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Cape cod roof insulation & venting: Hot roof designs, aka dense-packed insulated sloped roofs or ventilation designs for cape cod style buildings.
This article series describes various solutions for the un-vented or insulated portions of cape cod home roofs or ceilings or for similar under-roof spaces. The article offers advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible.
We provide data & design suggestions for cape cod home hot roof designs or for providing a cape cod style structure with roof and ceiling ventilation for this hard-to-vent roof design.
Special Venting (or "packed" hot roof) Problems for Cape Cod Roofs
Cape cod roofs present a special problem in that in the usual cape design there is a lower attic knee-wall enclosing what amounts to a "low attic" followed by the equivalent of a cathedral ceiling over the middle section of the roof, often penetrated by dormers when the second floor of the Cape Cod is designed to be occupied, followed again by an attic design for the upper third of the roof.
When I examine Cape Cods built in the Northeastern U.S. in winter, homes built in the 1930's through 1960's especially show ice dams on the lower roof, snow on the center roof, and all snow melted off of the upper third of the roof.
Those conditions mean leaks into the building walls, unnecessary heat losses (higher heating costs), and too often roof sheathing and insulation contaminated with several genera of mold.
And where we have removed insulation from between the rafters of such roofs we often find moisture has been trapped against the roof sheathing. Sometimes that has led to mold, delaminating plywood sheathing, or even rot.
In new construction we have an opportunity for better building design, implementation, and energy performance.
The bottom line (in my opinion) is that roofers and builders who like the "hot roof" approach trust their implementations of the hot roof approach and assert that it works fine but water or moisture leaks from either outside or from within the building into an un-vented building cavity (like a cathedral ceiling roof) cause more damage than leaks into a vented roof.
Watch out: when enthusiasts recount differences between the performance of a "packed" or "hot roof" design (presumably very high-R and "tight") and a ventilated roof design (more trouble, more cost, sometimes poorer insulation and higher energy cost) they sometimes may not have realized, or may fail to report that when a leak into the packed roof occurs, water stays there a long time - inviting rot, carpenter ants, mold. The result is a very large repair expense.
Steven Bliss offers a final warning on Hot Roof Designs:
I agree with you that, in the real world, this is not such a good idea. If there's a flashing leak or other roof leak, you could have a pretty soggy mess that stays wet for a long time and could cause structural decay.
Plenty of people are building hot roofs, but I wouldn't -- except maybe one with spray urethane which won't absorb much water like cellulose would. 
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