The slits among XPS boards, difficult to seal, will have a great influence on the thermal insulation performance .
这是我一个人感觉可以这么写的，却找不到依据，还请老师批改一下!
difficult to seal可以这么用的？是修饰slits吧!
改为定语从句最保险：between可以表示三者以上每二者之间。
The slitsbetween 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，一般表示良性影响。假如热绝缘是有利的原因，则用affect（负面影响）较好。
The slitsbetween XPS boards, which are difficult to seal, will affect the thermal insulation performance.
