fandyman

I don’t think I’m missing that point - I agree that uPVC frames are inherently more insulating due to their chambered design, and I’m not alleging a defective product. My concern has always been about the installation outcome, specifically at the threshold detail. Where I differ is that I’ve already tried to resolve this reasonably and practically with the installer. The installing team have attended multiple times, and the company owner himself has visited. On those visits, the issue was repeatedly dismissed as “normal” on the basis that heat rises, that the bottom of doors is always colder, and that condensation is caused by breathing or clothes drying indoors - which I made clear we do not do. Beyond additional sealant, no attempt was made to investigate or improve the threshold detail. I’m not disputing that installing onto outer brickwork is common practice in typical UK builds. I am questioning whether, in this specific case, the resulting internal surface temperature at the sill/frame junction - which is cold enough to repeatedly condense and grow mould under normal occupied conditions - is an acceptable outcome of reasonable care and skill. If insulating under the threshold is the appropriate mitigation, that’s exactly the sort of conclusion I was hoping to reach collaboratively. Unfortunately, that option was explicitly ruled out by the installer, who stated insulation under the sill is not required and that nothing further could be done. Given that, it seems reasonable to seek independent or regulatory review rather than continue a circular discussion where the issue is acknowledged in practice but dismissed in principle.

Agreed - I’m under no illusion this is cost- or effort-free. I’m proceeding on the basis that it now comes down to evidence and independent assessment. Just to clarify for anyone reading this later - warranties don’t really affect the process I’m following here. Under Section 75, the card provider’s assessment isn’t contingent on manufacturer or installer warranties. It’s about whether the goods/services were supplied with reasonable care and skill and whether the outcome is acceptable. Warranties can be part of the background paperwork, but they don’t limit or replace the bank’s liability. Photographic evidence and (if required) an independent expert report are what actually inform the decision, rather than whether a warranty exists or what it covers. I just wanted to make that distinction clear so people don’t conflate warranty routes with Section 75, as they operate independently.

Agreed on separating product design vs installation detailing vs the surrounding building fabric. For uPVC I’m not expecting an “aluminium-style thermal break”, but I do agree the right starting point is the manufacturer’s section drawings / data sheets / installation instructions for the frame + cill so the intended thermal performance and assumed installation detail is clear. On the “opening prep” point: the opening was already in this state when I bought the property and has not recently been altered. The installer measured and supplied the door set to suit the existing opening; I didn’t specify a brick-to-brick detail or instruct them to omit any clearance/thermal mitigation. From my side, the key symptom remains persistent localised condensation/mould at the internal sill/lower frame junction, correlating with low surface temperatures on thermal imaging. I take your point that there may also be an air leakage component under the cill if the underside isn’t properly sealed - I’m going to check/confirm that separately - but the issue presents primarily as a cold internal surface at that junction rather than general room humidity.

The “£30 brush strips” framing isn’t really applicable here. I’m not pursuing this via FENSA for compensation, and I’m not expecting token gestures. The escalation route I’m using is Section 75, which is joint and several liability for the entire contract value where goods or services aren’t provided with reasonable care and skill. That’s a very different legal framework from snagging or goodwill fixes. This isn’t about chasing draught brushes or minor sealing - it’s about establishing whether a persistent condensation/mould issue arising at a specific junction is an acceptable outcome of the installation, or whether it points to a defect in the way the threshold detail was executed. If an independent assessment concludes it’s acceptable, that’s the end of it. If it doesn’t, the remedies available under Section 75 are materially more than “£30”. I think it’s important to be accurate about the mechanisms involved, particularly for anyone else reading this thread who may be weighing up similar options.

I get what you’re saying about how most re-fitters operate and the reality of “fit the hole” jobs - I’m not disputing that this is common practice. Where I struggle is that we’re not talking about chasing passive-house performance or some theoretical improvement. We’re talking about persistent condensation and visible mould forming at a very specific junction, under normal use, that wasn’t there before the installation. That feels like an outcome problem rather than an expectations problem. I didn’t ask the installer to redesign the opening or go beyond what was quoted, but equally I don’t think it’s unreasonable to expect a newly installed door not to create a cold internal surface that repeatedly condenses and grows mould. Even if the trade norm is to accept that detail, it doesn’t automatically follow that the outcome is acceptable in every case. I’m not saying the installer acted in bad faith - just that something about this particular junction isn’t working as intended. That’s why I’m pursuing independent/regulatory review rather than arguing it out on forums.

We agree to disagree then. I am pretty sure that under UK consumer law a consumer does not need to instruct a trader on how to avoid creating defects and the trader must still perform the service with reasonable care and skill. You don’t need an architect’s spec to expect no persistent condensation, no mould and no unreasonably cold internal surfaces under normal use. If the outcome is defective, the absence of instruction does not automatically excuse it. That would be nonsense.

I do not think "it fits the hole" is correct. In UK law (Consumer Rights Act 2015) the supplier’s obligations include that the service is carried out: with reasonable care and skill fit for purpose as described I do not believe that fitting the opening dimensionally exhausts the duty of care. If that were true leaking windows would be acceptable if they “fit”. It would also mean draughty doors would be acceptable if they “fit” and cold bridging would never matter right? I am confident that is not how CRA works.

I’m here to understand the technical aspects of the installation and the causes of the observed condensation. I’ve tried to be clear and structured in explaining the issue, but I’m happy for people to disagree on the substance. If you think the installation detail is acceptable and that the symptoms are expected, I’d be interested in the technical reasoning behind that.

