Ventilating loft to lower temperature upstairs.

justwantedtosay

WIAWSNB said:

My suggestion of a fan is not to shift the air out of the rooms, but to cool yer bod. 

You wrote "Instead, drive out the warm air in the actual rooms.  Get a fan"

ComicGeek

There's a lot of bad science on this thread....

Insulation laid across the loft floor only reduces the rate of heat transfer from the hotter loft into the cooler rooms below. It does not stop it or minimise it, as eventually (without any other external inputs) the loft and the rooms will be an equal temperature. There is very little thermal lag through insulation, timber and plasterboard, so, while the rate of heat transfer is slowed by the insulation, there is no thermal mass to soak up heat and delay its inevitable transfer. It might take 6-8 hours for the heat to transfer through a concrete slab, but pretty immediate for a lightweight structure. 

The underside of the ceiling in the rooms is going to be warmer as a result, and operate as a very large radiator. That is a key part of why the upper floors of a house will always feel warmer than the ground floor in summer - it is not because hot air 'rises' from downstairs and gets trapped, it's because the internal surfaces of upstairs are warmer as there are larger areas of building fabric exposed to solar radiant energy, and 'trapped' heat within loft spaces also plays a key part.

UK buildings may cope for a couple of days of hot weather, but any extended period of hot weather quickly heats up the lightweight building fabric and the building overheats. We can only remove that heat by ventilating when the outside air is cooler, and that also means sufficient ventilation to remove stored heat from internal walls and floors, and reduce internal temperatures. It's a similar concept to heating a house in winter, where it may take a few days of heating to heat up the walls etc. 

So yes, ventilating the loft space sufficiently to remove excess heat will absolutely help reduce internal room temperatures below. The amount of ventilation needed to do this is going to be large though - much more than just a few small roof vents or a domestic bathroom fan. I have done it with Velux rooflights (controlled automatically with rain sensors and internal temp sensors), and that worked really well with the loft hatch removed at night time as well to draw air through the whole house. The rooflights had to have the glass replaced with non transparent material, otherwise that would just add to the problem, as well as surface condensation in the winter. What you don't want is controlled ventilation, as you don't want excess loft ventilation in winter.

But will it make enough of a difference? Always difficult to predict, but will certainly be cheaper to buy a portable air con unit and run it upstairs...

WIAWSNB

justwantedtosay said:

WIAWSNB said:

My suggestion of a fan is not to shift the air out of the rooms, but to cool yer bod. 

You wrote "Instead, drive out the warm air in the actual rooms.  Get a fan"

It was a flippant remark, but, fair do's, I can see how it came across. Sorry.
Yes, it is important to ventilate the actual rooms - opening up a through-draft is very effective - but a fan directed at the person is immediately effective. I wasn't suggesting using it to vent the room, tho' I guess it might help a bit. I doubt it tho'.
Right now, I am in my sitting room, with the windows wide open, and the door leading to the upstairs - where the windows are also open - leading to a good side draft playing over me. It's genuinely nice, and cooling. If I shut the windows, or the door, it'll quickly become very unpleasant.
If I didn't have this natural through-draft, then a fan would come close.
I hope you understand the larger principle involved; you need to sort your immediate environment, and not look towards your loft.

koalakoala

I am envious!
Here I am in the NE of Scotland with my electric blanket on before bed every night lol

WIAWSNB

ComicGeek said:

There's a lot of bad science on this thread....

Insulation laid across the loft floor only reduces the rate of heat transfer from the hotter loft into the cooler rooms below. It does not stop it or minimise it, as eventually (without any other external inputs) the loft and the rooms will be an equal temperature. There is very little thermal lag through insulation, timber and plasterboard, so, while the rate of heat transfer is slowed by the insulation, there is no thermal mass to soak up heat and delay its inevitable transfer. It might take 6-8 hours for the heat to transfer through a concrete slab, but pretty immediate for a lightweight structure. 

The underside of the ceiling in the rooms is going to be warmer as a result, and operate as a very large radiator. That is a key part of why the upper floors of a house will always feel warmer than the ground floor in summer - it is not because hot air 'rises' from downstairs and gets trapped, it's because the internal surfaces of upstairs are warmer as there are larger areas of building fabric exposed to solar radiant energy, and 'trapped' heat within loft spaces also plays a key part.

UK buildings may cope for a couple of days of hot weather, but any extended period of hot weather quickly heats up the lightweight building fabric and the building overheats. We can only remove that heat by ventilating when the outside air is cooler, and that also means sufficient ventilation to remove stored heat from internal walls and floors, and reduce internal temperatures. It's a similar concept to heating a house in winter, where it may take a few days of heating to heat up the walls etc. 

So yes, ventilating the loft space sufficiently to remove excess heat will absolutely help reduce internal room temperatures below. The amount of ventilation needed to do this is going to be large though - much more than just a few small roof vents or a domestic bathroom fan. I have done it with Velux rooflights (controlled automatically with rain sensors and internal temp sensors), and that worked really well with the loft hatch removed at night time as well to draw air through the whole house. The rooflights had to have the glass replaced with non transparent material, otherwise that would just add to the problem, as well as surface condensation in the winter. What you don't want is controlled ventilation, as you don't want excess loft ventilation in winter.

But will it make enough of a difference? Always difficult to predict, but will certainly be cheaper to buy a portable air con unit and run it upstairs...

Hmm, you may well be right.
Yes, our upstairs rooms become much warmer than downstairs, and I did largely put that down to 'hot air rises'. But with the outer covering being slate - which becomes scaldingly hot in sunshine - I guess some of that heat must get through, despite the insulation. 
As long as there's a breeze, tho', the rooms can be ventilated down to an acceptable level with no issues. With no breeze, using the chimney effect of the whole house does a similar job.
I wouldn't dream of considering an answer such as adding your level of ventilation to the remaining loft space; the cost would be prohibitive, with the result - whilst very possibly noticeable - unlikely to be a 'solution'. 

stuart45

It's a combination of both. Hot air rising up, and an extremely hot roof space above. The best method for any theory is to test it out. My guess is that replacing the air in the loft would make a slight difference to the room below, but probably not enough to be worth it. 
I was working in a loft when we had the last hot spell. I felt the difference as soon as I was halfway up the loft ladder.