Another avenue besides the rigid would be to lay 2x6 rafters over the existing roof, insulate with 3-1/2" batts, leaving an air space between the insulation and the new roof sheathing. A continous ridge venting coupled with continous soffit/eave venting will carry the hot air up from the eaves and out the ridgeventing and the batts will block a majority of the heat gain from entering the home. This will be pricy as well, not sure if it would be less than using rigid, which is a decent solution,imo as long as you have a decent thickness of ridgid similar to a sips panel.

1st, what is supporting your roof - rafters or beam system? T&G, sheathing, & shingles add up to a lot of weight requiring some type of support system. My cathedral ceiling is constructed as follows -- beam/purlin support, T&G, double layer 2" rigid foam w/seams staggered, 2x lumber secured face down on top of rigid foam (running from soffitt to ridge), sheathing, and shingles. This isolates my interior ceiling from outside temps and the 2x provides airflow up through the ridge vent.

A rafter system is usually cheaper than a beam system. Depending on price you may want to look at radiant roof sheathing designed to reflect the suns rays back off the roof (see Louisiana-Pacific & Georgia-Pacific websites). Coupling with fiberglass insulation between the rafters should significantly reduce the amount of heat coming through the roof. I also prefer full-width insulation baffles for the underside of the roof sheathing.

Not sure sierraeast's recommendation would work if your roof is sagging due to inadequate support. Keep in mind snow loading, if applicable, as well. Particularly with the shallow roof pitch.

Not an expert by any means...

Your problem reminds of a similar one that an acquaintance had with a newly-purchased house (WVA). Having no funds to spare for proper solutions, he put a bar about 10" above the eave peak and another along the gutter, and fastened white nylon tenting to drape over the roof. I don't know the details of how he fastened it, but it did survive numerous storms. Dropped the interior temperature about 15°.

Which reminds me of that house (in Georgia?) the architect designed with two roofs, one over the other, with airspace between. Effective natural cooling.

Seriously, if you are actually considering a total roof replacement, check into SIPs. I suspect that adding labor and materials together, they would be no more expensive than any other method of replacement, and considerably easier to do.

Many thanks to all of you for your great info! Core question for us is -- if we invest in this (regardless of which approach/materials we use) ... will it be worth it? That is, will it only reduce our interior temp by a few degrees (in which case, not worth it) - or does it have the potential to reduce the temp by alot (in which case, probably worth it). What do y'all think?

I would think it would be worth it ten times over. Just imagine living in an attic vs living in a home with a ceiling before the attic. Any insulation you can add up there be it a false roof or other would pay for itself over time. Not to mention the resale value should you ever decide to sell. You mentioned that the floor is not insulated? Is this a concrete slab or wooden floor over dirt? This could be another problem seeings how cold air falls.

A cost effective way to lower the solar gain from your roof is to install a light colored roofing material to reflect the sun's rays. You can also sheath the roof decking with reflective aluminum foil. There are foil products that cover plywood, or plywood that has foil pre-applied. Before taking that step, make sure that the roofing material you use can withstand the heat build-up from underneath.

On my new home I used the plywood roof decking with the attached radiant barrier. A lot of return for a small extra cost....

I'm considering using E-Barrier by Sherwin Williams. Has anyone used this product?

Here is a link that might be useful: Sherwin-Williams E-Barrier

Foil faced roof sheathing is only effective when there is a vented attic space, or when the rafter cavaties on a cathedral type ceiling has it's own indiidual air space, which the op's does not. Foil facing as an underlayment for the roofing will not be effective as well in the op's case. Running rafters, batts and draping radiant barrier down over the batts keeping an air space between the radiant and the new roof sheathing coupled with ridge/soffit venting would be effective as the hot air has to have somewhere to escape. Sips panels would need no radiant barrier.

You definately want a layer of vented air and insulation between the sheathing and the finished cieling. The current poor design is probably what warped and destroyed your current roof. On a sunny day, the temperature of those shingles increases 80-90 F above air temps with no where for the heat to go.

It would seem if you have to replace the entire current roof anyhow, most of the things you need to do to fix the roofing design problems will solve the heat problems. I wouldn't replace the roof the way its currently designed (without any ventilation). In developing a new roof design you can incorporate some type of ceiling insulation. I'm not sure what you mean buy building a false roof. Either way its a massive project, with numerous solutions depending on asthetics and other issues. How big is the cottage? I'd probably consider replacing the entire roofing system with a modern system.

