Debating All Aspects of Passive Houses - And May the Best Argument Win!
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I am currently working on a rather challenging retrofit project involving both interior (cavity) insulation in the walls and vaulted ceilings and exterior foam insulation. As per instructions, I am using exterior dimensions to determine the areas of walls, roof, floor slab, etc. Trouble is, when I add (or thicken) exterior insulation (on the roof, for example) other areas (walls in this case) increase, therefore their associated heat loss. As a result, the exterior foam does NOTHING. In some cases, it RAISES the heating load.
I do understand why external dimensions are used (to overcompensate on heat loss through each area, thereby avoiding extra thermal bridge calculations at every area junction) but if the building is adequately modeled with the external dimensions and exterior foam is then added, how can the exterior foam POSSIBLY lead to any extra heat loss? The effect is that I can help the heating load and make this building work with extra rafter insulation (which there's no room for because the ceilings are vaulted,) but nothing on the outside does much - COMPLETELY COUNTERINTUITIVE, IM(H)O!!!
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Graham,
You say "but if the building is adequately modeled with the external dimensions and exterior foam is then added" I trust that in both cases, internal and external retrofit, you are measuring your external dims in each case i.e. remeasuring after the foam has been added (from other discussions I'm pretty sure that you have - just want to make sure).
Geometry: Addressing the coffers could have a profound impact upon heat loss - but I suspect that it is not so much the foam thickness that is the issue but the wall height - Say the height of the external wall internal-retrofit, using external dims, is say 2.7m (0.3m floor, 2.1m wall [floor to u/s ceiling], 0.3m roof) but for the external-retrofit, due to the coffer is say is say 3.6m (0.3m floor, 3.0m wall [floor to top of old roof], 0.3m roof) then it is not surprising that the additional 0.9m of perimeter wall heat loss area will offset many of your moves undertaken to improve the roof especially if the wall is of a lesser U-value than the roof.
Insulation thickness and geometry: There is almost certainly a diminishing return on the insulation surface area vs. insulation issue - a friend of mine did some analysis of "Thick" and "Thin" insulation just to assess the geometric impact that insulation of varying performance can have the findings were quite measurable. I guess that this limit state could be encountered when you have very low U-values and some particular surface area characteristics - PassivHaus retrofit of a vaulted roof perhaps..... Perhaps this is a case where "mining" thermal bridge for energy benefits will be of particular value.
Hope these thoughts help,
Mark
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YES, the thermal bridge calc is the answer!!!
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