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Insulating a retrofitted slopped roof on level flat roof

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Preferred insulation and venting methodology for retrofitted slopped roof on level flat roof.

We are looking for suggestions, options and methodologies for adding roof insulation to a 1960’s era 1600 sqft home. Has daylight basement. Each of two levels is about 800 sqft. Residential home located in south central Alaska.

While this would seem a common an easy project, the twist is the home has two roofing and venting systems. The original flat roof, that is still intact, with very few penetrations, is a vented, level, flat roof with built up felt and hot tar. A trussed, site built, sloped, 4 ft peak, 1'-6" perimeter overhangs, asphalt shingle roof was added in the 1980's directly on the top of the existing flat tarred roof. This has worked well for almost 40 years. Customer cannot complain about the life span obtained from the original 3 tab Malarkey shingles. We are keeping with that same brand.

Alas, it’s finally time to remove and replace the 3 tab asphalt shingles, add ice and water shield over the entire slopped roof sheathing, add drip edge, update and modify the existing cold attic venting, and add insulation. The original 1960 flat, level, roof has 2x10 or 12 joists with foil faced ~2" fiberglass insulation. It has overhang vented soffits via both ends of the joist bays. The shingle roof on top of the flat roof has overhangs with proper screened eve soffits. It has small gable end vents but has no additional insulation or ridge venting. These roofing systems have been relatively trouble free but is obviously not insulated optimally.

We are considering offering the customer two options. Cold vented attic or warm unvented attic:

1. Cold Vented Attic
a.) Cut venting slots through the flat built up tar and sheathing in the attic with a circular saw. b.) Add blown or batt fiberglass/rock wool on top of the built up roof. c.) Seal the flat roof soffit venting. d.) Adding ridge venting. e.) Adding soffit vent baffles in the attic to prevent blown or bats from obstructing pitched roof soffit vents.

This seems pretty typical along with being tried and true, least expensive, with the exception of cutting slots in the built up tar roof? There will be a significant air gap in the insulation roof stack due to space in the flat joists. This is path I am likely leaning towards. for the customer. air gap is difficult to access without significant removal of the tarred roof and sheathing between the trusses or the conditioned space ceiling. What is recommended if anything for the unheated overhangs of the flat roof.

2. Hot Unvented Attic
a.) Spray foam (closed cell) bottom of sloped sheathing. b.) Seal both flat roof and sloped roof soffit and gable vents. c.) Cut venting slots through built up with circular saw. Is this venting still necessary?

This makes the attic warm. This effectively puts a hat on top of the building and provides for possibility of cathedral ceiling in some parts of the house in the future. Future accessible semi warm space for future M/E upgrades. Our understanding is there must be enough closed cell "R" insulation value for the dew point of the roof to be within the non-moisture suspectable insulation? Additional, rigid foam insulation on top of the sloped roof sheathing before the ice and water and shingles is a consideration? The air gap in the flat roof joists is less of a concern in this scenario? Insulating the vertical permitter of the flat roof rim joists will be impossible/difficult opening up for potential condensation? Seems a bit risky because of the lack of rim joist insulation.

3. Possible option of using dense-pack cellulose insulation in the flat roof joist space? Achieving no air gaps will be difficult. No venting required? The interior vapor barrier is poor, as it is thin foil on old paper. Seems lik a risky bad idea.

Any suggestions comments or considerations are welcome.



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