I am having poly spray in insullation done in the upper attic/second storey because I am having two bedrooms built up there.
Should I allow the sprayfoam to be sprayed right to the roof sheathing or does there need to be a gap to allow for airflow?
My concern is that without a gap it will trap air and not allow any airflow.
In roofs built using trusses here are the requirements: Where insulation depressors are used they must provide 1" of air space; where insulation depressors are not used you should have 2 1/2" of air space.
In roofs that are stick framed using roof joists or rafters, such as in an a-frame house, you need a minimum of 2 1/2" of air flow over the insulation.
As long as you are using closed cell spray foam, you don't need an air space. The reason why you need an air space with conventional insulation is because you need to get rid of the moist air around the insulation. As long as you have 4" of spray foam, you will never have issues with moisture.
Yes, you can spray foam direct on to sheathing, but ensuring a soffit to ridge venting system is installed between each truss on the sheathing prior to spray foam with an external ridge vent on the roof to ensure air flow. If this is not done heat during the summer will significantly reduce the life span of the shingles
Regarding all of the previous replies the winner Is........................................Kevin
Ventilation in attic spaces, particularly in climates where you have very cold winters and very hot summers, is there for two purposes.
1.) To keep the warmth in, in the winter, and
2.) to cool the roofing material in the summer. Without the air space that Kevin suggests there will be no cooling at all for the roofing material. Roofing material that is baked due to lack of air movement below it to cool it down deteriorates significantly faster than roofing material that is maintained at a lower temperature. 25 year asphalt shingles often need replacement after 9 or 10 years due to lack of ventilation.
Yes, spray foam works in the winter but why not install it such that there is a continuous open air flow channel above it to protect the roof as well. Of course it adds cost, but not as much cost as replacing your roof more than twice as many times as would be required the roof / ventilation system is installed properly.
Dan Brown, RHI
The reason roofing materials require the airspace below them to be ventilated is because without ventilation the heat buildup in the airspace will easily double the outside temperature or more to cook the asphalt shingles from below. The normal humidity in the airspace at that temperature will also destroy the glues in OSB and plywood and warp dimensional lumber over time. Here's the important part - where all the other pros get an F - if there is no air space then there is no heat buildup! or humidity issue! the hottest the roof will get is the same temperature the surface of the shingles get with or without an airspace, truthfully the vented airspace is always much hotter the the outside temp anyways so by eliminating the airspace altogether your shingles should last longer. Spraying to the underside of the roof deck is a good idea in a 1 1/2 story design. This creates what is known as a "warm attic". It's imperative that the insulation is closed cell, minimum 4", and continuous on all outside surfaces throughout the new attic's "building envelope" you are creating, including gable end walls, and must be properly tied into the main floor vapour barrier as well. If there are any voids in the insulation the moist air from your living space will condense and freeze in them, and melt and ruin drywall etc. This design also deletes the poly vapour barrier behnd your new drywall as the foam is now your vapour barrier.
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