改为定语从句最保险：between可以表示三者以上每二者之间。

The slits between XPS boards, which are difficult to seal, will have a great influence on the thermal insulation performance.

另外，have an influence on 带有褒义，通常表示良性影响。如果热绝缘是有利的因素，则用affect（负面影响）较好。

The slits between XPS boards, which are difficult to seal, will affect the thermal insulation performance.