Hi iggy,
Welcome to the Mattress Forum! 
This answer might be a bit more detailed than what you wanted, but I’ m hoping to assist you in optimizing your sleep environment as much as possible in preparation for summer.
While it’s not always possible to track down temperature regulation issues for any particular person on a specific mattress because there are so many variables involved (including your room temperature and humidity, your sheets, your bedding and bedclothes, your mattress protector or any mattress pads you are using, the firmness of your mattress, and where you are in the “oven to iceberg” range) and some people can sleep warmer on mattresses that most people are generally fine with … there is quite a bit of information about overall temperature regulation in post #2 here.
In very general terms … the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material.
Talalay latex tends to be more breathable than Dunlop latex. There are also variations in each category and less dense foams tend to be more breathable than denser foams while firmer foams tend to allow less sinking in which can mean there is less insulating foam material against your body. While the upper layers of a mattress are the most significant part of temperature and moisture regulation … deeper support components that allow more airflow can also have an effect and so innersprings will also tend to sleep cooler than foam support cores as long as the air can ventilate to the outside of the mattress.
All foams are insulators (rather than heat conductors) so to some degree they will all be warmer than mattresses that contain no foam at all (such as mattresses that only have an innerspring and layers of natural fibers on top), and this may be the information that you previously read on the forum.
Some of the other factors involved in how warm a mattress sleeps are how closely the foam conforms to your body (the more closely it conforms around you the more insulating it is), how soft or thick the foam in the comfort layers are (the softer/thicker it is the deeper you will sink into the more insulating materials), the type of quilting used in the mattress (natural fibers allow for more airflow and humidity control which translates into better temperature regulation), the type of ticking (cover) used (natural or more breathable fibers such as cotton or viscose or even some of the more breathable synthetics will wick away moisture and ventilate better and humidity control is a key part of temperature control), and any cooling technologies used in the mattress such as ventilating and moisture wicking materials, heat conductive materials, or phase change materials. Additionally, having the mattress elevated off of the floor will promote more air circulation.
In addition to this … the mattress protector you choose along with your sheets and other bedding and what you wear when you sleep will also have a significant effect on temperature regulation because they can either add to the insulating effect or to the ventilating and moisture wicking effect of your mattress. You can see more about the effect of different mattress protectors in post #89 here. Bedding made from natural fibers or viscose materials (like bamboo) will also tend to be cooler than synthetic fibers and linen sheets along with silk are probably the coolest of all the natural fibers for those where sleeping temperature is a main priority. There is more about sheets and bedding in post #7 here. In many cases manipulating the mattress protector, sheets, or bedding to cooler versions can make “enough” of a difference for many people who would otherwise sleep hot on a mattress.
And all of this is separate from any environmental conditions in the bedroom (temperature and humidity levels with higher humidity adding to the perception of heat), on the physiology and tendency of the person themselves to sleep warmer or cooler and where they are in the “oven to iceberg” range, and on their weight and body type which will affect how deeply they sink into the foam layers of the mattress. As you are eschewing air conditioning, humidity in the summer can have as great an impact upon your sleeping comfort as the temperature.
Ventilation and moisture wicking transports humidity and moisture (and the heat it contains) away from the body and either stores it inside a natural fiber (like wool) and away from the body or releases it to the surrounding atmosphere where it is dispersed. The water vapor is what transports the heat away from the body. If the water vapor and moisture is transported and/or stored away from the body, then the humidity levels closer to the body are lower and more water vapor can be produced (perspiration) in response to excess heat which keeps the cycle going. Lower humidity levels next to the skin are more cooling than higher humidity levels (just like going outside on a hot humid day feels hotter than going outside on a hot day with the same temperature where the relative humidity is lower). This is the reason that wool is a great temperature regulator because it traps air which is an insulator when it is too cold but also allows humid air and moisture to be transported away from the body or stores moisture inside the fiber which keeps it away from the skin and keeps the cooling cycle going (which is why it is used in both cold and hot climates).
In summary … it’s always a combination of several interacting factors that determines the sleeping temperature of a mattress in combination with a specific person and environment. Overall, the innerspring with wool on top (provided the wool wasn’t too thick or soft, enveloping you) would generally provide the best air circulation for you.
Subject to first confirming that any retailer or manufacturer on the list that you wish to visit is completely transparent (see this article ) and to making sure that any mattress you are considering meets the quality/value guidelines here … the better options or possibilities I’m aware of in and around the Montreal area are listed in post #276 here . I can’t keep track of the inventory in each store, so you’ll have to do some phoning to find out which stores currently might offer the items you are considering.
A forum search on Montreal (you can just click the link) will also bring up more comments and feedback from other members in the area that may be helpful as well.
I hope that information about temperature regulation (and some of the many factors that influence it) is helpful.
Phoenix