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?

  • The primary issue, as Lee mentions, is that the original insulation becomes pointless. You need a fully-insulated outer envelope, not a two-part thing. Really, though, we're blowing hot air here since we have no idea what your climate even is.
    – isherwood
    Commented Feb 16, 2018 at 2:42

1 Answer 1


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.)

  • The details of how this 1" air space is accomplished would be very interesting. One of the original 1-story houses in our 1970 tract development has just been demolished (including slab) and a 2-story is being constructed on the lot. The builder told me that the insulation (blow "on" cellulose with binder) would be under the roof. I wonder how the 1" air gap will be provided. I suppose there will be a continuous soffit vent to vent each rafter bay? Commented Feb 17, 2018 at 18:46
  • @JimStewart Interesting...where I live it’s mild but moist. We worry about moisture and venting. We would never consider not having an air space and providing cross ventilation. Soffit vents in each joist space with a ridge vent is common. Directly applying spray-on insulation to wood roof joists could cause dryrot...I think. However, I suppose in drier environments it could be effective. Walls and floors are different. Spray-on insulation is common there. The main issue we have is proper mixing, installation and adequate curing (out gassing).
    – Lee Sam
    Commented Feb 17, 2018 at 20:18

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