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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Building Regulations and Standards: Radiation heat losses

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    • CommentAuthortony
    • CommentTimeFeb 17th 2008
     
    Are radiation heat losses correctly modeled in the hot box test?

    I dont think so. The hot box is a double totally enclosed environment. Radiation heat losses from say flat roofs are sent to outer space some of the time where there is no incoming radiation -- at night. Therefore the heat loss will be a lot higher than the hot box suggests

    Windows too loose heat by radiation and I think that in the cold side of the hot box the radiation losses will be far less than in the real world for the reason outlined above but to a lesser extent as the window is oriented such that it can "see" some terrestrial things. Roof windows do not benefit from that factor though so will be significantly less efficient than predicted. Large areas of glass suffer similarly.

    This argument effects U values and these are the building blocks of nearly all energy predictive models --- do we have a problem?
    • CommentAuthorAlbert
    • CommentTimeFeb 17th 2008
     
    Isn't this the thrust of the argument from the proponents of thin film insulation (Tri-Iso etc.)? I was hoping that thread had died a natural.

    My not very scientific feeling is that where the real-world insulation is installed with a bit of a gap between it and the outer surface of the structure it probably won't have much effect. This is because, even if it is radiation frosted, the outer surface will be a lot warmer that outer space, so the radiation transfer between insulation and it will be relatively small.

    For warm roofs I'd expect more of an effect. As for windows I believe that they should have external shutters and thick curtains (preferably insulated inner curtains) anyway. As you said about some of the Grand Designs greenhouses, large areas of unprotected glass are pretty daft. (Sorry about the paraphrasing.)

    Albert:confused:

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