Icing the Cake

When dressing my seven year old son to play in the snow, I always remind him to where his hat. It's what my Mom always told me.  And the truth is that it's the most effective way to stay warm.  By keeping one's head warm, you're insulating your largest source of heat loss.

2x8 rafters leave little space
to add insulation

Now what does this have to do with home energy efficiency? Well, it's the top of the home that is one of the largest heat loss sources. Wouldn't it be great if we could fit attics with knitted caps?

Probably the single most discussed issue when it comes to improving energy efficiency in rehabbed homes is how to effectively insulate their attics.  This is especially true when occupied attics have complicated configurations.

I don't have all the answers, but here are some quick lessons learned:

To ventilate or not to ventilate... that is the question

There doesn't seem to be a wrong answer to this question as long as your decision is accompanied by the appropriate details.

If You Decide to Ventilate the Roof

Sometimes abandoning a formerly
occupied attic and filling with
insulation is the most cost
effective solution

1. Entirely abandon occupied attic spaces.  This allows you to fill the entire attic floor with insulation to as great a depth as possible.  By placing the attic outside the thermal envelop, you can avoid the whole issue of how to insulate the cathedral ceiling.  The attic now has plenty of space to breath.  Remember to use baffles so that ventilation flows from eave (low point) to ridge (high point) as code requires.

2. Insulate between the rafters. Attach rigid insulation to the rafters and fill the cavity with cellulose or fiberglass.  But remember to again use baffles in order to maintain a small 1" ventilation space between the insulation and the underside of the deck.  The space should be continuous from the ridge to eave. If moisture should enter the space, it now has a way of drying out without doing damage.

If You Decide to Install an Unventilated Roof 

3. Provide an air barrier on the underside of the roof deck and then insulate.  This is achieved by securing rigid foam insulation between rafters and filling gaps with foam (Notice we are not using spray foam only because historic preservation rules prohibit its use within walls and/or ceilings).  Now the space between the rafters and under the rigid insulation can be filled with dense packed cellulose or fiberglass.   The layer of rigid insulation creates an air barrier which prevents the dew point from moving into the dense packed insulation.    The challenge lies in squeezing as much insulation as possible between rafters that are only 8" deep or less.

4. Provide the air barrier on the top side of the roof deck.  This allows you to reduce if not eliminate the amount of  insulation being stuffed under the deck, but it has consequences for the outside of the building.  Fascia and eave profiles will be modified and the roofing contractor is now responsible for installing a layer of rigid insulation and sheathing.

Rehabilitating old houses is an art.  And seeing icicles forming on the eaves of recently insulated attics (see attached pictures) is disheartening.  The icicles are an indication of heat loss.  The good news is that with experience comes improved techniques and more effective solutions.




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For more detail and discussion checkout:
"Insulating Cathedral Ceilings", Fine Homebuilding Magazine, June/July 2012 (#228), pg. 66.