insulation/ventilation problem

i live in kentucky in a split level brick house. house has exposed ceilings 10 ft at peak. wood is 2 x 6 inch tongue and groove yellow pine with 6 4 x 12 beams running length of house. on top of 2 x 6 expose wood there is 1/2 inch black board and on top of that 3/4 inch decking and then shingles. so i have approximately 3 inches of wood for insulation and nothing else. i would like to insulate my house from the out side by building a frame on top of roof say 2 x 4 or 2 x 6 and using spray foam insulation then redecking and shingle. my question is currently i have no ventilation at all. will i need to put some king of ventilation? can i put frame work on top of existing shingles? anyone got any other ideas?

 

Here is some reading that may help. There are different opinions as to whether to vent or not and how much insulation is enough. Since you will be going flat on the roof, I would consider rigid foam as opposed to spray in between a frame, less thermal bridging and perhaps lower costs. Remove shingles, add 2 to 4" inches of rigid, 5/8" OSB, and then new shingles. Whether you add strapping in there to allow ventilation is somewhat determined by how much insulation you choose and your climate. I'll let you read.

RR-0404: Roof Design —

Bud

 

thanks bud i read the article, they are way over my head but able to so understand some. i think i might write the professor that designed the roofs he seems very knowledgeable.
you mentioned removing the shingles is that necessary?
and what is thermal bridging?
anyway thanks for reply and suggestions.
bram

 

Removing shingles is probably optional, although it makes a better (neater) installation. Thermal bridging is the heat lost through the framing structure which is not normally covered with insulation.

Bud