Recently purchased a house. I will be re-roofing next spring. Currently has asphalt shingles. The Mrs. has directed that the new roof will be metal roof. Looking for strategies or advice on insulation, pigments, strategies, &c. Here are the home details. Location, Southern Maryland, Zip 20736. Original plus addition, so virtually 2 completely separate homes. A Side = 2 story with finished basement. 1985 construction. 2x4 at 16" OC. Soffit and gable vents (powered fan, but based on some advice, I have the stat maxed out to limit use to extremes). Attic had Fiberglass batt at 2 x R19. I recently added about 3-4 inches of blown cellulose. Man J showed about 39,000 Btu/hr heat load. Just installed a 3 ton geothermal HP in basement. No ductwork in attic. One skylight. B Side = Addition of in-law apt. On Crawl space. Also had 2 x R19 and i added cellulose cap. Due to shallower pitch and space, my cap tapers a bit toward exterior walls. Soffit and ridge vents. 2 skylights. Man J showed about 18,000 Btu/hr heat load. I have a 2 Ton geothermal heat pump in the attic of the apartment. All ductwork in attic as well. So with that info, I'd appreciate advice on options. Best and later, Trent

First- decommission & remove the gable fan, close up the gable vents. The fan only depressurizes the attic and increases air leakage at the attic floor/ceiling plane. With soffit venting it's far better to use ridge venting as the exit air. Gable vents short-circuit the air flow, leading to very uneven flow across the roof deck. Soffit-to-ridge vent flow is well guided by the rafters, even without an air barrier on the inner side of the rafters. Making the ridge vent cross-section only 2/3 that of the soffit venting prevents the ridge vent from increasing the overall stack-effect pressures on the conditioned space, since it puts the attic at about the same pressure as the soffits (which is about the same pressure as the ceiling below) rather than the ridge. Oversizing the ridge venting relative to the soffit venting de-pressurizes the attic, sucking more condition-space air into the attic.

Double-layered/cross-layered R19s with a 3" overblow of cellulose with modest compression performs about R40, which isn't bad. Taking it up to R50-R60 might be worth it in the very long term, but it's not a priority. If the pitch of the roof is such that the attic is getting a lot of unwanted summertime gain that somehow makes for an over-warm ceiling in the rooms below relative to the wall surfaces (measure it with an infra-red thermometer), use a CRRC rated "cool roof" finish on the metal roofing (or better yet, "shade" the roof with photovoltaic panels. :-) ) See: http://www.coolroofs.org/

On the low pitch roof in the apartment side with the ducts above the fiber insulation, it probably makes sense to put 3" (R20) rigid iso over the roof deck, and under the new roofing and closing up the soffit & ridge venting. You could use ~3.5" Hunter or Atlas or other nailbase panels, or standard roofing iso and half-inch OSB over it, staggering the seams, through screwed to the rafters with pancake head timber screws on a wide spacing to limit thermal bridging through the screws. If the pitch of the shallow roof is under 3:12 it's probably better to go with a cool-roof finish to the roofing no matter what.

See: http://www.atlasroofing.com/tabbed.php?section_url=58 http://hpanels.com/images/stories/pdfs/lit_prod_color/english/H-Shield-NB.pdf

With ~R20 on the outside of the roof deck it puts the ducts at least partially inside the thermal boundary, and with a sealed attic duct leakage won't drive air infiltration, and the summertime outdoor air moisture can't get in to condense on the cool ducts. It's a triple- win, but it's not cheap. If the appt. side is a 1-story, it might even be cheaper/easier to route the ducts in the crawlspace rather than the attic, where the performance hit is less (especially if you turn it into a conditioned crawlspace.) Ducts in unconditioned attics are common, but a BAD idea on several counts.

On either side using a purlin-mounting scheme for the metal roofing keeps the roof decking drier and more resiliant over time.

