You have no way to achieve this without a major remodel.
Roof purlin metal building air gap.
Research has confirmed that creating an air space under a metal roofing system will increase energy savings during both summer and winter months.
But probably the best benefit that comes from using purlins is the air gap it creates between the metal roof and the existing shingle roof.
Metal roof purlins are installed horizontally under metal roofs.
Warm air holds more moisture than cold air.
Warm wet air rises and meets the cold roof and cools.
The colder the climate the more essential the air gap.
If i had to choose between a 1 2 inch gap and a 1 inch gap i would be inclined to go with the 1 inch gap if i were building in climate zone 5 or anywhere colder.
They are installed on top of the roof rafters with a felt underlayment or vapor barrier installed on top.
This lets any condensation on the underside of the metal roof evaporate.
Metal roof coatings with solar reflectance can help building owners save substantially in annual cooling costs.
Ventilation air follows exactly the opposite path.
You would have to seal the ridge vent it isn t working anyhow because your building does not have an air intake from enclosed vented soffits.
If you do not completely fill purlin cavities code requires airflow from eave to ridge over top of the insulation.
Warm air is actually lighter than cold air known as avogadro s law essentially condensation occurs at night when the temperature inside a building is warmer than outside.
Rising along the underside of the angled purlin until it reaches a gap then rising up to the next purlin and so on to exit at the ridge or gable.
They give added support to the roof and also provide a nailing surface for the end panels and drip edge.
Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels.
Getting a perfectly sealed vapor barrier under purlins would be nearly impossible to achieve.
Do all roofing projects require purlins.
You don t even want to go there.
In the case of sips it s essential in my view to have an air channel between the roofing underlayment and the metal roofing.
The air space allows natural ventilation when there is a vent at the low part of the roof structure and another at the high part of it.
The metal will be attached to at least 3 4 purlin and 1 2 plywood instead of just 1 2 plywood which should make the overall roofing system stronger.
Purlins are 2 by 4 feet and are installed much like metal roofing.
On a sloped metal roof there is a vent at the bottom to allow air in and natural convection takes air at the top out notes wingfield.
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