by R. Daniel Mann
Question: Is there a need for a mechitza between men and women when there is no minyan?
Answer: We must start our answer with some sources that serve as the basis for the need for a mechitza. Most explicit discussions on the matter are relatively recent, as the mechitza was taken for granted without halachic discussion until the 19th/20th century.”
The gemara (Sukka 51b) tells of structural changes made in the Beit Hamikdash to deal with the growing realization of problems of modesty between the genders. Rav Moshe Feinstein (Igrot Moshe, Orach Chayim I:39) is prominent among those who learned from the fact that such changes in the Beit Hamikdash are generally prohibited that the need for separation must be a matter of Torah law.
The only context in which there is any Orthodox unanimity that a physical separation is necessary is when davening in shul. It appears that the concept need not be linked specifically to davening, as the gemara says that Beit Hamikdash renovators based themselves on a pasuk relating to a funeral (Zecharia I:28:12). On the other hand, in practice there is not a history of anything close to universal separation between the genders. Rav Moshe (ibid., OC V:12) makes a distinction between settings that are private (i.e., by permission only), which do not require separation, and those that are open to the public, which require.
Since the setting of davening in shul is unique in its unanimity and its level of definitiveness, it is worthwhile to investigate the halacha’s scope by broadening your question. Does all tefilla require a mechitza? Does everything in shul? How do we define a shul? A man is not allowed to daven, learn aloud, or even make berachot when exposed to a lack of modesty (see Shulchan Aruch, OC 75 with commentaries). However, it is agreed that regarding davening in a place that is not set for tefilla, the formal requirement of mechitza per se does not exist. This is more obvious in a public place, like a plane. The need for a mechitza is more of an obligation to put one in the proper place than a prohibition to daven without it. Therefore, since there is no way to expect an airline servicing Jews and non-Jews to put up a mechitza, there is no problem. Even in places like sheva berachot and a shiva house, there is not a formal need for a mechitza (see Igrot Moshe ibid.).
If men are davening in a shul at a time when there is no minyan, it would seem that a mechitza is needed if women are present (one or two women are likely not a problem (see ibid.; Ishei Yisrael 9:28)). After all, they are davening and the shul has sanctity that elevates tefilla even without a minyan (see Shulchan Aruch, OC 90:9).
What about a place that is set for tefilla without a minyan? The gemara in Megilla 27b can be instructive. In explaining the various positions on whether a communal beit knesset can be sold to become a beit knesset of an individual, the gemara raises the claim for R. Meir that an individual’s shul does not have kedusha. Rashi (ad loc.) and others explain that this is because matters of kedusha (i.e., elements of prayer that require a minyan) are not recited there. On one hand, this downplays the status of a shul without a minyan, but many posit that even according to R. Meir it has some kedusha (Ramban, ad loc.) and at least the status of a beit knesset. We note that many places that have semi-regular davening but without a minyan usually have several other uses, which also makes it less like a classic shul, in which we know a mechitza is required.
Tying things together, we suggest the following approximate guidelines (there are many slightly varying cases). In a room that is treated like a shul, just that it belongs to such a small community that there is not usually a minyan, there should be a mechitza. In a multi-use room that has semi-regular davening but without a minyan, davening should be done with a separation between men and women, but a mechitza per se is not necessary (assuming it is done in a way that there are no modesty in dress problems).