray and you will usually find that water gets in sooner or later. mineral (rock) wool is the best fill .
    however, cavity fill alone will not give the sort of u-values we should be producing for minimal energy use and you will either have to insulate outside (best because of thermal storage) or inside. do not forget to do a dew-point calculation to ensure that condensation is on the out (cold)side of the vapour barrier.
    • CommentAuthorGuest
    • CommentTimeMar 15th 2007
    "however, cavity fill alone will not give the sort of u-values we should be producing for minimal energy use "

    Depends on the insultant used and the size of the cavity, surely.
    • CommentAuthorSaint
    • CommentTimeMar 16th 2007
    Mike, should we not be asking a ourselves nowadays if we go to the expense and trouble of incorporating a cavity with all the detailing, workmanship, structural issues and potential latter day problems then why fill it?
    Cavity fill is an expression originally derived from a remedial process.
    Surely we have two choices/opportunites in new build. Internal insulation giving a rapid response to heating or external insulation with the benefits of a thermal store and minimal risk of interstitial condensation.
    • CommentAuthorSaint
    • CommentTimeMar 17th 2007
    What I actually meant to say but somehow missed out was why build a cavity at all? Just go for a solid wall with internal or external insulation!
    Hi Saint,

    I can see where you are coming from with this, and there is an argument for doing as you say. However, I'm not entirely convinced about either for the following reasons

    Internal insulation, While a rapid response may be desired by some in the winter, summer temperatures can be less comfortable.
    External insulation seems ideal but a I am concerned about the durability of comparitively high density rendering systems on low density backgrounds. I wonder if there is a real risk of cracking of materials with disproportionate thermal expansion and contraction.

    Maybe my concerns are unfounded but I mentioned this on an earlier thread about passiv-haus and no-one seemed to contradict me other than to say there are other methods than render, which is of course true.
    • CommentAuthorGuest
    • CommentTimeMar 26th 2007
    My 100 year old house has a 115mm cavity between brick skins, which was foam filled 25 years ago with no ill effects. The house is single storey with a 300mm eaves overhang. Any slappings I have carried out have revealed complete cavity fill with no signs of dampness.
    Hi guest, Can I ask if your house in an area which is severely exposed or sheltered?
    • CommentAuthorGuest
    • CommentTimeMar 28th 2007

    My house is in the countryside and the walls are exposed to driving rain.
    Thanks Guest that is good to hear. I assume you are nice and warm with all of that insulation and that your energy bills are low. Do you know what type of foam was used?
    • CommentAuthorGuest
    • CommentTimeMar 28th 2007

    Foam was Urea fomaldahyde (hopefully correctly spelt). Despite American fears that it gives off minor irritable fumes when curing I have noticed no ill effects.
    I am very interested in this. I have recently bought a 1930's semi brick house with tile cladding just below the bay windows. I would like to have cavity wall insulation to help reduce our energy usage. The site is very exposed and has an open aspect to both front and back. The back faces SouthWest which is the prevailing wind direction, and it only needs to be slightly windy elsewhere in town for it to be very windy at the house. The kitchen extension has either been built without a cavity tray, or the drains are blocked as when its wet and windy the water gets in, a good few litres from one nights rainfall. Some of the window frames also leak in the wind-driven rain.

    Due to the fact that there is obviously moisture on the inside of the cavity, I have been put off the idea of cavity wall insulation as it may defeat the purpose of the cavity as a moisture barrier and cause damp issues.

    So should I worry about the damp getting in, or will the moisture not get past the first bit of insulation?

    I don't want to use and exterior render instead as its a nice brick house and it would spoil the feel of the property.

    Thanks for any comments! :)
    • CommentAuthorGuest
    • CommentTimeApr 6th 2007
    Being a former 'surveyor' in the loose sense of the term for 2 cavity wall insualtion companies, i can tell you an awful lot of properties have been insualted that probably shouldn't have, no cavities where checked before installation. I was asked by the company i worked for to 'inspect' a property that had been insualted. The guy had taken a window out and couldnt see any insualtion around his window!!! As i reported back to my boss that i couldn't see any insulation either it was reoted back to ciga that the guy had 'disturbed' the insualtion(in the agreement) not too! so nothing was done about it!!! I have a bee in my bonnet about how many houses are not insualted properly. I have been seriously thinking about buying a boroscope and offering a service to check, i doubt most properties have been installed fully!!!
    • CommentAuthorGuest
    • CommentTimeApr 17th 2007
    Is it not still the convention to have a cavity in most roofs as well as walls for the same reason- to safeguard the building through extreme weather conditions,states of repair,and over a long lifespan? Why alter this? This approach does not limit the amount of insulaton or air tightness the building can have.
    More problematic is the possibilities for bringing the current buildng stock up to todays (and future) U value requirements.I have read that replacement of same at todays rate of build would take 1,000 years -surely not an option!
    So, what is the likely hood of an exposed, 25year old ,stone/cavity/block house experiencing damp problems caused by bridging across the infill material?
    Would some materials be more likely to allow any moisture to run down (harmlessly ?) than others?
    Has any one any experiance of using "perlite" as cavity fill in existing buildings ?

