Does anyone know if ventilating the roof cavity is effective for cutting down on cooling bills? Every time I climb up inside the roof, it is hot as hell up there. It seems like a bit of ventilation would cool things down a lot. Kind of like the image below, with the air coming up from under the eaves and then out through the gaps in the ridge:

Some ventilation would certainly help.

The roof cavity should already be vented to some degree, the usual extra option is louver opening each end of a selected roof area or installing extraction fans on timers.

6" to 12" of insulation on the ceiling below is the most effective way of keeping the cooling bills down.

Ventilation is necessary for a number of reasons.  But, it really doesn't matter how hot the attic space gets if there is no heat conduction through the ceiling.  If your ceiling feels warmer than the room on those hot days, that's why.  The best way to prevent that is a reflective radiant barrier.  In my house, I got some "insulation" 90cm x 3m with maybe 3cm of insulation (an air gap is necessary) between the reflective surfaces.  That helped a LOT.  IMO additional insulation is superfluous as it does nothing to reflect the heat - cold climates different story.

34 minutes ago, bankruatsteve said:

Ventilation is necessary for a number of reasons.  But, it really doesn't matter how hot the attic space gets if there is no heat conduction through the ceiling.  If your ceiling feels warmer than the room on those hot days, that's why.  The best way to prevent that is a reflective radiant barrier.  In my house, I got some "insulation" 90cm x 3m with maybe 3cm of insulation (an air gap is necessary) between the reflective surfaces.  That helped a LOT.  IMO additional insulation is superfluous as it does nothing to reflect the heat - cold climates different story.

? If you are only trying to keep the heat out I agree the additional insulation is not required, but if its hot I use AC so the insulation is working in both directions - cool air in, hot air out.

43 minutes ago, bankruatsteve said:

IMO additional insulation is superfluous as it does nothing to reflect the heat - cold climates different story.

It's not unnecessary CGW is correct and it's proven, if the fibre is wrapped in foil and rolled out over a ceiling area there is a cooling difference as well as noise.

10 minutes ago, CGW said:

? If you are only trying to keep the heat out I agree the additional insulation is not required, but if its hot I use AC so the insulation is working in both directions - cool air in, hot air out.

Well, if that's good for you, there you go.  I've never been worried about cold conduction through a ceiling myself.

I am going to disagree with everyone I think.

I have vents in the roof, those fancy gable louvres....absolutely pointless and do nothi g but provide access for driving rain.

i fitted them to obviously aid crossflow but I have been here long enough to realise that wind of any magnitude only ever comes as a prequel to rain.

i have never had a day where its like the uk.....windy every day.

So heat that builds up in a roof is going to stay there until it gradually dissipates at 3am every morning.

 

Keeping the heat out in the first place is the better solution, foil under your tiles, foil wrapped insulation on a ceiling or failing that the mechanical extract fan...but vents...ok for decoration

 

13 minutes ago, eyecatcher said:

i fitted them to obviously aid crossflow but I have been here long enough to realise that wind of any magnitude only ever comes as a prequel to rain.

You question the Mythical "cross breezes" ? open up the windows and despite it being 38c outside with humidity ~75% the whole house cools down!

Your not alone! they have evaded me for many years, occasionally (very!) there is a nice cross breeze, open up the windows, close the insect screens, they then block any semblance of a cross breeze! & your back to cooking!!