Yes, thermal resistance/conductance is a two-way street, for passive conductive cooling devices, with equal speed limits posted for both directions.
OK, now for the caveats: If your heat sink is rated for forced air flow, and you have no forced air flow, then the heat flow through the heat sink will be different. Your heat sink is rated for certain conditions at both ends. If you meet those conditions (at both ends), except for reversal of temperature conditions, then you can expect equal but opposite heat flow.
Say that your 10C/W heat sink is rated 10C/W for conduction of heat from a 1W source to still air, with the contact area with air being fins. Now, you put those fins INSIDE your enclosure, in contact with still air, and you place the outside end of your heat sink in contact with a device (say, a cold plate) that will keep that end of the heat sink 10C cooler than the inside air. In that case you will get 1W of energy flow from the fins of the heat sink to the cooling device (cold plate).
You would want to pay attention to such things as: Warm air rises and cooler air falls. Air fins are most effective when hot air can rise from them and allow cooler air to come in contact with the fins. Cooling fins, on the other hand, would be more efficient when placed such that cooled air can fall away from them.
Total thermal resistance from junction to air is
Rja = Rjc + Rcs + Rsa.
Where: a = air, j = junction, c = case, s = (heat)sink
Rjc & Rsa are reasonably fixed by component choices.
Rcs is somewhat more variable - see below.
If your data sheet does not give junction to case thermal resistance (usually Rjc or similar)* then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.
Your comment about varying with heatsink suggests you don't really understand the subject. Finding one of the many many good tutorials on the net would be advised.
Thermal resistances are like electrical resistance - they can be added in series.Heat flows from junction to case outer, from case to sink (cia washer and thermal paste, and from heatsink to air. So you get
Rthermal = Rjc + Rcs + Rsa
Trise jc = Rjc x Watts etc
Trise = Tjc + Tcs + Tsa
see prior paragraph for meanings.
Rjc is specified by the device manufacturer.
Rcs is set by thermal washer material, heatsink compound, pressure of mounting etc. It should be a minor contributor.
Rsa is set by heatink design and size, air flow etc.
Temperature rise = Rja x Watts.
Search eg www.digikey.com for TO220 and look at a few Tjc ratings.
This datasheeet
http://ww1.microchip.com/downloads/en/DeviceDoc/22057a.pdf
gives an unusually large number of Rjc and Rja ratings for various package versions of the same part and is a good starting point.
For the TO220 package Rjc is 2 C/W.
Rja here is from about 30 to 60 C/W but the heatsinks are "wimpy".
Large heatsinks with fans can be around 1 C/W - much less with much care.
A well heatsunk TO220 can manage Tja = 10C/W without too much effort.
But it can be far worse without due effort.
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Edited:
The initial paragraph originally read as follows. It is evident that I wrote "case to sink" but intended "junction to case".
If your data sheet dies not give case to sink thermal resistance (usually Rjc or similar) then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.
Best Answer
According to the note just below the thermal data table from that datasheet:
The way I interpret that:
So if you are attaching the heatsink in the normal way, on the back, against the pad, use the junction to pad thermal resistance.