1K or 2K construction foam – which assembly foam suits your building site?
Contents
What you need
Project template “Door frame and installation with PU foam”In short: 1K construction foam cures with the humidity of the air, 2K foam through the chemical reaction of both of its components. That is why 2K is independent of weather and room climate and very quickly dimensionally stable – for door frames, stair treads and everything that has to be true to size after minutes rather than hours. For large quantities of joints, cavities and window installations with a time buffer, 1K gun foam is the higher-yielding choice: 500 ml of Soudafoam X-tra 750 delivers around 35 litres of foam, while a 400 ml 2K can is enough for one to two door frames.
The decisive difference: where the curing comes from
1K foam is a moisture-curing polyurethane foam. It draws the water it needs for curing from the air and from the substrate. That works reliably – but it takes time, and it gets slower the drier and colder it is. According to the manufacturer, the Soudal Soudafoam B2 straw foam is tack-free after approx. 20 minutes, cuttable after approx. 45 minutes and only loadable after approx. 24 hours.
2K foam brings the second component along in the can or cartridge. It is mixed on activation or in the static mixer, and the reaction proceeds independently of the ambient humidity. The Ottopur OP920 is as a result cuttable after approx. 9 minutes, can have its bracing removed after approx. 30 minutes and is fully loadable after approx. 3 hours (at a 20 mm bead thickness). That is the real reason for 2K: not a better foam, but a faster, weather-independent building sequence.
The two candidates compared directly
Soudal Soudafoam X-tra 750 gun foam B2, 500 ml
1K – moisture-curing, for the foam gun
- Yield approx. 35 litres from the 500 ml can
- Skin formation after 6 minutes, cuttable after approx. 15 minutes
- Bulk density approx. 20 kg/m³, compressive strength approx. 38 kPa
- Thermal conductivity 0.037 W/(m·K), joint sound insulation up to 62 dB
- Building material class E to EN 13501-1, corresponds to B2 to DIN 4102
- Fill joints only about one third – the foam expands afterwards
Soudal Soudafoam 2K assembly foam B2, 400 ml
2K – cures independently of the humidity
- Cuttable after just 20 minutes
- Pressure- and stress-free curing – important with delicate frames
- High dimensional stability without volume shrinkage
- Adheres to almost all building substrates – except PE, PP, PTFE and silicones
- Building material class B2 to DIN 4102-1, also suitable as bathtub-setting foam
- Can contents are enough for one to two door frames
Detailed comparison 1K versus 2K
| Criterion | 1K gun foam (X-tra 750) | 2K assembly foam (Soudafoam 2K) |
|---|---|---|
| Curing mechanism | Moisture from air and substrate | Chemical reaction of both components, independent of moisture |
| Cuttable after | approx. 15 minutes | approx. 20 minutes |
| Fully loadable | Only after hours up to a day (1K straw foam: approx. 24 hrs.) | Depending on the product in minutes to hours (OP920: approx. 3 hrs.) |
| Degree of joint filling | approx. one third | approx. 50 percent |
| Application temperature | Can +5 to +30 °C, substrate +5 to +35 °C | Can +15 to +25 °C, substrate +5 to +35 °C |
| Yield | approx. 35 litres from 500 ml | one to two door frames from 400 ml |
| Tool | Foam gun | Can with adapter or 2K cartridge gun |
| Fire behaviour | B2 to DIN 4102 | B2 to DIN 4102-1 |
| UV resistance | Not UV-resistant – cover or paint over outdoors | Not UV-resistant – can be painted, plastered and bonded over |
The 2K foams in the range differ considerably
2K is not the same as 2K. Between the can and the double cartridge lie worlds – above all in the time you have to work.
| Product | Soudafoam 2K, 400 ml | Soudal 2K door-frame foam, 210 ml | Ottopur OP920, 400 ml |
|---|---|---|---|
| Pack | Can with adapter | Double cartridge 2 × 105 ml with static mixer | Can with adapter |
| Cuttable after | approx. 20 min. | approx. 5 min. | approx. 9 min. |
| Bracing can be removed after | No manufacturer figure | approx. 20 min. (at +20 °C / 65 % r. h.) | approx. 30 min. |
| Fully loadable | No manufacturer figure | No manufacturer figure | approx. 3 hrs. at 20 mm bead thickness |
| Yield | 1–2 door frames | approx. 5 litres per cartridge, 1–2 door frames | approx. 10–12 litres per can |
| Application temperature | +15 to +25 °C (can) | +10 to +30 °C | Process within approx. 4 min. after activation |
| Special feature | Also as bathtub-setting foam | Propellant-free, no dampening of the substrate needed | λ 0.026 W/mK, EMICODE EC1 Plus, max. joint width 30 mm |
The double cartridge is the fastest way – and the least forgiving: pauses of more than 10 seconds let the foam cure in the static mixer. Anyone working with it lays everything out beforehand, wedges the frame beforehand and foams at least six fixing points in one go. With the OP920 about 4 minutes of working time remain after activation, after which the can is lost. With its 20 minutes until it can be cut, the Soudal 2K can is the most relaxed of the three.