Cool roof finishes come in many colors other than "white" these days, but you have to ask- compare the solar reflective index (SRI)

If yours is like most 1980s homes, the biggest hole in the thermal envelope is at the basement or crawlspace foundation, both literally a hole (air leakage at the foundation sill and band joist usually exceeds all window & door weatherstripping leaks), and the figurative "hole" of an uninsulated foundation. A couple inches of closed cell foam to insulate & air seal the foundation-sill & band joist is about right, and on the walls trapping 1" of XPS or unfaced EPS (seams taped or foamed) to the concrete, foam-sealed at the top/bottom edges with R11-R13 unfaced batts works well in that climate zone. Put an inch of foam under the bottom plate of the studs as both a thermal & capillary break from the ground temp & moisture, TapConned to the slab. It's not structural- so the pressure deformation rating of the foam is of no consequence- it's only holding up the wallboard, not the house. Alternatively, 1.5-2" of fire-rated Thermax held in place with polypropylene fasteners is a thin, quick & easy DIY that doesn't eat up floor area. See: http://blog.energysmiths.com/2011/08/basement-insulation-part-3.html

On the crawlspace section you may need to put down a 10mil poly vapor barrier, lapped 12" and taped at the seams, sealed to the foundation wall a foot or so above ground level. Then you can put up rigid foam and seal the band joist just like the rest. In the muggy mid-atlantic vented crawlspaces add far more moisture load to the house than they take away, and are just big infiltration & heat flow leak point. Insulating between the joists just puts the bottom edges of the joists at higher risk for rot, and it's nearly impossible to air-seal the underside of a floor, even with copious amounts of spray foam. Sealing & insulating the crawlspace walls brings it into conditioned space. So long as there is a ground vapor retarder and SOME minimal air communication with the rest of the space the mold risk falls by quite a bit by making it an insulated semi-conditioned space.

If the 2x4 walls have R13 batts that were installed decently it's not a priority, but if you're ever re-siding pounding them full of cellulose from the outside and putting up an inch of exterior insulating foam sheathing tightens up the place and adds comfort. (An inch of foam cuts the heat loss of a 2x4 wall by about a third, 2" cuts it by half.) If the wall cavities are empty (I think that was still legal in MD in 1985) insulating the walls makes the top of the priority list.

An infra-red imaging scan may be worth it to identify any gaps in wall insulation that needs rectification, but doing that should also be in conjunction with a blower-door test to find & fix all of the bigger air leaks, AFTER you've fixed all the air leaks that you know about (which includes the foundation sill/band joist leak).

Concentrate your initial efforts on air-sealing, which has the biggest bang/buck. If you have a bunch of can-lights penetrating the upper floor ceiling, that's usually an energy-disaster. But every plumbing & electrical penetration should get the 1-part foam or quality-caulk treatment, flue-penetrations need metal air-blocks (and if the insulation was installed with clearance from the flue, using rock-wool batts to fill those gaps is usually OK.) Watch out for any plumbing vents or wiring chases that provide a path from basement/crawl to the attic & beyond too- seal them at both the top & bottom. If you have a lot of larger gaps to fill you might spring for a 12 board foot FrothPak kit (~$40 at box store home centers) which is a 2-part 1.5lb density closed-cell foam sealant/insulator, which beats buying 20 cans of 1-part foam. If you have a lot of seam sealing to do it's worth buying a low-end pro-type foam gun that takes the 20oz+ screw-on cans. The pro-tools work a lot nicer than the throwaway straw-nozzle type you get on the small cans at home centers. (Shop online if you don't have a ready source for that stuff.)

39K + 18K is a fairly substantial heat load for a US climate-zone 4 location with a 99% outside design temp that's probably in the high-teens or maybe even +20F unless it's pretty big place, even with 1980s 2x4 construction. Do you have a lot of glazed area comprised of single-pane windows/sliding doors etc without storm-windows? If yes, that's probably worth dealing with, but the best or most cost effective approach depends on the particulars. But understand this: Every square foot of single pane window loses as much heat as 10 square feet of 2x4 batt-insulated wall- 2 average sized single pane double-hungs on a 15' section of wall will dominate the heat loss of that room. Adding low-E storms cuts the loss out of those windows by almost 2/3, and it's a lot cheaper than replacement windows. But in some instances it'll be worth springing for better windows no matter what- it just depends.

Dana,
Thank you for the detailed response. Some follow up info and follow up questions.
I had thought the powered vent was definitely going. Thanks for confirmation. The attic on the main house is fairly tight. Prior to blowing the 3” cap of cellulose, I got the professional 1 part foam and gun. I probably used a half dozen cans. No can lights penetrate ceiling, so mostly electric runs and a couple bathroom fans. My biggest liability was the attic stair. I fashioned a box of 2” dow foil faced board as an attic stair enclosure. I got it fairly tight. When new roof goes up, we can plan to cut in a ridge vent on the main house with a 2:3 ridge:soffit ratio.

