Building Science Deep Retrofit Detail
Feel free to challenge me on this subject if you wish. I was reading the debate and discussion attached to one of Martin Holladays blog entries about ‘Deep Retrofit’ details. In the discussion following Martin Holladays blog entry, John Brooks asked a very simple question – where do you define the drainage plane on the Building Science Corporations detail? Well, I agree with BSC’s initial response. The piece referred to as ‘metal flashing attached to furring strips’ does not need to do anymore than is shown in the BSC’s detail. It’s purpose is to shed out any condensation that may form on the back of the siding material. I remember when in Ireland we began to place polystyrene (and later polyiso) insulation batts into our masonry cavity wall construction. All of the young architects is school were asked to swallow a complete mis-interpretation of the cavity wall construction. We were all asked to draw our details showing the DPC (damp proof course) over window lintels, going from the inner wall leaf to the outer wall leaf. This meant that with insulation in the 4-inch cavity, the architectural draftsman or woman was asked to show the DPC cutting through the cavity insulation. We were all told at architectural school this was to transfer water from the inner leaf of the wall construction to the outer leaf. Beep! Problems.
As one architect who worked in an actual construction company later informed me, if you get water on the inner leaf of your construction, you might as well give up. Because you wall is finished anyway – you have no way back. He informed me, this fairy tale about transfer of moisture from the inner leaf to the outer leaf was nonsense. His architectural details over windows always showed a proprietary cavity tray installed on the outer leaf of the cavity wall. Its purpose he told me was to shed out any water which may condense on the inner face of the outer leaf of the construction. His analogy always was to imagine a water fall, on the inner face of the outer leaf. He informed me, this exaggeration was only that, an exaggeration. He told me, for 90% of days in the year, and depending on the orientation of the wall, it might be bone dry and therefore the cavity tray is redundant. But why go around the building site, and have different rules for different walls. It would only confuse the contractor, and would be impossible to manage.
So you can take it, that in 95% of situations the moisture that forms on the inner surface of the outer layer of construction can be taken care of by ventilation and evaporation. It is just that unlucky 5% of cases, where a river of water can actually occur – on certain days of the year – when the air is saturated with moisture, and over the cold nights the air sheds all of its water content on the inner surface of the outer layer. Nowhere in the above, do we need to consider this notion of water on the inner layer needing to be transfered via weepholes or anything, to the outside. As my friend from the building company told me, if you have water on your inner leaf, inside the insulation, the wall is already destroyed and there is no come back from that. He also told me, it is useful to have wings, or dams to close the ends of your cavity trays. Because, you can stop the water travelling down the back side of the outer layer, doesn’t mean that water if sufficient in quantity (that 5.0% case of the ‘river’), cannot find its way horizontally into someplace else. So the stop-ends, or dams at the end of the cavity tray are necessary to prevent that horizontal escape of the water.
But as architects and designers we all have to get away from this notion of transference of water from an inner to an outer leaf. In real life, that simply isn’t the way to go. I think the Building Science Corporation have figured that one out already – fair play to them. But the problem is, the poor training about water management has been taught to successive generations of architects. So that false science is lodged in the system, and you meet it time and time again.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part