Is that open cell foam , or closed cell foam , can you suck and blow through it / breath through it ?
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I would bet that insulation type rockwool is not food grade and probably has unwanted chemicals in it. It would not necessarily affect the growth of the plant negatively, but the plant would become unhealthy to consume. Also, cultivation grade rockwool is relatively cheap, if it is more expensive that insulation rockwool, then that would be because it HAS to be made as it is made.
i think the grow Rockwool is rinsed more that’s all ie less chalk lime making it more stable for growing straight from the bag although it still needs a soak and good flush
@ifish Yes, its open cell… we can blow through it.
@zem its not about be more expensive or no… i just had a lot of this rockwool and i’m not using so just i want to try it.
i let the rockwool 3 days in water to “clean it”… its just a experiment… lets see how will be the end of this history 
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to experiment is ok as long as you dont flower harvest and smoke from such material. All around you, is material that could probably grow a plant’s roots however, they are all toxic for humans to eat.
what’s in builders Rockwool that’s toxic @zem ? Acustic just means it’s denser , and in fire situation with builders Rockwool ( all Rockwool just two ingredients ? ) no toxic released , as none there to begin with ? Same deal with perlite bought from builders store v perlite from grow shop kind of , just paying more for slightly better quality and cuz growers are willing to spend a lot more money
: )
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Construction rockwool is water-repellent… That would be my main issue. It won’t hold that much water (if any) as RW from grow shop.
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i did asked myself exactly this
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@LemonadeJoe I work in a builders yard , we sell Rockwool and with a leak in store roof the ones that get dripped on or sit in puddle deffo get/absorb a lot of wet , breaking it into chunks will also rearrange the flow direction of the fibres in pot helping even more absorption plus adding extra air
If worried about absorption levels , add some yucca to the first few feeds or when pre soakin
Apart from stabilising ph in ( grow ) Rockwool , the pre soak is there to also break water tention or grow Rockwool won’t perform as well , this must be done even tho it’s grow Rockwool
It’s the stone/ Rock in Rockwool that makes it water resistant and fire retardant not chemicals @zem
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I’ve opened roofs before to fix leaks and the rock wool insulation rolls have soaked in the water and held it like a sponge. If it hadn’t been absorbent the ceilings would have likely collapsed. 
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