I bought an old home. There is new blown in insulation on the floor of the attic. The attic will not be a bedroom BUT will eventually be a kid's playroom. My plan is this:
Build a subfloor over the blown in insulation.
Put batts between the joists under the roof.
Put drywall up on the underside of the roof so the kids don't see or touch the insulation.
It won't be anything special – thick plywood floors with a rug and unfinished drywall for the roof (building a very short wall from the roof down to the floor so I don't have to drywall into the corners where the roof and floor meet.
But I keep reading that you're not suppose to put insulation under/against the roof. There are no leaks, it's a relatively new roof. I want to put some insulation along the ceiling/walls of the play area because we are in an area that will get cold and hot enough that kids won't want to be up there.
But what are my options if I keep reading about not putting it against the roof? And do I need to leave a gap for the soffits if I do this?
Best Answer
As heat (vapor) escapes up through the ceiling and insulation in the joist space, it turns from vapor to liquid when it reaches its “Dew Point”. (You can google Dew Point. )
That moisture (liquid) will cause dryrot and mold if it is not dried out. The easiest way (and customary way) to dry this moisture out is by ventilating the space between the insulation and roof sheathing.
In order for ventilation to work, it has been determined that you need a 1” air space between the insulation and the roof sheathing. (See ICC Code R806.3.) Also, cross ventilation is recommend (soffit vents on both sides of your house or gable end vents).
The amount of venting is 1/150th of the area of the attic, unless you install not less than 40% and not more than 50% in the top one-half of the attic (gable vents or ridge vents) See ICC R806.2, then it can be reduced to 1/300th the area vented.
After you install the insulation in between the roof joists, the existing blown in insulation in the ceiling will not help thermal resistance. (However, it will provide sound control between floors.)