Do Roof Shingles Insulate?

I have a question about roof replacement. My composition shingle roof was installed in 1984 and is starting to curl at the edges.  Should I remove the shingles or would it be okay to apply a new layer on top of them? Leaving the old one on seems to provide an insulation in my mind, but I would like your opinion.
Mike P, Oldsmar, FL

In most cases, it is always best to remove old roof shingles before installing new roofing.  Old roof shingles offer absolutely no insulating value.  To insulate, a material must trap air.  Roof shingles won't do this.  In fact, they can actually have the reverse effect since old layers of shingles can act as "heat sinks" trapping and storing the sun's heat. That heat will radiate back into the attic space, making that space warmer and potentially raising your cooling costs. 

Another economic factor to consider is that a 2nd roof layer will usually not last as long as a single layer.  In my 20+ years as a professional home inspector, I often observed that two-layer roofs seemed to have about 2/3rds of the life of their single layer counterparts.  The reason?  Heat build up is greater on a 2-layer roof.  The additional mass the old layer contributes stores heat and lets the new layer to dry out all that much faster. 

So, here's the math: If you expect to be in your home for most of the 20+ year life span of a new roof, you are almost always better off spending a bit more to have the old layers stripped off.  However, if you are re-roofing to sell the home or you expect to be there for less than 10 years, save the cash and add another layer.

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