Metal roof and condensation
Okay. This is driving me crazy and I can’t seem to find an answer no matter how hard I look. Is there anyone with a background in building science that could help?
Back story. I recently purchased a standing seam metal roof in 24G galvalume panels 1.5 inch seams, to go on a residential house with a “normal” attic. House has drywall on the ceiling and kraft paper beyond that on the attic side, on top of that is 16-24 inches of fibre glass batt insulation (2 layers) and then a normal attic air space before the joists and sheeting. For the roof, (after much research and talking to several contractors) the best idea seemed a recommendation to go with Gaf deck armour on top of existing 1/2 inch OSB seathing then apply the 10mm thick rain screen, then the metal panels–this is what we did. The roofer recommended the rainscreen to provide a channel to have the roof dry in case of condensation. The roof is vented in the attic with ridge vents and at the soffits with soffit vents. The roof was screwed down every 12 inches with clips through the mesh and underlay, and no sealant was used on the screws that were used to screw down the sheathing.
My question, I live in the pacific northwest where it is very humid and cold and rainy, and I woke up several mornings this Autumn to find the TOP surface of the roof literally drenched with condensation, or in one case frost. However, does this mean that the bottom side of the steel panels is also “drenched” with condensation because of the air cavity or the nature of metal? I have no way of seeing into this cavity between the steel panels and the 1/4 inch mesh underlay, but I am dying to know if there is any significant condensation in there? Or does this roof behave like a normal asphalt shingle roof–with condensation on the surface facing the atmosphere only? Does warm/moist air have to migrate from my living space to the underside of the metal panels to cause condensation? Or is this a phenomenon that will happen regardless of how well my living space is sealed when the dewpoint is reached outside and the attic air is hitting the underside of the seathing and working up towards the metal?
PS. I have read the thread on here where it was suggested that a metal roof not be laid on cedar breather (which is essentially what I used) because of oil canning concerns. This info appears to me to definitely be FALSE. If anything, laying the metal panels on the cedar breather naturally crowns the panel and there is very little oil canning at all. It provides a sort of “cushion” for the panels so that any imperfections in the roof sheathing are not noticeable at all in the panel. This was also told to me by the roofer who said that the cedar breather (10mm rainscreen) helps a lot with oil canning. Even with 18 inch wide panels and two very small strengthening ribs running the length of the panel, there is very little (almost none) oil canning on my roof. Just to add to this, several different people worked on my roof, and there was one particular person who laid the panels quite tight in one area and these panels are very slightly more crowned than the other ones–they literally bow a bit when I walk on them and step off–it’s a nice effect. Laying the panels quite tight with a fastner every 12 inches, and using a mesh underlay with a heavier guage of steel (24G) seems to be a good way to go. I also think the screws need to be driven in at a slight angle at the top of the panel (by the hips and ridges) to help with crowning the panel lengthwise. In areas where the screws were driven flat/hard at the hips to secure the panels, some oil canning exists.
In any event, was my roof done right? Will there be a lot of condensation under the metal panels??? It seemed to me that allowing the metal to breathe underneath was a good idea–hence the rainscreen and breathable underlay. Won’t this allow some movement of air and drying toward the attic side and between the mesh and the Gaf underlay? Some have said the mesh won’t allow significant movement of air, but doesn’t air move from low to high pressure? So when the wind picks up or air conditions change, or the metal heats up from the sun, wouldn’t this suck the moisture out from under the panels through ANY available gap no matter how small?
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