Hi frostfire,
While it’s not possible to quantify the sleeping temperature of a mattress for any particular person with any real accuracy because there are so many variables involved including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials … there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
Support systems that don’t provide for any airflow under the mattress can be one of several risk factors in terms of developing mold or mildew under the mattress (see post #10 here) but it’s unlikely that it would be a significant issue with temperature regulation since the layers that are closer to your body will have a much bigger effect on your sleeping temperature than the deepest layers of your sleeping system.
I’m assuming that you’ve read the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that you will need to make the best possible choice but two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
The tutorial also includes several links to several lists of the better online options I’m aware of including a link to a list of most of the “simplified choice” mattresses (in the optional online step) that include a range of different types and categories of mattresses that would be inside your budget range. Many of them (besides the simplified choice mattresses) also have very good return or exchange policies.
Testing mattresses locally has the advantage of being able to compare many different types of mattresses based on your own personal experience so you can see which one is a better “match” for you than the others based on your own personal experience rather than just trying one mattress and not knowing how it compares to the other mattresses that you could have tried.
When you can’t test a mattress in person though then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
The better options or possibilities I’m aware of in and around the Seattle area (subject to making sure that any mattress you are considering meets the quality/value guidelines I linked earlier in this reply) are listed in post #2 here.
Phoenix