The answers you have gotten are right on the mark.  As one of the  authors of the AES and ISO published Standards on magnetic tape storage, I can affirm that there was consensus by all of the experts on the Technical Commissions that wrote the Standards that storing magnetic tape, long-term in plastic bags, is not a good idea (for a number of reasons).  The one exception I would note is, in specific circumstances (similar to what Richard mentioned), it can be beneficial to store magnetic tapes in sealed plastic bags containing non-dusting packets of desiccant (such as silica gel crystals).  This allows for a low-humidity micro-environment around the tape regardless of the environmental conditions in the vault.  Note: this practice is NOT referenced in the Standards but has proven beneficial in a multitude of test environments.

Peter Brothers
[log in to unmask]
Audio and video restoration and re-mastering since 1983

-----Original Message-----
From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of Mark Campbell
Sent: Wednesday, December 13, 2017 12:07 AM
To: [log in to unmask]
Subject: [ARSCLIST] Plastic bags around tape reels inside cardboard tape box

I have a question regarding dew point and open reel tape playback and subsequent vault storage.

It is a given that playback of tapes will occur in temperature and humidity conditions that are far from ideal for tape storage...(ideal being something approaching 16 deg C and 30% RH and a good studio for playback is set to be comfortable for humans not tapes  ---22 deg C and  60% RH)

The problem thus arise:

If after playback of a 10" master tape...I dutifully place in back in its plastic bag and then into its cardboard tape box doing so...I have capture a bubble of air which surrounds the tape inside the bag which is at 'studio conditions" (60%RH)

That tape (well bagged and boxed) goes to an hypothetical cold store at (8 degrees 30% RH)

My question is...will the plastic bag hold that 'studio-air' around the tape...and cause dew formation?

And if so...

Would it be best not to use the plastic bag at all...and allow the tape to 'breath' in its cardboard tape box?

I hope to hear from you soon.

Mark Campbell