Old Stone House

Dead Air Space

August 1, 2006

Filed under: Day to Day,Energy Savings @ 10:52 pm

One of the more interesting discussions about our house has to do with ‘dead air space’ and energy conservation. Our house, like all others of its era, was never insulated – despite this it keeps temperature/heat quite well. Naturally it is more expensive to heat then new homes, but I was surprised to find that it wasn’t as bad as expected. An older contractor explained that the reason for this was that the house was in fact insulated somewhat with ‘dead air space’.

The air space between the plaster and lathe in the house, and the exterior walls (stone) is known as the dead air space. So long as this cavity of air is kept from circulating and remains totally sealed, it acts as a form of insulation.

The air space in our house varies, but in generally measures about 2″ deep – offering a paltry R-value of about .80. In contrast, if this same 2″ cavity was filled with fibreglass insulation, this R-value could be increased substantially to about 8.0.

So why is our house not colder then it is?

It seems that the Victorians understood that the house would be cold, so the installed doors between every room and relied on localised heat such as parlour stoves. In effect reducing the overall square footage which needs to be heated. In our day to day routine, we generally keep all the doors in the house shut, which seems to help a great deal with heat conservation. Like wise, we have a number of different heaters that augment our forced air furnace such as a natural gas and electric fireplace. Generally, we keep the house at a constant temperature of 18C/64F. While quite cool, when coupled with localised heating sources such as the aforementioned fireplaces, or a portable radiant heater, we are quite comfortable.

As well, we live in fairly close proximity to the both of our neighbours, and have a series of mature trees lining our property, all of which help shield the house from the elements and thus, offer some relief from the elements.

Finally, someone did insulate the attic with R40 – which undoubtedly helps a great deal!

So while we renovate each room in the house, would it be worthwhile to gut each room and insulate it? As far as we’re concerned – no. First, the loss of architectural elements such as original plaster and mouldings in each room would only but further degrade the historical significance of the house. Second, the projected cost of insulating vs. current energy expenditures means that over a decade would pass before we would begin to see a return on our investment. Finally, we’re just not quite that ambitious… For now, we are happy to keep the house cool, put on sweaters and wear slippers.


  1. Attic insulation is much more important than wall insulation. Also, how can you have only 2″ of dead space? We have an old house with 2×4 walls and the air space is thus about 3 3/4″ (old 2×4 were bigger).

    “We don’t have framed walls – the plaster is strapped onto the stone!”

    Another compounding factor is just how dead is the air in your wall? For example, a well insulated attic helps prevent the gradient of cold to warm air in the house as you go up the walls and a weather-tight wall helps prevent air infiltration. You will still get convection cells of cold and warm air in your walls but truly dead air has an R value of 5.7 per inch. I like to think even the spider webs help! When you add up the R values of all the layers in your wall such as sheetrock, lath and plaster, shingles, wood, etc., the resulting total R value might be as high as R-5 using R1 for dead air. If there is very little convection or infiltration in your dead air space, the total could be higher.

    “Exactly! Although the house is uninsulated, we seem to maintain the temperature at a moderate expense.  This suggests that as you state, we are getting a modest, yet effective R value.” 

    As a final note, fiberglass R values are never fully realized because of air leakage (infiltration). You might see R values cut in half in poorly installed insulation and in areas where wiring and plumbing disrupt the fiberglass.

    Comment by JB — May 29, 2008 @ 3:46 pm

  2. “You will still get convection cells of cold and warm air in your walls but truly dead air has an R value of 5.7 per inch”

    I’ve been scouring the web to determine what the R-value of a 1″ thickness of “dead air” space is and in the previous comment it suggests an R-value of 5.7 per inch.

    Can you, or the reader who posted the 5.7 R-value figure, recommend where I might learn more about this?



    Comment by Mike — January 15, 2010 @ 8:09 am

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