To maintain airflow, leave a space of 38 to 50 mm (1 1/2 to 2 in.) between the top of the insulation and the underside of the roof sheathing.
The recommended space between the roof and insulation varies by material. Fibreglass or mineral wool typically requires a 50mm gap for ventilation, while foil insulation needs about 25mm. Spray foam doesn't need any gap as it forms a sealed barrier.
If the insulation is installed properly, there should be a 2" air space above the insulation to the sheathing. In all practical purposes, this is should be easy to do if the rafters are 10" deep and the insulation is only 8". If that is the case, the random touching of the insulation buckling up will not matter.
Building scientists recommend, and building codes require, that the insulation installed between the rafters must be in direct contact with the underside of the roof sheathing. An air gap between the sheathing and the fluffy insulation (or the spray foam insulation) is not permitted.
Not less than a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.
To maintain airflow, leave a space of 38 to 50 mm (1 1/2 to 2 in.) between the top of the insulation and the underside of the roof sheathing.
This should be confirmed in the membrane manufacturer's BBA certificate, though 25mm to 50mm is typical.
Remember that a two-inch breathing space between the insulation and the roof sheathing must be included to allow for ventilation. If I leave room, it's hard to fit more than R-13 without going to rigid foam board.
In roofs, the air gap should be larger, typically around 50mm (2 inches), to allow for proper ventilation and prevent condensation. It's important to note that some insulation materials, such as spray foam, do not require an air gap.
You can put all the insulation on top of the roof deck, all the insulation on the underside of the roof deck, or some on top and some below. You can put it all below the roof deck but use two different kinds. Your insulated roof can be a cathedral ceiling or it can be above a conditioned attic.
You definitely don't want any insulation to touch the roof membrane as it'll pull water inside. Netting shouldn't be necessary - if the insulation doesn't touch when it is installed it'll only settle in time and won't touch later.
A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel.
The sheathing should be spaced 1/8” apart along the edge and end joints for expansion and contraction. Proper nailing and nail spacing will go a long way for a more aesthetically pleasing finished roof.
The recommended maximum gap between roof sheathing boards for low-quality, 3-tab shingles is 1/4-inch. If asphalt shingles are installed over sheathing with gaps exceeding 1/4-inch, over time, these gaps may telegraph through and become visible.
Yes it can touch. As long as there is nothing obstructing flow from the soffits - which sounds like the baffles are.
Ideally you want about 3 to 6 inches of space at the top and 3 to 6 inches of space at the bottom.
You can't simply leave a gap between your insulation and the surface and call it a day. This is where sizing comes in, and it varies depending on the material used and where it's installed. It's recommended to leave an air gap insulation of at least 25mm or 1 inch for the walls and ceilings.
The general advice we give is that you should always insulate between and above the rafters (warm roof) or between and under the rafters (cold roof).
A universally accepted guideline for roof ventilation is the 1/150 ventilation rule. According to this rule, a roof must have ventilation that is at least one square foot for every 150 square feet of attic or crawl space. A minimum of 13.3 square feet of ventilation is needed for a 2,000-square-foot attic.
Depending on the specifics of the roof, it may be possible to fully fill the rafter depth or, in most cases, leave a 25mm or 50mm air gap between the top face of the insulation and the sarking membrane.
(Source: NRCA.) In the event you are erecting a “cold building” (such as a refrigeration building) that stays at 32 F (0 C) degrees or less inside, you will need a vapour barrier on the outside of the insultation to prevent warm exterior air from entering and potentially damaging the roof's insulation. (Source: NRCA.)
Yes, too much loft insulation can cause dampness in some situations. Loft insulation works by trapping warm air inside a building, preventing it from escaping through the roof. This helps to keep the interior of the building warmer in cold weather, reducing the need for heating and saving energy.
Insulation is often laid in contact with the roofing felt. Where the felt is impervious any contact condensation will run down the felt and make the insulation damp.
But it's not the only important player that helps keep you comfortable throughout the changing seasons. In addition to having proper roofing insulation, your attic space must also have an effective roof ventilation system that maintains a flow of ambient air throughout.
Never pull your roof membrane under tiles too tight
The ideal measurement of this will sit between a minimum of 10mm and a maximum 15mm. If the membrane is taught, there is risk of tearing and the water could become trapped behind the tiling batten.