Typically, code requires 2 inches between insulation and roof sheathing. If you have 2x8 rafters, they are 7-1/4 inches deep, subract the 2 inches and that leaves you with 5-1/4 inches allowed for insulation.
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.
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.
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.
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.
A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel.
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.
(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.
Brushing against, moving or touching the insulation can dislodge those glass fibers, allowing them to lodge into skin, eyes, and be breathed in to irritate the throat and lungs.
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.
Unless otherwise specified by the material manufacturer, or this code, siding shall have a clearance of at least 6 inches (152 mm) from grade and at least 1/2 inch (13 mm) from other adjacent surfaces (decks, roofs, slabs).
Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.
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).
Overall, the second layer of insulation plays a crucial role in improving insulation performance, reducing heat loss, and maintaining a comfortable and energy-efficient living space.
The optimal R-value for most homes is 30, but you can go as high as 38 if you live in a tropical climate. Anything higher than 38 indicates an over-insulated space. While you might think that too much insulation can't be bad, there are some significant consequences to consider.
Can I put plywood over insulation in the attic? Yes you can install plywood subflooring directly over the floor joists and insulation on your floor. You need to install rigid insulation boards on top of the attic flooring joists and then install plywood over the top.
As a general rule, insulation should be replaced or retrofitted every 15 to 20 years. However, it may need replacing or retrofitting sooner depending on the type of insulation installed, the R rating of the original insulation, compression and movement over time, and whether any damage has occurred.
Exposed insulation is at risk of catching fire if not covered properly because it is extremely flammable. Installing a fire retardant plastic sheeting over insulation slows the fire damage rate in the case of an adverse event.
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.
Don't: Put Vapor Barriers on the Bottom of the Floor Joist in the Crawl Space. Because vapor barriers are good at retaining moisture, putting plastic sheeting on the joists can cause moisture to build up under the floor.
Yes it can touch. As long as there is nothing obstructing flow from the soffits - which sounds like the baffles are.
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.
This should be confirmed in the membrane manufacturer's BBA certificate, though 25mm to 50mm is typical.