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0 pointsmannykannot4y ago0 comments

This is going off-topic somewhat, but I am thoroughly confused by the article you link to. The author insists we should put the rain control layer inside of the insulation, even on roofs.

He does not go into much detail on roofs, but in his wall examples, the outer cladding and the drained cavity behind it appear to be a de-facto rain control barrier outside of the insulation layer (which could be, he says, rock wool or glass fiber, which are surely a big problem if they get wet, right?) If this is so, then what does it mean to say the insulation is outside of the rain control barrier? Furthermore, if we combine these diagrams with the one showing a roof-wall junction, there does not seem to be any barrier against rainwater reaching the wall insulation that way, and I cannot see how this is not a problem.

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throw0101a4y ago· 2 in thread

> If this is so, then what does it mean to say the insulation is outside of the rain control barrier?

Yes, the cladding is acting as a bulk water control layer, but there is still moisture in the air from humidity.

* In a cold climate the warmer/moister inside air will at some point reach the edge of the building: by having the insulation on the outside of the control layer(s), the air will be kept warm, and so there won't be a cold surface for it condense on. If the insulation was on the inside then the control layer would be cold, where the air would condense, possibly causing mold growth.

* In a hot/humid climate the humid outside may get by the insulation and hit the cooler control layer and condense, but will just roll off the control layer outside of the building (and not cause mold inside).

> Furthermore, if we combine these diagrams with the one showing a roof-wall junction, there does not seem to be any barrier against rainwater reaching the wall insulation that way.

This is probably regarding Figure 5. All the various layers are connected to each other, so what's on the inside cannot get out, and what's on the outside cannot get in.

The black line (air/vapour layers) is continuous up the wall, around the corner, and onto the roof. The light blue (insulation) goes continuous up the wall, around the corner, and onto the roof. And the cladding is also continuous so that bulk water and UV rays are also blocked continuously.

See related articles:

> RR-0410: Vapor Barriers and Wall Design

* https://www.buildingscience.com/documents/reports/rr-0410-va...

> BSD-106: Understanding Vapor Barriers

* https://www.buildingscience.com/documents/digests/bsd-106-un...

mannykannotOP4y ago

That is what I originally thought it might mean, but the article uses the term "rain control barrier", and has a separate item for vapor control.

With regard to their relative placement, see, for example, the caption to figure 2:

"Figure 2: "The Perfect Roof"—The perfect roof is sometime referred to as an “inverted roof” since the rainwater control layer is under the insulation and ballast." [my emphasis.]

The author also writes, in the caption to fig. 1: "The claddings function is principally to act as an ultra-violet screen", thus seeming to differentiate it from any of the control layers he listed at the start of the article.

In addition, the author also lists a separate item for air control, but later writes "What about this air control thing? Well air can carry a lot of water and water is bad for the structure" - further muddying the issue, as now he seems to be discussing water vapor.

I guess this could all be attributed to being sloppy about definitions, to the point of being unambiguously wrong when it comes to roofs.

throw0101a4y ago

> The author also writes, in the caption to fig. 1: "The claddings function is principally to act as an ultra-violet screen", thus seeming to differentiate it from any of the control layers he listed at the start of the article.

The cladding can not stop any water and, if the control layers beneath the insulation are properly installed, it won't make any difference. The fact that the cladding does help in blocking rain is just a nice bonus to its primary function.

In some locations it may be difficult for the cladding to do this: think of any place very foggy. There's no way to make the cladding air tight, so high humidity (100%) air will get behind it (stopped by the c. layers). Or if you have shingles on your roof that leak (or get blown off in a storm), they can still block UV, but there will be dripping. And that's why you want the control layer(s) further in.

Water can destroy a structure very quickly if you don't allow for easy/quick drying. Defense in depth.

> In addition, the author also lists a separate item for air control, but later writes "What about this air control thing? Well air can carry a lot of water and water is bad for the structure" - further muddying the issue, as now he seems to be discussing water vapor.

At the top of the article he lists the control layers in order of importance. Controlling for air is more important than control for vapour, as this video explains:

* https://www.youtube.com/watch?v=FXXgjvOJcYI

It's just if you control for air you also just happen to do a lot of controlling for vapour. There's can be a lot of moisture in air, i.e., humidity.

Further there can be some subtleties with moisture/vapour: in many case you want to stop any flow either in or out of the building; in other cases you want to control air but let moisture out:

* https://www.youtube.com/watch?v=zhkGcklWB_Q

* https://www.buildingscience.com/documents/building-science-i...

* https://www.buildingscience.com/documents/insights/bsi-077-c...

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