It is always a problem trying to pick the right warmth horse rug for the conditions of the day. Just because it is winter does not mean every day is going to be cold, some days will be very warm some very cold. It is even harder to predict between seasons when the weather can quickly turn hotter or colder.
Without having a tack room full of different weight winter horse rugs then what can you do.
One solution is layering your rugs.
By having available a range rugs that can be added in layers you have the flexibility to match the conditions on any day. The range of horse rugs needed really depends on the climate in your area and the extremes in temperature you experience.
In a mild climate you may just need a cotton rug, a polar fleece or wool rug and a waterproof synthetic rainsheet. In colder climates you would add a waterproof synthetic blanket.
In colder times of the year this range of rug provides options like below:
Icy cold day - Waterproof synthetic blanket over a padded underblanket over a polycotton rug
Very cold day - Waterproof synthetic blanket over a polar fleece over a polycotton rug
Cold day - Waterproof synthetic rainsheet over a polar fleece over a polycotton rug
Cool Day - Waterproof synthetic rainsheet over a polar fleece rug
Warm day - A polycotton rug
These are just suggestions and there are many different other combinatiosn you can try. Another advantage of layering is that the layers trap air and provide a higher level of insulation and warmth than a single rug alone.
Another variable is the blanket thickness and also whether you purchase a rainsheet with a polar fleece lining. GoHorse has a range of winter rugs with 200g polyfill and 300g polyfill padded blanket lining and also has a range of waterproof synthetic rugs with polar fleece lining. These rugs provide even more options to mix rugs so your horse is not too cold but not too hot - just right whatever the temperature of the day.