Debating All Aspects of Passive Houses - And May the Best Argument Win!
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Does PH acknowledge the effect of convective loops within wall cavities when air permeable insulation (fiberglass batts) is used? If so, how is the difference accounted for?
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Whether you have these loops does not only depend on the material, but predominantly on how it is fitted. PHPP assumes insulation to be installed correctly, i.e. tightly fitted without circulation around batts; for the certification someone needs to assume responsibility for this and sign it off. Correctly installed, there shouldn't be any loops, insulation being air permeable or not (assuming it is actual insulation material that has been tested to work otherwise, and not collected locks of your pet's hair or grandma's destructed PJs - trust me, people install those things!).
Mark is the prime authority on loops and insulation, maybe he likes to chip in?
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As Kerstin says PHPP assumes that you have installed suitable matterials in a professional manner with a high standard of workmanship. I don't know that I can live up to the title of being an authority in anything but from my digging around on the subject it is obvious that thermal bypass mechanisms can have a significant impact upon performance. For instance recent research by Leeds Metropolitan University has found that homes in the UK are failing to perform by 40-70%! There is even one instance of a semi-detached home built to comply with current 2006 Building Regs (well that's what was hoped) and it performed like a 1950's semi! Whilst such radical examples are a result of systematic problems with both the design and delivery of a building a large proportion of this failure is due to poorly installed insulation. Learning the lessons it seems that there appears to be at least four key concepts that good insulation will adhere to:
1) Create, support and protect an Air barrier (warm side)
2) Create, support and protect a Wind barrier (cold side)
3) Use the above to Encapsulate the insulation (use full fill techniques to ensure that there are no gaps in front or behind the insulation)
4) Use multiple layers of insulation, with staggered joints, to ensure that there are joints running though out the depth of the insulation, or use suitable, carefully specified, blown insulation.
Really this is where experience, knowledge and the auditing of drawings, specifications and workmanship can play a strong role.
I hope this is illustrative.
Mark
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Just found out about a great way of publishing stuff online (www.scribd.com.) So I've taken the opportunity to upload the product of my research and digging around on the subject of thermal bypass. Constructive criticism would be appreciated.
http://www.scribd.com/doc/17039330/Ther … erformance
The article was first published in Green Building Magasine http://www.greenbuildingmagazine.co.uk
Also I was thinking that it would be good if people could upload photo's showing examples of good and bad insulation installation. The broader the variety of site locations, construction types and details the better. The idea is that they will help to form a freely accessible catalog of what to do and what not to do. Photos could be of timber frame and masonry, good and bad installation. The more the merrier. Examples of the kinds of things that would be useful include:
* Gaps between batts
* Compression (“tucked in” insulation)
* Electrical cables squashing the insulation
* Gaps in insulation due to services installations
* Slumped insulation (partial fill cavity walls)
* Perfectly installed insulation no gaps (or layered staggered joints), encapsulated (supported both sides)
* Thermographic images could also assist the development of this image library
Details that are worthy of consideration include, but are not limited to Walls, floors (including perimeter edge), roofs, corners, eaves, gables and window reveals. The more examples that can be collated the better. Please post them here, on this thread, or email to me at m.siddall (at) devereux.co.uk.
Hopefully this will serve to put discussion into pictures. Images could be uploaded onto www.flickr.com with a link pasted through to this site.
Cheers,
Mark
Last edited by Mark S (Sat, 04/07/2009 21:30:53)
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Thanks for the info. I am fairly familiar with the effects of poor insulation installation and the need for quality work, but are you saying that a wall that is "properly" insulated (full filled, air sealed) with, say, fiberglass batts will exhibit NO convection loops, despite the fact that the insulation itself is air permeable?
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It depends upon the temperature gradient and the height of the insulation - to allow stack effect and looping to develop. As noted in the article CFD simulation for insulation at a depth of 0.5m suggests that, in attics, convection within the insulation will not occur until temps fall below -40C when the density is 30 kg/m3 for rockwool and 15 to 18 kg/m3 for glass wool [Ciucasu 2005]. Thus roofs are fairly safe from convection. I have not found any similar information/studies discussing convection within fully filled wall units consequently the the temperature gradient that would result in convection must be considered unknown [to me at least.] That this does not seem to have been studies surprises me.
Mark
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Yes, and walls tend to be a good deal taller than 0.5m, yes?
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Exactly.
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Aye, there's the rub...
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For full download, rather than read-only, the publisher has now ensured that for a small fee the article can be purchased online:
http://www.greenbuildingpress.co.uk/pro … tem_id=174
Mark
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