Insulating and venting new metal roof
Looking at the archive for metal roofs, it appears that many recommend allowing a 1″ or more air gap to vent the roof to allow condensation and excess heat to escape. However I wonder if the air gap would invite condesation under the roof on cold evenings (when roof metal roof cools faster than air beneath it). I live in northern Illinois, so it would be good to vent the roof in the summer, but wonder about frost (underneath) and such in the winter.
I was also thinking it would be a good idea to break the thermal bridging across sheathing and nails by laying an inch of rigid poly on top of the sheathing, then insulate below (approx R-36 to get a total of 42 not counting any R of the plywood sheathing..maybe flash and batt to get good seal). Goog or bad idea? And if I do use rigid, should it be foil faced… the actual effectiveness of a radiant barriers seems mixed, but should I use foil faced poly?
Thanks.
Jeff
Replies
Condensation and frost under steel roof.
About 200 times a year a warm wet wind arrives and condenses on all surfaces that are below dew point, this leads to condensation, melted frost and ice dripping off steel roofs. At other times with homes that do not have insulated, air tight and water vapor proof ceilings it happens almost every night.
There really is no point in venting a roof as the passing wind pulls the air from within the attic and from inside the heated comfort zone adding to the cost of heating and cooling your home. An air tight roof is best.
On sunny days, at any time of year a steel roof can get very hot and this will radiate heat downwards as well as upwards. A white, silver or light coloured roof does reflect more radiation back into space. A reflective barrier, foil covered polystyrene or similar, will cause more radiation to reflect back upwards into the roof.
Insulation, is only air bubbles trapped inside a plastic sheet. This will hold the heat back for a time, but it will soon become as hot as the roof. The only thing that works is external shade. Plant some tall trees.
A lot of heat will be conducted through the frame of the building, having a building made of brick, concrete or stone does help as these materials can absorbe a lot of heat and they work well, as long as they can release their heat overnight. A clay based tile is probably the best type of roof, next is slate.
You can loose a lot of heat by having a polystyrene lining under the ceiling joists and on the room side of the frame, cover this with drywall.
Uh, a ventillated roof IS "external shade", for all intents and purposes.
But, this is a metal roof
Yes a floating clay tiled roof does provide shade, but in this instance we are writing about a metal roof that, while providing shade....will also pass most of its heat into the frame and space below by conduction and radiation and some by convection.
A clay tiled roof plus 8 inches of polystyrene sheet (more is better) takes longer to warm up, and thereby offers a longer period in the morning before the heat gets into the home.