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?

Thanks,

Mike

“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.