It will ferment faster, and if temps are too high, certain beneficial bacteria will die off. I think 75 is the top for optimal results.

That said, if you find a clean spot on a cold basement floor, you can probably get the temp down. We fermented some beer this summer in 95 plus degree weather, but the temp on our crawlspace floor kept the carboy at a very consistent 68-70.

Per NCHFP:

Store at 70 to 75F while fermenting. At temperatures between 70 and 75F, kraut will be fully fermented in about 3 to 4 weeks; at 60 to 65F, fermentation may take 5 to 6 weeks. At temperatures lower than 60F, kraut may not ferment. Above 75F, kraut may become soft.

Dave