    With regards to the brick built 1930's house - does it have a cavity or is it solid?
    • CommentAuthorGuest
    • CommentTimeApr 18th 2007
    Dear last posting. If you can see how the bricks are laid. Then this can give a clue. If all the bricks are laid length ways then it is more than likely to have a cavity. If some are length ways and some end on then it is more likely to be solid. Sometimes the thickness measured at say the front door will also give a clue. Solid brick walls in 1930's were around 220mm thick wheras brick cavity walls were around 250mm. Solid stone walls tend to be even thicker say 500mm. Hope this helps.
    Regards Wessex Energy Services.
    • CommentAuthorgnewman
    • CommentTimeApr 19th 2007
    I had my cavity walls filled with Rockwool 30 years ago. The heating bills dropped but the act of blowing the Rockwool blew off plaster in three places. I have had no damp problems.
    • CommentAuthorGuest
    • CommentTimeMay 8th 2007
    We have very porous bricks just in side the limit for housebuilding. Our Garage walls were running with water, we have coated them with a water repellent liquid. We do not want to do the property in this way. We therefore assume that water is running down into our cavity and then drying out. Wouls you recommend cavity wall insulation, if so what type.
    • CommentAuthorGuest
    • CommentTimeMay 12th 2007
    PDobson (in the process of registering)
    I am building a new house in Surrey and I am interested in upping the wall and roof insulation - The construction is block & brick with a 100mm cavity & 150mm rafters (warm roof). All space in side is at a premium because the planners made us downsize. Any advice on what insulation I should use - I have looked at Tri-iso super 10 on the roof in conjunction with something like 70mm celotex but NHBC wont touch it - They are OK with Thinsulex ? apparently - anyone got experience of that? The full fill interlocking EPS system looks interesting - haven’t got a definitive answer on that from NHBC yet.
    • CommentAuthorSBourke
    • CommentTimeAug 1st 2007
    Hi, I would appreciate some impartial advice on cavity wall insulation in older properties.

    Our house was built ~1920 with porous bricks and lime mortar (was told 1 cement, 2 lime, 6 sand...). The mortar is weathered and flaky on the surface, but not recessed. There is a 3 inch cavity, unusual for the period of construction, with very solid bar ties - they have been inspected and show only superficial surface corrosion. The property is large and I have only noticed 2 bricks with cracks (both near the highest visible course). No significant problems with damp (slightly high readings in a few areas, but not enoungh to cause concern - no visible sign of damp anywhere).

    I am interested in installing cavity wall insulation and believe the polystyrene beads are better in this situation. I have had conflicting views as to whether this is sensible - concerns raised about accelerating corrosion and failure of the wall ties in particular. What would you advise?

    Many thanks,

    Newcastle upon Tyne
    • CommentTimeFeb 11th 2015
    I thought I would resurrect this thread as I plan to build brick and block with 300mm fully filled cavity with mineral wool batts like Tony has done, However I have discovered a map that shows our area (North cornwall/devon coast) is classed as exposed, and I have read somewhere of the possibility of bricks blowing their surface off due to frost in wet brickwork. It was suggested that with a narrow cavity some heat from the house stopped the frost damage as some heat still got to the outer skin but with a larger cavity full of insulation no heat got to the outside hence the possible damage.

    If I had 250mm of insulation and left a 50mm gap between insulation and outer skin how would I maintain this air gap?

    I would appreciate any experience in this field
    • CommentAuthorringi
    • CommentTimeFeb 11th 2015
    Bricks have frost ratings for a reason.

    I don't think a 50mm gap will help protect the bricks from frost, it may protect the mineral wool from a bit of dampness, but the general view seems to be that the dampness goes less then 50mm into the mineral wool anyway.
    • CommentTimeFeb 11th 2015
    Thanks ringi, what "level" of frost protection would be acceptable for this level of exposure.
    • CommentAuthorCWatters
    • CommentTimeFeb 11th 2015
    FL bricks.... F=Frost resistant. L=Low salt.

    F1 = Moderately Frost Resistant
    F2 = Frost Resistant

    I believe that's from BS EN 771-1: 2003 "Specification for Clay masonry units"

    F2 suitable for severe exposure. Low Salt helps stop them going white but doesn't seem to guarantee they won't.
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