Best/cheapest insulation method for flat roofs in new construction?
I am an architect in Atlanta, GA (climate zone 3) mostly designing modern/contemporary homes that typically have flat roofs. There seems to be a lot of conflicting information and opinions out there as to the best way to insulate these types of roofs. Typical construction would be 12″ deep ceiling/roof I-joists or 2×4 open-web trusses with flat OSB decking above. The most common potential options (all unvented scenarios)are:
1. Cheaper route that many builders will generally favor: Build and additional layer of sloped OSB decking on ripped 2x furring strips for slope to roof drains and place R-30 fiberglass batts between rafters. Not particularly smart or green in any way and requires extensive caulk air-sealing but as with most things, the most-common way is most-affordable (up-front cost at least). Other potential downside is that if ceiling plane is not adequately air-sealed (for instance at recessed light fixtures) then warm air at underside of deck could cause condensation in colder months.
2. Place average 4″ of XPS rigid insulation on top of the decking (~R-20) and 4″ of closed-cell foam (R-24)doubling as air-sealer at underside of deck. Total R-value = ~44.
3. Place average 4″ of XPS rigid insulation on top of the decking (~R-20) and 2″ of closed-cell foam (R-12)doubling as air-sealer at underside of deck and then R-30 batts. Total R-value = ~62.
4. Forgo XPS insulation above deck and used sloped framing/decking as in option 1 but use 2″ of cc foam (R-12) add fill out remaining 10″ of cavity with dense-packed cellulose (~R-35). Total R-value = ~R-47
I know there’s a lot of variables here and the answer is often “it depends on priorities” but just looking to get some feedback on best balanced approach taking into account some of the following concerns:
– What scenario is going to provide best R-value for the money?
– Is using closed cell foam on underside of roof deck a real problem in the event of trying to track down and fix a potential roof leak? Is open-cell a better alternative on this count despite the lower R-value but with added benefit of also being cheaper?
– Will the weight of10″ of cellulose cause deflection of the drywall ceiling over time or it remains pretty self-supporting?
– Recessed can lights can be reduced but seem unavoidable in the ceiling plane. What factors to consider due to this?
– For considering cost trade-offs between the various options, I would assume the value of adding the extra layer of sloped framing/decking as in options 1 and 4 to average maybe +/-$1/sf of roof.
– On most projects, the builders seem to say that doing sloped XPS insulation is more expensive than having the framer build the slopes but not sure of the exact cost differential.
– Many of my clients would want to take into account the ‘green-ness’ of various products as a factor as well.
Any help in assessing these options and especially in comparing real-world costs would be greatly appreciated.
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