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Our builder has recently installed a large aluminium sliding door. The frame includes a "thermal spacer" that is between the inside and outside frames. The manufacturer claims this will insulate the inside frame and stop it getting cold. Following installation, we have found that we frequently get condensation on the frame (but not the glass). If I clean the condensation, it returns in about 24 hours. The builder says this is because we have too much humidity and not enough ventilation. I have purchased a humidity meter and read up on the science of dew points and and the installation instructions for the window system: Our humidity level is typically 50-55%. This seems pretty normal and nothing exceptional. The calcs suggest that with an internal temperature of 20°C and humidity of 50%, the dew point is 9.3°C. This means condensation is expected to form below this temperature. It is colder than this outside for most of the winter. The installation instructions for the window system say the cill should be on plastic packers to insulate it from the masonry and the frame should be fixed through the thermal spacer (plastic bar). I think our cill sits directly on the masonry and is held in with metal clips that connect it to the masonry. Taken together, I think the problem is that our frame isn't correctly insulated from the masonry, the frame gets cold and this leads to the condensation. Please can I ask: a. Is it normal for aluminium frames to be very cold on the inside? Is condensation normal? b. Is it most likely caused by our (excess) humidity, poor frame design or incorrect installation? c. Is my explanation plausible or is there something else I have missed?

I was speaking to an engineer today about cooling our house should it need it. He was talking about fan coils when i mentioned cooling the slab with underfloor heating. His advised me not to due to condensation and the risk to the floor and concrete. He stated that it was due to the dew point/humidity in the house and the condensation around the pipes and on the floor. He stated it would form not just on the floor surface but under tiles and around the pipes Now i don't pretend to know anything about this but i am sure that those who have looked into cooling a slab must have considered this issue and mitigated against condensation forming. Can anyone help with this issue(if it is one)