How should I think about rooftop insulation versus attic?
I’m replacing my 15yr old asphalt shingle roof with a metal roof and must decide whether to insulate the roof on top or in the attic. The home is a 2004 vintage 24x40ft manufactured home on cinder block foundation with a dirt Crawlspace and a 4/12 pitch truss roof made of 2×4’s. All the ductwork and some electrical runs thru the attic and there is currently no access panel for service. There appears to be R13 or R11 batts laid across many sections of the attic – but not all. We have 3miles of open water that makes exposes us to severe winds at times. Our goals to improve comfort, do the right thing and save on energy.
We expect to add 500-1000sq ft in the near future, at which point I will need to upgrade or supplement the current furnace, AC (HVAC) and water heater (HWH). The current furnace and hot water heater are small enough to share a coat closet – with the furnace on top and tank below. Only one system of the two housed here can expand in the current space. it’d be tricky to move the furnace ducts to another location because there’s no storage or closet space.
#1 I can add blown in insulation for much less than $1000 but due to the small attic space, but I’m concerned about flexibility and serviceability challenges because a foot of blown in insulation eats up half the volume of the space. When I need to grow the HVAC and HWH for the addition this might obstruct moving the furnace to the attic.
#2 I can add 4” of polyiso to the roof now for $3000 and get an R value in the low 20’s
#3 I can do a combination of the #1 and 2 or possibly spray foam portions or use other materials in the attic.
I’m looking for trade offs of one option va another and what risk I face having insulation on top of the roof as well as in the attic.
any advice is welcomed ! Including referrals to other FHB and other articles.