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How to add insulation to existing roof - Ice dam prevention

I have a 54 year old Cape style home in Nerw England. Plaster ceilings run along the base of the roof rafters (2x6 rafters). No soffits - have Hicks vent drip edge & Full ridge vent. Need to re-roof, want to add insulation, build out vented soffit system,- by adding rafter tails, extend the rakes out, Ice & water 6 feet from edge & valleys then re-roof.

One option presented is to remove existing roof , deck & what ever insulation might be there, then spray in foam, add 2 to 4 inches of rigid insulation, new roof deck & then re-roof. Second would be blow in cellulose below existing deck, then add rigid foam , new deck etc.

third is just add rigid on top of existing deck, add new deck & re-roof.

How do you get a cool deck using the rigid - Do we need to add 2 x 2 screwed through rigid to the rafters?

Any thoughts / feedback greatly appreciated

Asked by John Shea
Posted Oct 10, 2011 8:47 AM ET


5 Answers

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If you want a ventilated roof over rigid foam, the standard solution is to install 2x4s flat on the foam to create 1.5-inch-deep channels. The 2x4s run from soffit to ridge, with one 2x4 above each rafter. Attach the 2x4s to the rafters below with long screws. Then install plywood on top of the 2x4s, followed my asphalt felt and new roofing.

Answered by Martin Holladay
Posted Oct 10, 2011 10:03 AM ET


Hi Martin,

Thanks for your response. One other question, Is it still best practice to vent the rigid / roof interface using the 2x4 as you described or is a "hot" roof acceptable? Plan is to use asphalt architectural shingles with energy star rating.

Answered by John Shea
Posted Oct 10, 2011 10:45 AM ET


I believe that either approach can work, as long as you have an excellent air barrier that prevents the exfiltration of humid indoor air, and as long as the total R-value of the roof assembly meets or exceeds up-to-date code requirements.

However, a vented roof assembly is more forgiving of installation errors. If you can afford the added cost, the installation of ventilation channels is good insurance.

Answered by Martin Holladay
Posted Oct 10, 2011 11:04 AM ET


A Radiant Barrier mounted above R-19 insulation or equal value would greatly enhance your thermal value. the only requirement of the product is an air gap. mounting on rafters is okay but constant contact with any product cuts into the thermal value. when installed correctly the radiant barrier will reflect the heat created in the home Back into the living space. 1st cutting your fuel costs , 2nd keeping the snow from melting and a constant source of water and ultimately ice from forming. I will be glad to assist further but I don't want to preach.

Answered by Thomas Cimini
Posted Oct 13, 2011 11:51 PM ET


The radiant barrier adjacent to an air space that you propose will add, at most, R-3 to the roof assembly. And it will only work until it gets dusty; once the radiant barrier is dusty, the R-value of the assembly will go down.

There are less expensive -- and more dependable and long-lived -- ways to add R-3 to a roof assembly, so I don't recommend a radiant barrier in this case.

Answered by Martin Holladay
Posted Oct 14, 2011 4:38 AM ET

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