Try low-ex foam, such as the Soudal Flexi, works well...
Wedge a 310ml cart nozzle on to the end of the foam-gun, and flatten the tip, to ensure a good depth of foam inclusion....

Can you rely on just foam for air sealing???

Even the flexible foams (flexifoam, thermofoam) don't expand, merely deal with contraction.

Do you mean once cured? I guess it might do a little, would be interesting to learn more... but can't imagine it's much until the adhesive gives way.

Sealed all our 3g windows direct into the stone mullions with Soudal and worked really well but had to include lead flashing barrier between 3g unit and foam as the foam reacts with the seal material used on the 3g unit. To make air tight traditional burnt mastic used to point glass to stone mullions both inside and out.

It depends on the foam. The canned foams seem as through they're a two part foam with a solvent that keeps them liquid. When blasted out of the can it evaporated off and the foam hardens. Most of the expanding is done by the time it's left the can but you'll see some further expansion. The faster it left the can, the more it expands

Other urethane foams I've used behave differently. You can get it as two part, relatively inert liquids that when mixed look like a glass of baileys for a minute before going berserk and ending up at around 50 times the volume. You'd be a brave man to seal windows with it but it would work excellently. You'd need a delivery mechanism such as a cola bottle with a cartridge gun nozzle rammed through the cap and you'd pour it at the top of the window, letting it run down the profile in a stream, coating it. Eventually it would reach foaming temperature and cease running down the window and expand sideways. You'd also need somewhere to dump your bottle as by this time any unused contents of the bottle would be actively spraying out under its own expanding pressure and going eveywhere. The foam quality is excellent and closed cell, could be used for a good air seal

As per the above but far less aggressive is something like D4 urethane adhesive. Like golden syrup, it eventually expands and becomes a foamed urethane. Cell structure isn't quite as good

Best of all are the two part foams where each part comes in its own gas canister, about the size of a small gas fire bottle. The foams these things produce are controllable and the cell consistency is excellent. You'd need to fit the glass and put packers in the middle of any spans though because they push like crazy and would definitely distort he frames if unsupported. One of my aluminium frames ended up bent because of this


Of all the little can foams are used most because they're the best compromise- not airtight but the uk buildit industry doesn't care, controllable, portable and totally mess free if installed with care. You could paint the foam with something like liquid radon barrier to get a gas tight

Posted By: cjard. You could paint the foam with something like liquid radon barrier to get a gas tight

There are also some paints sold as airtight now... Not sure whether you can paint over the top.

Posted By: gravelldCan you rely on just foam for air sealing???

The answer is definitely yes. My whole house is sealed with foam of different types. The wall and roof with Icynene and the windows and doors with Soudal Flexifoam in conjunction with Compiband expanding foam tape. I had an an tightness test result of 0.47ACH.

Of course the interesting question (for tapes or foam) is what actually happens to the air tightness over time. It would be so interesting to have our sub 1ACH builds retested after 10, 15, 20, 30 years and see what happened.

Like Wooky we used both to be sure. The winter winds make a good natural wind test - we found a leaking door (the door to frame, not how it was fitted into the reveal) seal that way. Tiny leak, but you could hear it. Going to be using the natureal wind test in future years and will report back.

Posted By: Ed Davies

Posted By: Greenfishwe found a leaking door (the door to frame, not how it was fitted into the reveal) seal that way.

If that was the door used for the blower-door test then the test wouldn't have measured that, if I understand how they're done. Is there a separate check for how the door replaced by the blower plugs the frame?

No separate test, no. It's worth noting that for our PH-level airtightness test, the blower is small enough that it fitted in a shower room window. We opened the window, cut some Panelvent to fit (could use any board, just what we happened to have), then cut a hole in that for the blower, put it all together and apply lots of sticky tape to seal it all together.

Posted By: GreenfishNo not that door, anyway my actual air test was a total joke. Came out around 1 (units I can never remember) at 50Pa, but the MVHR was left open, and the fan poorly fitted to the door with lots of gaps. All very slap-dash. The tester could not get that I was not interested in just passing, I knew we would do that, but I wanted to know how good we were. I still don't know, the measured result is clearly a worst estimate

Glad I used Paul Jennings, then. He does know what he's doing.

The leaking door was a patio door, and the air test didn't show the problem, took a 70mph westerly gale pushing against it to reveal that. Door adjustments have improved things, but in hindsight I wish we had an outward opening door in that location. The pressure of the wind is immense, it knocked over a wall during construction, so this door will always be vulnerable in high winds.

I think the 50 Pa measurement corresponds to about a force 5. whilst 70 mph is force 11, violent storm. which appears to be over 660 Pa. So no airtightness test is going to show that and no door will be airtight at that pressure. (except presumably a nuclear blast-proof door!) The Passivhaus test goes up to 100 Pa, but they limit it at that for some reason that I forget.

Posted By: Monty Gerhardymechanical fastening

I would certainly agree that it is.

The original assertion by @djh surprised me. Are we talking very very fine leakage here? I would hope a decent door could withstand a gale force wind, but maybe my intuition is off - is it to do with the larger area of a door (in the same way that carrying a large board in wind is surprisingly difficult)?

Just trying to "grok" this.