greetings,

a contractor friend is building a strawbale house with a ridge-vented metal roof for which the architect specified insulating both the underside of the roof deck (btwn rafters) and the ceiling. the house is in central california where it's hot and dry in the summer and fairly chilly, and sometimes quite moist, in the winter. my intuition tells me they should only insulate the living space, but they argue they want to keep the heat from coming through the roof.
any ideas on this?
thanks,
stephan

Yes. It won't.

I would like to hear the architect's reasoning for insulating at the roof level. It makes no sense to me. If he is trying to isolate the absorbed heat of the metal roofing from the rest of the structure, I would think it should be done at the roof deck, not between the rafters.

i think i understand his rationale. the insulation in the roof will keep the attic temperature lower than it would be without it by blocking some of the radiant heat (in the case of summer). i'm sure he has observed this in like situations. the attic temp. would never be below the outside ambient temp. but it would still help the lower, ceiling insulation do its job.
what i need is a way to clearly articulate the thermodynamics involved--rather than trying to explain my innate understanding of the matter--to the builder, architect, and owner. how do i explain the increased benefit of placing the insulation that they plan on putting in the roof rafters on top of what they plan on putting on the ceiling instead.
thanks for your feedback.

Insulation doesn't block anything. It simply delays it. Let's say you put R-19 insulation under your roof. R-19 has a U value of O.05 plus a little. The U-value tells you how many BTU's per hour per square foot will get past the insulation. If your climate is hot for a long period of time, sooner or later, enough BTUs will penetrate the insulation and heat your attic.

Probably a better idea would be to make the roof as reflective as possible. If your roof is dark, it will absorb the heat and try to trasmit it to the attic. The roof itself will always be hot. The closer to white you can get, the more heat is reflected, and the less there is to be transmitted to the attic.

But once the heat is in, it's in. The insulation will only delay the effect, not stop it.

A light-colored roof and good positive ventilation is the way to go, and insulate the heck out of the ceiling.

To add on this, that is why ventilation is so important. It removes the heat that builds up against the face of the insulation by venting it out. Thus making it easier for the insulation to do its job.

an update: i just talked with my builder friend. he said when the owner asked the architect about the insulation scheme (insulating roof and ceiling) recently, the architect said, "what? why would you want to do that?" so he knows better, but somehow the design ended up on his drawings..
thank you all for your input.

regards,
stephan

i agree with richard but i would insulate the heck out of the ceiling with spray foam insulsoy