Here is a link that might be useful: ventilating asphalt shingles,

Yellowpups, You didn't mention wanting to keep the look of the open rafter/t&g so im assuming you do inquiring about another topical roof system. If you dont mind covering up the existing ceiling, you can use the existing rafters and add insulation still incorporating an air space, ridgeventing and eave venting, covering the lid with drywall. You can also put radiant barrier in place. The best way would be to use styrofoam baffles against the roof sheathing in each rafter space, line radiant barrier on the underside of the baffles, insulate and drywall. You want to make sure the eave venting and ridgeventing isn't interefered with, but the baffles should make sure of that. You are planning a re-roof anyhow, so that would be a good time to ridgevent if you go this route. With that venting method you could also skin over the t&g with radiant barrier plywood,(techshield), and not have to radiant the underside of the baffles. just so long as you have that air space.

Will it be worth it? Definitely!

We did this exact thing on my sister's cottage. It used to be unbearably hot when the sun was beating down on the roof. So we added a false roof with rigid insulation and a layer of vented air and it is amazing the difference it made.

GREG H -- can you tell me what the construction was that you did on your sister's cottage roof? (i.e., did you use a water memberane on the plywood decking). Would you mind sharing what the total cost was? Sounds like your situation is exactly what we're looking at. Thanks!
SIERRAEAST - yes, we want to keep the exposed rafters and t&g ceiling, so insulating from inside is not an option. This is why we're exploring how to build up the roof on the exterior. But the est $$ we got from the builder was outrageous. We want to do a good job - but this is a seasonal cottage, and we can't break the bank. Then again, this current roof (not the shingles, but the roof decking and roof) has lasted about 50 years since it was orig built,...so maybe we're over-thinkg this?

I have 3,000+ sq ft ranch with Cathedrals on each end & Regular ranch roof in between with 5/12 Pitch. I chose to use Radiant Guard Ultra preforated w/ the black Plastic Vapor barriers you can get in HD (not the foam ones).
I installed the system myself a year ago using an electric stapler; 5/16 staples and the job went very easy. And in the regular ranch house I merely stretched the Radiant guard perpendicular to the joists & ran it the full length of the house , stapling it to the underside of the joists.
This year my electric bill with the AC on hasn't crossed 150. a month. Plus we find that the 3 ton A/C unit gets "too cool" with the programmable Honeywell thermostat at night . So we cut it off at 8PM & restart it in the morning, we have a Brick home and our setting is for 76 degrees. It can get too cold in here!!!
Have (8)1000 sq ft rolls of Ultra (preforated) left over if you want to tackle the job yourself & SAVE BIG $$$.

Here is a link that might be useful: Radiant Guard.com

Yellowpups, I have no idea what the cost was for my sister's cottage because I didn't pay for it. I was just cheap labour. I work for beer and steak.

Basically, the construction of the cottage has three main roof beams running the length of the cottage and the roof is made of tongue and groove 2x4s (or 2x6s, I forget which). There are no rafters. The shingles were placed directly onto the tongue and groove boards.

What we did:
(1) Stripped the shingles.

(2) Laid down a layer of rigid foam. This particular rigid foam had a layer of paper-thin fibreglass stuff on it. Not sure why or what it is called. As I said, I was just cheap labour not the brains. Tape the seams.

(3) Laid 2x4s flat on top of the foam, spaced 2 feet apart and running from eaves to peak. These were screwed down through the foam and into the tongue and groove boards. If you have rafters, you'd probably place these boards above the rafters and screw them down to the rafters. This will create the air path from the eaves to the peak on top of the foam.

(4) Laid down a layer of plywood.

(5) Add the soffit vents and ridge vent.

(6) Put down the roofers felt and shingles.

As I said before, it made a HUGE difference.

Maybe this has been mentioned but I would consider replacing the sheathing then applying 1 or more inches of polyisocyanurate insulation board then another layer of sheathing, then light colored asphalt shingles. (If you place it between thicker strapping you can create a 3/4" airspace so foil-backed sheathing will create a radiant barrier.) You could also use insulation laminated to sheathing board (nailbase). This gives you a good increase in R-value and doesn't require a new ceiling finish. Most asphalt shingle manufacturers accept unvented "nail-base" designs but check the specs to be sure.