Thanks — I understand why that might help with draughts or water ingress. My concern here isn’t primarily air leakage, but internal surface temperature. The condensation and mould are forming because the sill/frame junction is becoming cold enough to drop below dew point under normal conditions. Adding a thick internal bead may improve airtightness, but it wouldn’t change the underlying thermal path through the sill into the masonry, and therefore wouldn’t raise the surface temperature that’s causing the condensation. There’s also a risk of sealing moisture into the junction rather than resolving it. That’s why I’m trying to understand whether the installation detail itself reasonably anticipated and mitigated this thermal bridge, rather than looking for post-hoc cosmetic fixes. I appreciate the suggestion though.

Thanks for taking the time to respond. I agree this wouldn’t be an issue worth discussing if it were about chasing passive-house levels of performance — that’s not what I’m aiming for. The concern here is that the internal sill/frame junction has become the coldest surface in the room and is repeatedly dropping below dew point under normal winter conditions, leading to condensation and mould. I’m not suggesting the door should have been made short or that it should sit on compressible packers alone. My question is whether, in a conventional retrofit like this, best practice would normally allow the sill to bear directly on masonry with no thermal break or mitigation, given the predictable internal surface temperatures that result. I accept that the solid floor and external brickwork form part of the thermal boundary, but the thermal imaging shows the cold concentrated at the sill and frame junctions rather than uniformly from the floor. That suggests a localised bridge associated with the installation detail rather than the slab alone. I’m also not proposing post-installation tweaks or DIY fixes — sealing or adjusting locks doesn’t address a conductive cold path, and I’m conscious of knock-on effects. This is about understanding whether the original detailing reasonably anticipated and avoided this condensation risk. Appreciate the alternative perspective — I’m mainly trying to establish where the line sits between “unavoidable compromise” and “avoidable detailing issue” in this type of installation.

Thanks for sharing that — I can definitely relate to the frustration. I agree that large, heavy sliding doors and fully flush thresholds are particularly difficult details, and I can see why installers default to solid bearing in those cases. In your example it sounds like the constraints of door weight and flush internal/external levels severely limited the options. My situation is slightly different in that the installation is a uPVC patio door with sidelights rather than a very large sliding system, and there doesn’t appear to have been any discussion or design decision around managing thermal continuity at the threshold — the sill simply bears directly on masonry. What I’m trying to understand is whether, in a relatively conventional retrofit like this, best practice would normally expect some mitigation to prevent the internal sill/frame junction becoming the coldest surface in the room and repeatedly dropping below dew point, rather than accepting that outcome as unavoidable. I completely take the point about knock-on effects, and I’m not looking to “tweak” the installed doors myself — this is more about whether the original detailing should reasonably have anticipated and avoided this condensation risk in the first place. Appreciate you taking the time to reply — it’s useful to hear real-world experiences, even when the outcomes aren’t ideal.

Thank you — that’s really helpful and aligns closely with what I’ve been suspecting. Just to clarify one important point about the opening: When I purchased the house over three years ago, the opening already existed at full height exactly as it is now. There were no windows, doors, or dwarf walls present, and no structural changes have been made since (there was a conservatory that has been demolished since then and new patio paving laid in its place). The installer therefore worked with a pre-existing opening and was responsible for detailing the door installation within that condition. The sill appears to be bearing directly on brickwork, with no packers or thermal break beneath. Internally, the cold line corresponds very precisely with the sill/frame junction and the side-panel-to-door frame junction, which is where condensation and mould are occurring. Thermal images show localised internal surface temperatures at those junctions significantly lower than adjacent areas, suggesting a concentrated cold bridge rather than general floor temperature influence. The floor itself does not show the same thermal signature away from the frame. Your comment about Compacfoam is interesting — that’s broadly what I would have expected in terms of maintaining thermal continuity at a load-bearing threshold, even if not explicitly mandated by Building Regulations. As you say, “I’d do it anyway.” What I’m still trying to understand is whether, in a retrofit scenario with a pre-existing opening, best practice would normally allow a sill detail that results in the internal frame/sill junction dropping below dew point under normal winter conditions — even if the regulations themselves are not explicit. For clarity, the patio paving was installed after the door, and the final external level is approximately one brick (~75 mm) below the sill. Thanks again for taking the time to respond — this has been extremely useful.

Hi all, I’d appreciate some advice from people who know more about window/door installation than I do. We had new REHAU Rio patio doors with sidelights fitted earlier this year by a small FENSA-registered company. We are getting very noticeable cold and draughts around the bottom of the frame/sill junction, especially now that the weather has turned colder. After taking some thermal images, there is a very clear cold line exactly where the sill meets the frame. It’s a consistent pattern across the whole width. I’ve attached the thermal photos below. I also looked underneath the exterior cill and it seems like the sill is resting directly on the brickwork with no insulation or thermal break. The installer told me: and also: The floor is finished up to the frame, and the thermal camera shows the cold coming specifically through the frame/sill junction, not from below. The coldest areas appear most pronounced at frame junctions and corners rather than uniformly across the opening. My questions for the forum: Is a sill resting directly on cold brickwork with only tiny packers (1-4mm) and without insulation acceptable practice? Should a proper installation normally allow some allowance for insulation or a thermal break beneath the sill, rather than bearing directly on masonry? Would resealing or adding more foam internally achieve anything, or is this fundamentally a threshold installation problem? Is this something that FENSA would normally get involved with? Has anyone here had to escalate a similar issue under the Consumer Rights Act? I’m trying to resolve this with the installer first, but they say the installation is correct and that nothing more can be done. Any professional opinions or similar experiences would be really helpful before I decide how to proceed. Thanks in advance.