If your franking machine has an automatic sealing unit, gummed envelopes are almost always the better choice, because the machine wets and closes every flap at full running speed. If you seal by hand, or you run a compact desktop machine with no sealer, self-seal envelopes will be quicker and far less messy. Both types frank in exactly the same way and both attract the same Royal Mail Mailmark prices, so this decision is about how your post gets closed, not what it costs to send.
It sounds like a small detail. In a mailroom sending a few hundred items a week it is anything but.
Gummed or self-seal: what is actually different
The envelope body is the same. The adhesive on the flap is not.
Gummed envelopes
A gummed flap carries a dry, water-activated adhesive, usually a starch or dextrin gum. It does nothing until it is moistened. That is a feature, not a limitation: the flap stays open and inert for as long as you need, then seals firmly the moment water touches it. Gummed envelopes are the traditional choice for high-volume mailrooms because they are designed to be sealed by a machine rather than a person.
Self-seal envelopes
A self-seal flap uses two strips of latex-based adhesive that bond only to each other under pressure. There is no water and no licking. You fold the flap, press, and it is closed. Self-seal envelopes are the practical option for offices where post is stuffed and sealed by hand throughout the day.
One terminology note, because suppliers are not always careful with it. A peel-and-seal envelope has a silicone release strip you remove before pressing the flap down; a true self-seal has two exposed latex strips and nothing to peel. Peel-and-seal holds strongest, self-seal closes fastest, and neither can be sealed by a franking machine.
Sealing speed is where the real difference shows
Most mid-range and high-volume franking machines include an automatic sealing unit: a small tank of fluid, a wick or brush, and a moistening path the flap passes through as the envelope feeds. Feed a gummed envelope in flap-down and the machine wets, folds and franks it in one pass. Feed a self-seal envelope through the same unit and nothing useful happens, because latex does not respond to water.
That is the crux of it. With a sealer, gummed envelopes turn a run of 300 letters into one continuous operation instead of 300 separate presses. Without one, gummed stock is the worse option, because moistening flaps by hand with a sponge is slow and unreliable.
The same logic applies to folder inserters. Those machines expect gummed stock and a moistening tank, which is why we stock sealing solution alongside our folding inserting machine envelopes. Tap water works in an emergency, but it leaves limescale in the tank and encourages mould in the wick, which is the sort of problem that ends in an engineer visit. Sealing solution contains a wetting agent and a biocide, so the gum activates faster and the tank stays clean.
If you are still choosing a machine, or you are unsure whether yours has a sealing unit, Mailcoms supplies and services franking machines across the UK. Green Franking supplies the consumables; Mailcoms supplies the hardware.
Cost: closer than the price list suggests
Gummed envelopes are usually a little cheaper per thousand, because the flap construction is simpler and no silicone release liner is involved. The gap is real but modest.
The saving that actually matters sits elsewhere, because postage dwarfs stationery on almost every mailing budget. Under the rates that took effect on 7 April 2026, a 1st Class letter costs £1.80 with a stamp and £1.77 franked with Mailmark, and a 2nd Class letter costs 91p against 88p franked. On a Large Letter up to 100g the gap widens sharply: £3.30 by stamp against £2.86 franked, a saving of 44p on one item. Getting the format right matters far more than the flap, which is why it pays to read our guide to letter and large letter size limits before standardising on an envelope.
So choose the seal type that removes labour. If gummed stock plus a working sealer saves twenty minutes a day, that beats a few pounds off the box.
Shelf life and storage
This is the argument most people have never heard, and it catches out businesses that buy in bulk. Gummed adhesive is essentially stable: kept dry, a box will still seal properly years later. Self-seal latex is not. It oxidises slowly and loses its grab, typically over roughly twelve to eighteen months depending on how warm and humid the storeroom is. Heat accelerates it, so a pallet next to a radiator or in a sunny window fails sooner.
So if you order envelopes a year at a time, gummed stock is the safer buy. And never stack anything heavy on self-seal boxes: sustained pressure can bond the flaps together inside the carton. Store flat, keep them cool, rotate oldest first.
Print quality and Mailmark scanning
Seal type has no effect on the franking impression, but envelope quality does. The Mailmark barcode is read automatically by Royal Mail's sorting equipment, and a poor impression can be delayed or surcharged. Smooth, uncoated white paper lets the ink dry quickly; glossy or coated stock smudges whichever flap you chose, and no cartridge will fix it.
Two habits help. Do not overfill, because a bulging envelope lifts away from the print head and produces a faint impression. And keep the ink in good order, since a tired cartridge is the most common cause of unscannable mail; our guide to franking machine ink cartridges covers what to look for. Where an item is too thick to feed, use a franking label rather than forcing it through.
The sustainability question
If you are trying to reduce the environmental weight of your post, gummed envelopes have a genuine edge. The gum is a plant-derived starch and the envelope recycles with the paper without complication. Self-seal envelopes carry a synthetic latex adhesive, and peel-and-seal versions add a silicone-coated release strip that is thrown away at the point of use. Small per envelope, but multiply it across a year of mailings.
Paper weight is the bigger lever, though. Dropping from 100gsm to 90gsm on a large mailing cuts material use and occasionally the postage band too. Pair that with recycled-content paper and cartridge recycling through our ink refill and recycling service, and the mailroom becomes measurably lighter on resources. More on that in our guide to sustainable mailing for small businesses.
A short decision guide
- Franking machine with an automatic sealer, or a folder inserter: gummed, every time. Keep sealing solution in stock.
- Desktop franking machine with no sealer, sealing by hand: self-seal or peel-and-seal.
- Bulk buying twelve months of stock at once: gummed, for the shelf life.
- Confidential or high-value contents: peel-and-seal, for the strongest bond and visible tamper evidence.
- Sustainability is a stated priority: gummed, on recycled paper.
- Mixed volumes and ad hoc post: keep both. Self-seal DL for the front desk, gummed for machine runs.
Whichever you land on, buy envelopes that are accurately squared and cut for machine feeding. Badly trimmed stock jams, and a jam costs more in downtime than the envelopes ever saved. Browse our franking machine compatible envelopes, and see the full current tariff published by Royal Mail.
Frequently asked questions
Can a franking machine seal self-seal envelopes?
No. A franking machine's sealing unit works by moistening a water-activated gum strip. Self-seal and peel-and-seal flaps use a latex adhesive that bonds under pressure and does not react to water, so the machine will frank the envelope but will not close it. You will need to press each flap by hand.
Do gummed and self-seal envelopes cost the same to post?
Yes. Royal Mail prices by format and weight, not by how the envelope is closed. A standard letter up to 100g costs £1.77 franked with Mailmark at 1st Class and 88p at 2nd Class under the rates effective from 7 April 2026, whichever flap type you use.
How long do self-seal envelopes last in storage?
The latex adhesive on self-seal envelopes typically holds its strength for around twelve to eighteen months, and less in a warm or humid store. Gummed envelopes have no meaningful shelf life problem provided they are kept dry, which makes them the safer choice if you buy in large quantities.
What can I use in my franking machine's sealing tank?
Use a purpose-made sealing solution rather than plain water. Sealing solution activates the gum more reliably, contains a biocide that stops bacterial growth in the wick, and avoids the limescale build-up that tap water leaves behind in the tank and moistening path.
Are gummed envelopes better for the environment?
Generally yes. Gummed flaps use a plant-based starch adhesive and recycle cleanly with the paper, whereas self-seal envelopes use synthetic latex and peel-and-seal versions add a silicone release strip that is thrown away. Choosing recycled-content paper and a lighter paper weight makes a larger difference again.