Minimising radiant heat in loft space?

[ruggers]

What are the best options for reducing the summer heat that builds up in a 'cold' loft space on sunny days which comes from radiant heat heating the roof slates? Is air movement the most important, preventing the heat from entering or both?

 

I'm planning to start a self build & will be siting my MVHR unit in the loft on a gable end so I'm looking at ways to reduce the heat up there.

There will be 400mm of rockwool but the roof construction will be standard trusses which can make it harder to insulate to the underside of the rafters. 

 

I was wondering if adding something like a superfoil roll to the undersides of rafters would help reduce radiant heat, or be a waste of time if it's not a complete taped and sealed envelope. Possibly adding an extract fan controlled off a switch or thermostat.

[Adsibob]

You need to create a barrier to stop or rather delay the heat from passing from the outside into the inside. This is usually achieved by insulating with materials which have a high decrement delay. Do a search for decrement delay on this site, but effectively you want to utilise quite a lot of wood fibre products. From memory, you want to aim for a phase shift of about 11 or 12 hours.

[Olf]

30 minutes ago, ruggers said:

Is air movement the most important, preventing the heat from entering or both?

Everything helps bit by bit:

- prevent roof from heating (reflective paint or coating)

- remove hot air that collects in the attic (ventilation)

- prevent top surface of insulation to capture radiant heat from underside of the roof (reflective film)

- maximise insulation thickness (reduce speed of heat entering rooms)

- increase decrement delay (so the heat flow created by day high temperatures is so slow that becomes cancelled out by nigh low temps)

 

Superfoil is overpriced for what it does. Your 400mm of Rockwool does good job with stopping conduction, just lay something shiny on the top (emergency/space blankets).

 

I'm not sure if a fan will improve flow a lot: temperature difference produces strong enough pressure to make a decent draught, it is the cross section of the roof vents that restricts the flow. Anectodal evidence is that when I open the loft hatch on the hot day the blow rips my head off (before roasting the rest)

 

 

[ruggers]

So the issue I want to try and control is the over heating of the non-habitable loft space where my MVHR ducts will pass between the unit to the rooms. I can try making a an insulated cabinet around the unit. The radial ducting will be insulated and it will also be under the 400mm of mineral wool. The ceiling between the upstairs bedrooms and the loft I will use PHS Hi-Thermia membrane as a VCL. https://passivehousesystems.co.uk/wp-content/uploads/2020/05/PHS-hi-thermia-membrane-2020.pdf

 

The roof orientation will be North facing rear, South facing front with East West gable ends. It's actually rotated 20 degrees clockwise from North. I wasn't going to continue the PIR insulation up above the first floor but the S.E side brickwork might warm up a lot more than I think. The roof slates are what seem to bring in most of the heat. Looking at your suggestions has made me think that the fans or vents won't be too effective if the thermal mass keeps transferring the heat into the loft again. 

 

Standard trusses have braces all over the place making it extremely hard to make a neat job of sealing any foil to the underside of the rafters from eaves to ridge level, but if it doesn't need to be completely sealed and will still be effective, I don't mind adding some to the South Facing side.

I don't want it to be like fitting a VCL & then having gaps between all of the joints & it's an expensive waste of time.

[Adsibob]

5 minutes ago, ruggers said:

I wasn't going to continue the PIR insulation up above the first floor but the S.E side brickwork might warm up a lot more than I think.

As I was just saying on another post this evening, PIR is useless for heat protection. U values are important to for keeping the warm in, but not for keeping the heat out. To try to illustrate this, consider that a caravan and a stone cottage have the same u values, yet the stone cottage will keep cool in summer whereas the caravan most certainly won’t. That’s because the thermal mass of stone is very high. It has a high specific heat capacity. 
you therefore want to insulate with wood fibre and PIR, not just PIR.

[ruggers]

Do you have an example of the wood fibre or products your referring to that could be used in a loft situation?

