Attic - Heat - Global change

[utterlydiy]

I have a 4 bedroom detach house.
Brick outer and board inner with rockwool/fibrewool cavity.
The attic has 10” insulation with 5 vents in the roof, with tiles on felt.
During the summer months the attic gets really hot and even with 10” of insulation, the heat comes down through into the bedrooms.
With the global change with the weather with increase heat.
Suggestions of to reduce heat in the attic, whether foam insulation boards fitted in the rafters of the roof against the felt is good or bad. Advice please…

 

[RandomGrinch]

Fit solar panels - you'll be taking energy from the Sun, reducing the amount that heats the loft, with the opportunity to decrease your energy bills (or help pay for the extra A/C!).

 

[StephenStephen]

Is there ventilation at the eaves?

 

[^woody^]

IIRC, heat rises not goes down.

I suspect the bedrooms are hot because of that scientific principle.

 

[RandomGrinch]

IIRC, heat rises not goes down.

I suspect the bedrooms are hot because of that scientific principle.

Technically, I'm afraid not - heat is a measure of a molecules energy - air masses may rise and fall though, before mixing and tending towards an equilibrium

 

[Dereekoo]

And that why in hot weather all the velux on attic mods seem to be open.Fit a velux.

 

[^woody^]

Technically, I'm afraid not - heat is a measure of a molecules energy - air masses may rise and fall though, before mixing and tending towards an equilibrium

The loft is insulated, the molecular energy in that air is not going to be heating up the air the rooms below as much as the molecular energy in the air below the insulated ceiling.

This equilibrium you mention does not apply when there is an insulation layer to break up the equilibrium.

Back in the day, people used to open the loft hatch on the warmest summer days for this very reason, and the hot air never came streaming down through the hatch!

 

[^woody^]

And that why in hot weather all the velux on attic mods seem to be open.Fit a velux.

Or, open the bedroom windows

 

[RandomGrinch]

The loft is insulated, the molecular energy in that air is not going to be heating up the air the rooms below as much as the molecular energy in the air below the insulated ceiling

Yes, and I didn't say otherwise.
I was only commenting on your...

scientific principle.

The equilibrium also doesn't apply because, as with most things, this is a dynamic system.

Back in the day, people used to open the loft hatch on the warmest summer days for this very reason, and the hot air never came streaming down through the hatch!

If there were no other factors involved then the warm air would come (slowly) down through the loft hatch and the house would reach an equilibrium temperature.
The difference being, the loft still has more solar energy gain than the rest of the house (through the roofs emittance of blackbody radiation), keeping that air mass warmer; the temperature differential also leads to the stack effect, creating upwards air movement - an effect that could be enhanced by an open Velux as @Dereekoo suggests.

 

[^woody^]

I've just checked with my engineering mate Scot downstairs and he said something like "Ye canna change the laws of physics"

 

[wessex101]

The British summer lasts no longer than 2 weeks. I'm sat in my office in the middle of August looking out at drizzle and pondering whether I should put on a jumper..
Bring on the global warming.

 

[utterlydiy]

Seen some use similar to the kingspan Insulation Board fitted between the rafters/trusses to reduce the heat from the inside of the felt/tiles on the roof.

 

[RandomGrinch]

Seen some use similar to the kingspan Insulation Board fitted between the rafters/trusses to reduce the heat from the inside of the felt/tiles on the roof.

If you wanted a cooler loft, then you could use insulation boards between the rafters - you would need to ensure adequate ventilation of the loft space and cross ventilation between the boards and the roofing felt/membrane.
However, if as you say, your ceiling is properly insulated, there should be little benefit to the comfort of the downstairs rooms.
It would be a large cost, for little return.

If you can objectively say that the heat in the loft is having an effect, then firstly investigate if your insulation is laid correctly and look for gaps in the sealing of the loft (i.e. around the loft hatch, or down lighters etc.).

Ultimately, the best way to reduce the solar gain on a property, is to reflect the energy away, before it reaches it - hence the suggestion for solar panels, that may also have side benefits.

I've just checked with my engineering mate Scot downstairs and he said something like "Ye canna change the laws of physics"

Exactly!