Using for loft at cottage. Fits between rafters. Using a 10-inch charcoal gel-foam mattress (bought through Wayfair) for main twin and currently using use a 10 inch one with sheets and light cover if you wiggle jiggle OR slightly lift up main to roll out the trundle). With the 10 inch the siderails are just about 2-3 inches higher than the mattress, which barely keeps the pillow there if a violent sleeper. Anyhow, bought two of these trundle sets for this loft space. Its just the right mix of modern and rustic and wont worry so much about humidity and heat or cold with the materials.