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For a comparison of the processes I’d refer you back to BS 8417: 2011+A1:2014 and the footnotes to Table 4 “Preservative recommendations as given in Table 4 are based on penetrating processes (see 6.5.3) for which laboratory and, where appropriate, field tests and service experience provide a high degree of confidence in performance.....Developments in treatment techniques using superficial processes….. might justify the use of such processes” (my italics). Hardly a ringing endorsement for superficial processes such as flow-coating. Further, observations by Joran Jerner of SP Technical Research Institute of Sweden can be found here http://www.svanemerket.no/PageFiles/9770/Background%20_windows_consultation_proposal.pdf… but for a taster he states “ It is naive to think that a high share of heartwood can compensate for permeation.” Regarding desired service life data. You second post referenced BS 8417 in relation to a 60 year service life – “requires at minimum timber classified as durable (class 2)”. This is taken from table 3 and is for untreated timber. Table 4. for timber treated using preservatives in accordance with BS EN 599-1, asserts that permeable wood (softwood& heartwood are indistinguishable) with penetration to class NP3 (6mm lateral, x10 longitudinal) will provide for a 60 desired service life. In other words slightly/moderately durable timber such as Pinus sylvestris is good for 60 years with the appropriate treatment. Without that level of preservative penetration then 15 and 30 year maximum service lives are suggested depending on the treatment. “30 years before maintenance” sounds ridiculously optimistic. Any window or door should be inspected at least annually - and more frequently in exposed environments. At a minimum hinges will need to be lubricated. DVC standards are designed for the Danish market …and Danish conditions. BS Codes of Practice (such as BS 8417 Preservation of wood. Code of practice) have been developed with the specific requirements and experience of the British market and the climate/geography of the British Isles in mind. It’s a similar situation with security testing of windows and doors – the UK maintains a requirement for additional forced entry testing - PAS24 - in addition to the EN 1627 standard to accommodate the techniques typically used by UK burglars. The move towards superficial preservative treatment processes (such as flow coating) has been driven primarily as a cost-cutting exercise (and to a lesser degree by workplace elf n’safety factors). It is a lot quicker (cheaper) to make windows if you eliminate the vacuum impregnation process. With the minimal preservative penetration (≤1mm) achieved by flow-coating the durability and continuing integrity of the surface finish is extremely important. In such circumstances even the sanding of the frame can reduce the preservative depth. As Rationel make very clear in their O&M manual ( http://www.rationel.co.uk/media/1620007/om-manual_2014_sept14_web.pdf ), “Any cracks in the paintwork or timber, will allow moisture to penetrate into the product and over time cause decomposition of the timber” (my italics). I’d treat the Wood Window Alliance claims with a degree of caution. The kernal of their durability claims are based on academic studies conducted on vacuum impregnated windows of a specific design. They have ‘carried over’ the results from these studies – as you have noticed – to any generic window that their (paying) membership produce that complies with some basic guidelines. Those guidelines do not have a requirement for vacuum impregnation. The excellent TRADA book Wood windows: Designing for high performance (Patrick Hislop) notes that there are numerous designs of timber windows never mind the range of constructions that are described as ‘composite’. In the real world the preservative treatment is essentially a safety net. A well designed & constructed window, correctly installed with interface details suitable for its environment should shed the water that will facilitate rotting and have a service life of 60-70 years if not longer. The presence of Pine heartwood is certainly no guarantee against rot – remember at best it is only ‘moderately’ durable. Furthermore, heartwood is difficult to penetrate with preservative treatments. Nordan make good windows (SW Norway actually gets more rain there then we do in here in Ireland, never mind the sepearte arctic climate further north) hence their popularity with our Caledonian cousins. UK manufacturers are moving towards flow-coating on cost grounds. Penetrating processes would still be common amongst Norwegian and Swedish window manufacturers – the rest of continental Europe is flow-coat or dipping. Preservative treatment is just one factor in assessing the quality of a timber window but I would suggest that for anybody considering installing timber windows in areas of severe, or very severe, exposure it would be imprudent not to use a vacuum impregnation preservative. Good luck with whatver you choose. It will look a lot better then plastic and should last a lot longer if looked after.