You can seal the outside of conduit with closed cell expanding foam (as you suggest) or silicone caulking.
You can seal the inside of the conduit with silicone caulking or duct seal putty (as you suggest).
The conduit does not have to breathe or be open if the wiring is rated for exterior/wet, e.g. THWN. Wire that is THHN should not be in a wet location, and therefore the conduit is best kept open at the interior end.
When selecting a silicone sealer, note that
most sealers cure only to a limited depth, perhaps 0.5in. So capping a conduit should be done with at most 1in deep plug, allowing cure depths from both sides, the open end and the tube side. Those with a curing agent or accelerator will indicate so on the label, e.g. for
most silicone type sealers (this in contrast to acrylic or putty sealers) are corrosive curing unless noted otherwise on the label. The fumes released during the curing process may corrode the conduit or the wiring. This likely poses no problem in short open ended stubs of PVC conduit, commonly used to penetrate a house wall from an outside box. But for metal conduit (galvanized steel) and in junction boxes with exposed metal these sealants must be avoided, or -in the case of boxes- left to cure 48hrs with the box lid open.
This is a related and relevant answer, by "ConKbot" (2014):
Your normal every day RTV silicone is 'Condensation cured' where atmospheric moisture is what makes it cure. There are 3 general kinds. Acetoxy cure, which is the cheapest, bonds well, but releases acetic acid on curing (the acid ingredient in vinegar) And corrodes copper and lots of other metal. Even sealing an enclosure with it, the trapped fumes inside can corrode the items. The other two kinds are neutral cure, but still slightly different. Oxime cure which releases a ketone on curing. This can corrode copper on contact, but isnt as bad to use near electronics, just not directly on copper. The most desirable for electronics is Alkoxy cure, which releases methanol. Mind the fumes from all of them, but if you have a large surface area covered, Acetic acid fumes can be pretty nasty. Though your nose will give you plenty of warning that something isnt right.
This is a more thorough contribution by Dr. Barry L. Ornitz of Eastman Chemical Company Research (1997):
The corrosion is entirely dependent on the particular type of silicone sealant used, and most common ones generally available to the public still release acetic acid when curing.
To begin this discussion, however, it should be stated that silicone polymer is a term much like "plastic" - it covers a rather wide range of materials and properties. The chemistry of a particular polymer determines it characteristics, from hard and brittle varnishes, to soft and flexible rubbers. The initial work on polysiloxane chemistry dates back almost 60 years and it continues today, so "getting the formulation right" is hardly a meaningful expression.
Modern silicone elastomers are generally linear polymers of methyl silicone "oils" of extremely high molecular weight which will remain largely independent of one another but which may deliberately be cross-linked at a number of selected points. Two methods are commonly used to obtain such structures:
Benzoyl peroxide or other strong oxidizers can be used to oxidize side groups and so establish cross links.
Some vinyl or other organic groups groups can be incorporated into the siloxane structure, and then a catalyzed condensation polymerization of these is accomplished to establish cross links.
The first of these two methods is common in two-part silicone formulations such as potting compounds. In particular this is used in situation where a deep-section cure must be obtained.
The latter of the above two methods is generally found in the room temperature vulcanized (RTV) sealers generally available to the public. When the organic groups polymerize, a volatile species is generally released - the most common being acetic acid. Other species may be released if appropriate organic groups have been incorporated into the silicone structure; these include alcohols and even amines. These compounds are released as part of the cross-linking (curing) process. Most eventually evaporate from the cured silicone. They are NOT absorbed by the polymerization.
Single part silicone sealers generally contain a cross-linking catalyst (such as a tin compound) that is activated upon exposure to moisture in the air. These single-part sealers should not be expected to cure deeper than about a centimeter as the diffusion of moisture through the cured silicone, and the diffusion of the acetic acid or other compound out of
the curing silicone is limited by the thickness. To demonstrate this, leave the cap off a tube of silicone sealer for a few days. The silicone around the neck of the tube will be hard, but the rest of the tube will be uncured.
Since the acetic acid is released during curing, it can attack the underlying substrate material. This can cause corrosion of certain metals and prevent the proper adhesion of the silicone. However, on other materials, the acid can etch the surface slightly and increase the adhesion. Aluminum is one such material. Copper and zinc, however, are corroded by the acid. Thus brass and galvanized steel should not be used with silicones which release acid. Dissimilar metals can form electrolytic couples and corrode severely underneath a covering of acetic acid releasing silicone. Silicones do not adhere well to all other plastics either.
General Electric's Silicone Division can provide information about the compatibility of their products with different surfaces. GE manufactures silicones for industrial as well as domestic use. The following condensed table is from a GE publication on silicone sealants for domestic use. N-R means not recommended.
Adhesion Characteristics of General Electric Silicones
Commonly Available to Homeowners*
Silicone II Silicone Paintable
Rubber Sealant 5091
Alclad Aluminum Excellent Excellent Excellent
Anodized Aluminum Excellent Excellent Excellent
Brass Good N-R Good
Chrome Excellent Excellent Excellent
Copper Good Good Good
Galvanized Steel Fair N-R Fair
Carbon Steel Excellent Excellent Excellent
Stainless Steel Excellent Excellent Excellent
ABS Excellent Excellent Fair
Fiberglass Excellent Good Fair
Formica Excellent Excellent Good
PVC (rigid) Excellent Fair Good
LEXAN polycarbonate Good Fair Good
Vinyl (soft) Good Fair Good
Acrylic/Plexiglas N-R N-R N-R
Glass Excellent Excellent Excellent
Rubber (any type) N-R N-R N-R
* Note: The contractor grade silicone is similar to the Silicone II adherence properties, however, contractor grade silicone differs from Silicone II by having a longer tooling time, faster curing time and different formulations and ingredients.
GE Contractor Bath/Plumbing products, GE6040 (clear) and GE6070 (white) are specifically formulated to be non-corrosive to metals. In conclusion, using common silicone sealants around antenna connections is asking for trouble. Much like the Plasticizers in the vinyl jackets of coaxial cable, if you do not know they are corrosive, assume they are as the special non-corrosive grades are normally labeled as such.