I always thought that foil backed insulation like PIR was supposed to be as effective at keeping the summer heat out as it is at keeping the heat in when its winter, although my experience has always been as you described, it works in winter but still gets too warm in summer and asking what else can be done to prevent this.

 

Because PIR is basically insulating foam sandwiched between two layers of foil, does that not mean that reflective foil alone on the undersides of the rafters would be a complete waste of time? I was looking at this for example. https://www.toolstation.com/construction-insulation/foil-insulation/c626 

[Adsibob]

Steico and Pavatex/Pavatherm are the leading wood fibre insulation products. I did a combination of wood fibre products. Here is my build up, but on reflection I should have foregone another 40mm of ceiling height to increase the wood fibre thickness.

 

[Olf]

16 hours ago, ruggers said:

The ceiling between the upstairs bedrooms and the loft I will use PHS Hi-Thermia membrane as a VCL

 

The reflective component only works with air gap - will you have one either side?

If sandwiched, it will be just a fancy airtight membrane.

 

16 hours ago, Adsibob said:

PIR is useless for heat protection. U values are important to for keeping the warm in, but not for keeping the heat out. To try to illustrate this, consider that a caravan and a stone cottage have the same u values

I disagree: from U value perspective alone, heat is not clever enough to distinguish what stops it from flowing and which direction. In steady state the same U cases the same flow. But life s not perfect and we have days and nights

What stone cottage benefits from is high gravimetric thermal capacity (aka thermal mass) that delays the spread of heat energy (aka phase shift). And by the time it would try to get inside, it will start flowing back out in the night. Also a cottage usually benefits from uninsulated floor, with free underfloor cooling at steady 8'C

[Adsibob]

9 minutes ago, Olf said:

I disagree: from U value perspective alone, heat is not clever enough to distinguish what stops it from flowing and which direction. In steady state the same U cases the same flow. But life s not perfect and we have days and nights

What stone cottage benefits from is high gravimetric thermal capacity (aka thermal mass) that delays the spread of heat energy (aka phase shift). And by the time it would try to get inside, it will start flowing back out in the night. Also a cottage usually benefits from uninsulated floor, with free underfloor cooling at steady 8'C

The point I was trying to make is that PIR does not have very good thermal mass compared to something like stone or wood fibre. 

[SteamyTea]

On 01/08/2022 at 13:02, Adsibob said:

you want to aim for a phase shift of about 11 or 12 hours

That would increase heating in summer, we have more hours of daylight than that.

 

On 01/08/2022 at 22:40, Adsibob said:

U values are important to for keeping the warm in, but not for keeping the heat out.

That will take some explaining for a homogenous product like basic insulation materials.

On 01/08/2022 at 22:40, Adsibob said:

consider that a caravan and a stone cottage have the same u values,

I doubt there are any, and then you have the window to surface area and the volume to surface area to consider.

On 01/08/2022 at 22:40, Adsibob said:

That’s because the thermal mass of stone is very high

Cough, cough.

You are talking about thermal inertia.

 

 

If you want to reduce the solar radiation absorbed by your roof, stick some PV on it, that will convert about 18% of the energy into electricity.

You can use that to run some A/C.

 

You could cover your roof with tin foil, on the outside, that is effective.

 

Or wait for a cloudy day to come along.

[ruggers]

On 02/08/2022 at 16:08, Olf said:

 

The reflective component only works with air gap - will you have one either side?

If sandwiched, it will be just a fancy airtight membrane.

I wasn't aware of this, so for all upstairs rooms this would be pointless then, except the bathroom if I batten the ceiling to create a gap for down lights into a upvc ceiling to maintain airtightness between loft and bathroom.

Are we saying PIR does or doesn't prevent the transfer of heat into a building? If it holds the heat in when it's cold outside, I'd expect it to prevent it from allowing the outside blocks transferring the heat inside on warm days.

