How hot will it get in the attic?

[laurenco]

We're planning on using our attic space as a playroom and I can't work out whether the temperature will be balanced or if it's going to be too hot up there in the summer and too cold in the winter. The house is an MBC build (0.11 walls, insulated roof and slab) which is currently in build, with triple glazed alu-clad windows (which are fixed apex windows in the attic). We're putting UFH on the ground and first floor but nothing in the attic. The MVHR unit is going up here so we can add vents into this space and there will be a number of rooflights, probably double glazed. Will this be enough to create a usable temperature up here? If the whole house is 3G, are we made to put double glazed up here (to cut costs)?

Edited February 21, 2019 by laurenco

[Jeremy Harris]

Do any of the roof lights get direct sun?

 

If so then I think you'll find that it will get very hot up there on sunny days, as it takes very little solar gain to raise the temperature, just a couple of hundred watts is enough on a warm day, and roof lights, especially ones with a high inward transmittance (2G worse than 3G usually in this respect) can easily add maybe 500 watts or more each in sunny weather, if facing the sun.

 

If you do have roof lights that are likely to get direct sun, then I'd suggest looking at fitting external shutters/blinds, or perhaps having the external pane coated to reflect heat outwards.

 

Well worth doing a careful over-heating analysis, as when I did this on one of our preliminary designs (from an architect) that had four roof lights I found that there was a high probability that the bedrooms would reach around 30 deg C in sunny weather.

[laurenco]

Thanks @JSHarris The rooflights are south westerly facing so yes, this could be an issue....

[Gone West]

We have a triple glazed roof window in our bathroom and I chose low (32%) solar transmission glazing. As a result we don't have a problem with overheating but it is slightly darker in there in the winter. The room faces ESE.

Edited February 22, 2019 by PeterStarck

[puntloos]

Reviving an old interesting thread.

 

 

What about rising heat? I imagine that hot air from the rest of the house would also accumulate in the loft. Even if you carefully shuttered the rooflights, would this be an issue? 

Putting it differently, assuming no rooflights at all, and good insulation, is it almost guaranteed that the loft will be fine?

 

(I was even thinking of going for a Velux Cabrio, but I wonder if that can even be shuttered.. or are there awnings that could work on a pitched roof?

 

 

[Cpd]

I have put 4 velux in my shed when I was re roofing it as a way of increasing light and ventilation. What I did not expect was the huge heat increase ! It really gets hot in a very short time when the sun is out and I am just glad they can be opened to get some cooler air in ! I am slowly insulating the building so hopefully in colder weather that solar gain can be utilised.  I will also look at getting some velux blinds. 

[Declan52]

I put some blackout blinds on my roof lights in the bedrooms upstairs and they make a massive difference. Got them from eBay and was a 2 min job to fit them.

[Cpd]

1 hour ago, Declan52 said:

I put some blackout blinds on my roof lights in the bedrooms upstairs and they make a massive difference. Got them from eBay and was a 2 min job to fit them.

@Declan52 Ooo do you have a link, might be cheaper than going with the velux ones. 

[Declan52]

20 minutes ago, Cpd said:

@Declan52 Ooo do you have a link, might be cheaper than going with the velux ones. 

I got mine from this eBay shop. 

https://www.ebay.co.uk/str/cheapestblindsuk

[puntloos]

1 hour ago, Declan52 said:

I got mine from this eBay shop. 

https://www.ebay.co.uk/str/cheapestblindsuk

 

I assume you mean internal blackout blinds? I’m surprised that helps a lot. (in particular given that I have them too in a previous place and it was still sweltering..)

Is it because the rest of your house is well-insulated?

 

[Jeremy Harris]

We have blackout blinds in the bedroom windows and they don't seem to do much to reduce solar gain, TBH.  What seems to happen is that the inner pane of glass gets very hot and then continues to release heat for hours after the sun has gone down.

[puntloos]

5 minutes ago, JSHarris said:

We have blackout blinds in the bedroom windows and they don't seem to do much to reduce solar gain, TBH.  What seems to happen is that the inner pane of glass gets very hot and then continues to release heat for hours after the sun has gone down.

 

Could this be due to my ‘warm air from the rest of the house rises into the loft’ theory?

[Declan52]

7 minutes ago, puntloos said:

 

I assume you mean internal blackout blinds? I’m surprised that helps a lot. (in particular given that I have them too in a previous place and it was still sweltering..)

Is it because the rest of your house is well-insulated?

 

Yeah it's well insulated. They stop all the sun from shining in with no gaps around the edge so get no direct sunlight into the upstairs bedrooms. Mine are on the north face of my roof so it's only for the first 5ish hrs of the day the sun would be on them.

[Jeremy Harris]

5 minutes ago, puntloos said:

 

Could this be due to my ‘warm air from the rest of the house rises into the loft’ theory?

 

 

I doubt it, as the ground floor is pretty cool, as the slab gets cooled by the ASHP much of the time in warm weather.  It's consistently a degree or so warmer upstairs, but 90% of that seems to come from the windows as far as I can tell (they get very warm inside).