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We learnt how to keep our house cool this summer and thought we'd share what we've learnt in case its of use to others. We're in a timber frame building built to the Passivhaus standard but not certified. We don't have an ASHP, so no slab cooling for us but without a doubt our (not so secret) weapon are the external blinds. All our east and south facing windows (with the exception of 3 east facing windows - more on them later) have external blinds and as predicted by the PHPP these are the game changer. Our secret weapon is the little personal weather station (PWS) in the garden and with it's help this is how we've kept the house cool: In the evening when the outside temperature according to the PWS is below our target internal temperature of 21 degrees all doors and windows are opened and the MVHR ramped up. Overnight all the inward tilting windows are left open with the MVHR staying at a higher level and it stays like this until the PWS shows that the external temperature is rising towards our target 21 degrees - that happened at 07:50 this morning. As soon as the exterior temperature reaches our target then all doors and windows are closed and the MVHR is reduced to its lowest setting, we monitor the air quality with a couple of CO2 monitors as recommended by @Adrian Walker and increase the MVHR speed as required to keep good air quality. All sun facing windows have the blinds down, we have the mesh style blinds that don't block off the light but stop lots of UV. With this technique our hallway temperature has peaked in the early evening at around 21-22 degrees and the kitchen/dining room which has lots of east and south facing glazing has stayed below 24 degrees. As soon as we open up we get rid of most of that unwanted heat. The hallway starts in the mornings around 19-20 degrees. The reason I think the PWS is a secret weapon is as far as I'm concerned you need to accurately know the exterior temperature and when to let it in and when to keep it out. On a windy day, 26 degrees can feel cooler than 21 degrees but if you let that stuff in the house you're gonna regret it. If you don't have a PWS you'll probably be able to find one situated close to you on the Weather Underground website. Oh, and the 3 problem east facing windows without external blinds: 1 is the utility room. That has an internal blind and as it's the utility room the internal door to the rest of the house is kept closed. That room gets warm 25-27 degrees but the heat stays in that room which of course has an MVHR extract so the heat is slowly being pumped out of the house until it's door/window opening time then we throw the heat away with vigour. 2 is the ensuite which we deal with in exactly the same way as the utility. 3 is the main bedroom which has a great big window and curtains. The curtains remain closed while the sun is coming in that window but the doors are open so as that room warms up its heat is not being contained in that room. As the sun moves round and that window is no longer in direct sun the curtains are opened and a big old chunk of hot air falls out to be slowly extracted until window opening time. To me, its like the house taking a long overnight gulp of cold air and then holds its breath throughout the day until the temperature is 'just right' again.