As far as adding the rigid iso to the shallow roof and sealing, I have seen numerous posts where folks seem to be pretty particular when you seal an attic, you need to remove the existing fiber or cellulose. Is this a valid concern or not?

On the main house, basement is already finished, so other than a small closet with well pump, all basement walls are built out. I do not know what lies behind. The crawlspace will be one of my DIY targets. Currently has a poly vapor barrier, but torn/gapped in areas. I can plan do over of the poly. I was looking a two part c.c. spray foam kit for 600 board feet, which I estimate would give me 1.5” coverage. Sound OK? As far as “SOME minimal air communication”, is that covered by the routine cable/pipe/electric penetrations, or are you talking something more, like a floor grate open to below? Would this also require me insulate and put in an air tight access hatch?

For whole house, all windows are double pane, and not an excessive amount of glass. The 2x4 walls do have fiberglass. Fiber-cement siding is a future goal, but probably 5-10 year horizon, so will take a look at exterior iso at that point. House is about 3000 SF addition about half that. Load calcs were done prior to the added insulation, so not sure how close they are. Just moved in and did ghp this past May, so have not wintered over, but I know I never went second stage on cooling. We’ll see how it keeps up on heat.
Any good advice on skylights
Thanks again,
Trent

Foam installers insulating attics from the interior take a dim view of leaving the floor insulation in place because it interferes with getting a good seal at the soffits.

Building codes take a dim view of insulated partitions within the outer insulating layer due to potential dew-point/mold & moisture issues on the intervening space. But as long as the exterior R to interior R ratio keeps the attic space above the wintertime outdoor dew point, leaving it there is fine, and it's doing considerable good from a heating & cooling load point of view. Were all the insulation at the roof deck, R15 on the exterior would be good enough, but because there is a high likelihood of air leakage between the conditioned space and the attic space you'll want to go with a higher R. The cellulose buffers some of that moisture, but it can't buffer enough to overcome say, a small duct leak. There is some heating of the space by the ducts (leaking or not) during the heating season just from duct-wall conduction, so it'll run warmer than a simple-model of R values would indicate, but with 1/3 of the R outside the roof deck, the rest on the attic floor, with an average January temp in the 30F range and a 70F ceiling temp, the attic would average about 43F. If you keep the conditioned space at 35%RH/70F the dew point of any leakage air into the attic would be ~40F. If you want more margin than that, add an inch to the exterior foam. Alternatively, apply a flash-inch of closed cell spray foam to the roof deck & rafters from the interior which would provide a non-wicking condensing surface and another ~ ~R6 outside the condensing surface, and a ~1 perm vapor retarder on the potentially susceptible wood.

Whatever exterior insulation you go with, you can monitor the temp & humidity in that attic the first winter after the fact as a sanity check. Odds are it'll average higher than 45F with the ducts in there, maybe over 50F, even with just R20 iso on the outside. AccuRite makes some cheap ~$10 battery operated humidity & temp monitors that log both the peak/low temps & RH. The key to watch would be the min-temps on colder nights. If it's getting down to the freezing level it means there is probably still substantial air leaks to the exterior, or your duct insulation is REALLY substantial. If it doesn't drop below 40F even on nights when it's in the teens outside your risk is pretty low.

At 57KBTU/hr for 4500' of fully conditioned space is ~13BTU/square foot, sounds is in the right range for tight 2x6 construction with U0.5-ish windows. The attic-R and air sealing may have knocked a half-ton off the total heating load (but probably not much more unless it was REALLY leaky), but maybe getting onto a ton off the cooling load, depending on the roof orientation & color, and the humidity of the air leakage.

Adding higher density cavity insulation and exterior foam at the point of re-siding would usually be worth it on comfort grounds. The economic case would a longer term bet, depending on where your power rates are trending, and the as-used efficiency of the geo. At the VERY least you'd want to find & fix any voids in the cavity insulation before putting the new siding up. You may want to fix those even before you go to re-side if you find any colder spots on walls with an IR thermometer when it cold & dark outside. If that seems to be in the cards, use a weatherization contractor who uses blower doors & IR thermography to find & fix 'em all at once rather than attacking it piecemeal- it'll be cheaper & better that way.