Recommendation by application
- Door frame, stair tread, window sill: 2K. Pressure- and stress-free curing and quick dimensional accuracy are exactly what you pay the premium for here. For tight scheduling the 210 ml double cartridge, for calmer working the 400 ml can.
- Window installation with a normal building sequence: 1K gun foam. More material for less money, and the curing time does not matter if work carries on the next day anyway. Details in the guide Which PU foam for window installation?
- Large cavities, wall openings, pipe penetrations: 1K. With these quantities it is the yield that decides, not the curing time.
- Dry heating air in winter or very dense substrates: 2K. This is exactly where 1K struggles, because the humidity needed for curing through is missing – the most frequent reason why PU foam does not cure.
- Increased fire protection requirement: Neither the Soudafoam 2K nor the OP920 goes beyond B2. If B1 is required, the route is via the 1K foam Soudal MF 167 Click & Fix B1 – flame-retardant to DIN 4102 and additionally workable down to −5 °C.
- Occasional use without a gun: straw foam. What speaks for and against it is explained in the comparison Gun foam or hand-held foam.
What else to look out for when buying
The price per can says little – what matters is the price per litre of foam. A 500 ml can with a 35 litre yield is cheaper than two 750 ml cans with a bulk density of 25 kg/m³ each and a correspondingly lower yield. For 2K the opposite applies: there you pay for time, not for volume. Work out your requirement beforehand – the construction foam calculator does it in a few seconds. And check whether your gun suits the foam: 2K cartridges need a 2K cartridge gun, can foam a normal foam gun.
You will find all the products named here in the category 2K foam & door-frame foam. How the foam is then applied cleanly is shown in the How to: apply PU foam correctly.
Frequently asked questions
What is the difference between 1K and 2K construction foam?
1K construction foam is a moisture-curing polyurethane foam: it cures with the water from air and substrate. 2K construction foam contains a second component, which is mixed in when the can is activated or in the static mixer of the double cartridge. Curing therefore proceeds independently of the humidity and becomes dimensionally stable considerably faster.
When is 2K foam really worthwhile?
Whenever the component has to be true to size and loadable quickly: door frames, window frames, stair treads, setting bathtubs. Also in dry or cold surroundings in which 1K foam does not find the moisture it needs. For plain filling and insulating of large cavities the premium is not worthwhile.
How quickly is 2K foam loadable?
That depends on the product. The Soudal 2K door-frame foam in the 210 ml double cartridge is cuttable after approx. 5 minutes and can have its bracing removed after approx. 20 minutes (at +20 °C and 65 % relative humidity). The Ottopur OP920 is cuttable after approx. 9 minutes, can have its bracing removed after approx. 30 minutes and is fully loadable after approx. 3 hours at a 20 mm bead thickness. The Soudafoam 2K in the 400 ml can is cuttable after 20 minutes.
Can I apply 2K foam with a normal foam gun?
Only the can foam. The Soudafoam 2K and the Ottopur OP920 are activated and applied as a can with an adapter. The Soudal 210 ml double cartridge, by contrast, needs a 2K cartridge gun and the static mixer supplied. Important: pauses of more than 10 seconds let the foam cure in the mixer.
Is 2K foam more fire-safe than 1K foam?
No, not automatically. The Soudafoam 2K, the Soudal 2K door-frame foam and the Ottopur OP920 are all classified in building material class B2 to DIN 4102-1 – just like the common 1K foams. Flame-retardant to B1 in the range is the 1K foam Soudal MF 167 Click & Fix.
How much foam do I need for a door frame?
As a guide value, one 400 ml can of Soudafoam 2K or one 210 ml double cartridge of Soudal 2K is enough for one to two door frames. For door frames at least six fixing points should be foamed once the frame has been wedged cleanly.
All technical figures come from the manufacturer data of the products carried in the shop. We are happy to answer questions about the choice: Baustoffheld, Lindenstrasse 56, 47623 Kevelaer, +49 (0) 2831 - 916 77 03.
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