Insulating low-slope residential roof – GreenBuildingAdvisor

Hello and thank you for advise. I really appreciate everything I have learned from this site so far. I have read insulating low pitched article and comments after the roof already has been installed.

We are full house remodeling 1930 flat roof Spanish house in LA County California climate zone 3. 2.5 inch eps insulation on the walls with peel and stick wrb over osb sheeting. Installing new roof and the addition both 1/2 per foot slope with torch down roll roof membrane 3ply. Unvented crawlspace with 15 mil vapor barrier sealed to the foundation with dehumidifier. Questioning my attic ventilation. The attic is 4ft at the top and 2ft at the bottom withh 2×8 ceiling joists and 2×8 roof rafters. Main house is 24ft wide by 40ft deep and addition is 24ft wide by 13ft deep. The plan was to have vents between every roof rafter blockings(3 – 2inch holes) coming through soffit. Insulation is r30 mineral wool between ceiling joists and additional blown in to make it R50 and big enough gap(15 inch at least)between under rafters and top of the insulation. The roof is already installed.  It will be airtight house so we are installing broan ai erv and make up air for the hood in the attic and minisplits in the house.

1 For the main house can we skip the dog house with so many vents in each bay between roof rafters and the width of 24ft? I can install the dog house after the roof already installed but rather not to. I understand it needs to be 2 by 2 but how height and how much vent opening? I am leaning towards skipping dog house and watching the roof deck through out the year to see if there are any moisture problems.

2 Addition is no attic 2×8 roof/ceiling rafters with 2 inch poliyso spray foam under the roof deck and r23 mineral wool to fill the rest of the cavity. Any concern with that?

Thank you,

Georgiy