Covered knees = knee pain?

worth speaking to a phyiso about this? its really unsual to have such pain as a 'warm' muscle

As some others have said maybe try warmer tights.  I suffer a lot of knee pain, i wear Castelli I find them pretty good.  If it's bothering you, maybe get your Knees checked out in case there is a underlying problem.

Yes I agree that your conclusion is correct, I have had the same issues and come to the same conclusion.  Wearing looser tights or cycling pants, or just shorts, no knee pain, but with tight fitting tights, after 20-30+ km the discomfort will start.  So I've stopped wearing classic cycling tights.  If you notice orthopedic knee braces have a round cutout in the middle where the kneecap is.  Just hope that there hasn't been any permanent damage.  I ride year round and so was in tights 6-8 months of the year.  Probably not many people notice the phenomenon as most people, even professionals, don't ride much in cooler where you're going to be in tights all the time.

Agree with last comment - seems like it is coming from the knee cap being pushed onto the joint behind. Is your pain coming from behind the kneecap?

One solution may be to try cycling trousers rather than tights. You can get some fairly good ones from manufacturers like Pearl Izumi, which allow bib shorts underneath, have a longer backside, are stretchy, and remain fairly narrow in the leg while not being compressive

Sounds like it may be the compressive force of the tights putting pressure into your patella. This in turn may cause undue friction on the articular surface of the patella and femur causing discomfort and pain. Much the same as if you had tight quads.

The compression bit's curious, but as a matter of elimination could you beg/steal/borrow some windproof tights to try out ? I never have a problem with roubaix longs on an upright, even in snow + ice, but on a recumbent I seem to get a lot more windchill and achy kneecaps. On one particularly cold march Friday-Night-Ride-To-The-Coast, London to Southend, I just couldn't get any warmth into them, took a day or two to stop aching. Solution seems to be gore windstopper-fronted longs.

I have this issue too, no pain when in shorts and sometimes pain when in longs. It's as though the material is compressing the knee cap and it feels like it is bruised, but lifting the material off is instant relief!
Someone suggested to me it was perhaps linked to hydration, which does seem to have some truth in it as I have noticed less "yellow straw" coloured when suffering.

I have the weather issue in one knee, and this is can be worse with the knees covered. The reason for this is that the knees get wetter as the tights hold water (or sweat) in contact with them. However if you had this problem you would probably have noticed off the bike already.

Are you wearing tights over your shorts? This could be raising your position higher than is good for your knee. Or if your tights pads are thicker than your shorts pad, then the same effect is possible

I have a bad knee and noticed whilst doing the festive 500 that if I pulled the tights away from the painful area it would immediately go away. As if it was bruised and the tights were applying too much pressure.

it would be interesting if the OP has tried riding in "tights" conditions (by his reckoning, which is all that counts) in his shorts? tights shouldn't (unless sold as compression wear) be tight enough to be causing any restriction

Try leg warmers/knee warmers with the shorts you prefer.
Maybe Castelli Nanolight leg warmers for example

I think the most likely explanation is that you wear tights when it's cold, and your knees hurt when it's cold. Try warmer tights perhaps?

March Knee!
I get a pain in my knees when it's cool and shorts are on. Someone explained it as the cold air rushing to the back of the knee and tighening the ligaments. As result unless it's 12 deg or above the shorts stay inside.There is an element of different clothing as some pads are thicker than others, especially a set of well worn ones. This may alter your saddle height by only a few millimetre but it could enough if you are already on the upper "limit".

Other than that it's a multitude of possible sins.
Best advice I've found is Steve Hoggs way of raising and lowering the saddle by 3mm. Well worth checking his website. Gearing, some of us using compact chainsets tend to spin a little too small a gear, some using 53/39 may tend to grind too high a gear slowly. Either way can lead to over use of the knee and subsequent pain. Find out if your right leg is the same length as the left, apparently most people have one leg longer than another. If so try extender plates on the soles to even out.

Another thing could be the float on cleats. "Normal" pedals like Shimano, Look etc play hell with my knees, so I switched to Speedplay and will not go back. Some people are able to maintain a static postion, other of us move around alot on the pedals.

Odd, maybe trying a couple different pairs of tights to see if its still a problem. sometimes the smallest adjustment can cause an issue.