Published on April 13th, 2017 | by Henri


Adding more insulation in attic

Q.  I live in a ranch on one side with a split level on the other side. I want to put more insulation in the attic. I am capable of rolling out insulation on top of what is already there. Will that be okay? Will it set up any water or other issues if I do that? And should I roll out the insulation in the same direction as the insulation that is already down or should I go in an opposite direction?

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A.  Adding insulation on top of the existing one is an excellent idea, but you need to keep a few considerations in mind.

You have the choice of adding new batts (since this is the type of insulation you plan on using) on top of the existing insulation, using the R-factor of your choice, or of running the new insulation across the floor joists, which is the better way to do so because it also insulates the floor joists themselves.

If the roof is built with trusses, it is too difficult to do an effective job running the batts across the bottom chord; it is best to insulate between the bottom chords. You can cut strips of insulation to cover the top of the bottom chords if you have the patience.

But if the existing insulation does not fully fill the entire depth of standard floor joists, the remaining space must be filled with insulation of the right thickness before running the new batts perpendicular to the floor joists. Otherwise, there will be an air space between the old and the new insulation in each joist bay and cold air from the eaves can infiltrate these spaces, negating much of the benefit of your efforts.

Whichever way you choose to add new insulation, be sure not to obstruct any eaves ventilation.

Regarding the water issues, you are referring to potential condensation and mold on the roof’s sheathing and rafters. It is an important consideration.
Adding insulation lowers the attic’s temperature, which means that the colder air’s relative humidity will rise and the dew point can be reached, leading to condensation. The positive side of this is that a colder attic will reduce snowmelt from the roof cover and may reduce the formation of ice at the eaves.

To prevent the formation of condensation and mold, attic ventilation in the form of continuous soffit and ridge venting with an uninterrupted air space between them is helpful, but may not be enough. All convection paths that allow warm, moist air from the conditioned living areas to enter the attic must be sealed; and often, there are many. These paths are found around recessed and surface ceiling lights, exhaust fans, any electrical wiring and receptacle, any plumbing pipes, any cracks in the ceiling and wall finishes, etc.

If you are not sure where these paths are, the best way to find them is through an energy audit that consists of a blow door test and infrared thermography.

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