 

On 02/08/2022 at 23:15, SteamyTea said:

If you want to reduce the solar radiation absorbed by your roof, stick some PV on it, that will convert about 18% of the energy into electricity. 

If I can afford it, I will be adding some solar panels to the South facing front roof, but they will be the inline type so won't have the air gap behind them which shades that part of the roof. The total of panels spread gable to gable will only cover 20% of that half of the roof. I don't feel the need for aircon up North with 2 weeks of heat wave per year but it doesn't take a heat wave to heat the loft space enough to possibly affect my lagged MVHR ducts supplying rooms.

[SteamyTea]

3 hours ago, ruggers said:

inline type so won't have the air gap behind them which shades that part of the roof.

That has nothing to do with taking the power away.

Energy is always conserved, to if the PV module receives 1 unit of energy, converts that into 0.2 units of electrical energy, then only 0.8 units are passing though to raise the temperature of 'something else'.

As you say, 'up north', and to be honest, down here in the far west, we are not bothered too much by excess temperatures.

So it may be worth looking at your house design, and orientation to the sun, to work out if it is really the roof causing the problem or the gable ends i.e. if one faces SW and gets a good solar bashing in the afternoon.

[ruggers]

I was meaning the panels sitting over that area of roof slate means that the slate behind doesn't take the full impact of the heat so the radiant heat is reduced within the loft whilst also creating energy via the PV absorbing sunlight. But most panels sit off the roof on rails with an air gap behind them causing a shadow and separation from the slates. An inline PV system fixes direct to the battens via plastic panels so i think the heat stored in the black PV panel will again travel back into the loft space as radiant heat.

 

Thinking about it now, anything fixed to the underside of the rafters to try and divert or radiate the heat back outwards from the loft space probably won't work unless it's sealed from edge to edge like the rest of the thermal envelope being airtight. Using the foil type VCL to the underside of the rafters would have been a nice way to achieve this but unless sealed it would have no where to redirect it too. As it's not a habitable loft with expensive attic trusses, It would cost me probably £5k extra to do it properly. The slates will get very hot on sunny days, we can't stop that, so it's finding the most cost effective solution to diverting that radiant heat back outside.

[Temp]

The tiles get hot then radaite the heat across the gap below them. I think the solution I woul like to try is a well ventilated gap under the tiles with a shiny membrane. Problem is are there any shiny membrane that will stay shiny under tiles? Probably not. I think dirt and dust will accumulate. So perhaps it need a regular membrane with ventilated void below it and then shiny insulation?

 

[TonyT]

What about a mushroom vent from the loft space to outside and let the heat out that way.

 

Some states in the USA have extract fans in the loft to vent this space to outside.

 

I have a velux in the loft of the original house (1860’s build) that I open up in the evening and let heat out.

 

In my office I have skylights that open to dump excess heat should the BMS computer deem it necessary.

 

all working along the same theme….

[ruggers]

@TonyTAs in a standard vent slate that just fits into a pitched roof? I've seen the mushroom type on flat roofs but theres others that look much better. 

 

It's why I added this post incase others had tried similar things. I could add lots of vents and they don't even work, its a bit of trial and error. I don't mind fitting a vent on each gable or a couple of slate vents up near the ridge on the North side if it helps, I was just unsure if it makes any difference or not with them being so small compared to the size of the roof. A velux is a significant opening size so it will let a lot of heat out compared to a few vents.

 

@TempI think unless I'm going to fully insulate the underside of the roof to keep heat out as per comments above which isn't very cost effective for my use of the room, My best bet will be to add some vents to get the air moving and consider a radiant heat barrier to the underside of the rafters to prevent as much entering and leave a gap at ridge level where the vents are to help steer it in this direction. The rest of the heat reduction will have to rely on the loft insulation on the floor.

[Adsibob]

To answer your original question, air movement within the roof build up, is unlikely to make a big difference in my view. You are much better off putting in as much wood fibre as possible, and also installing a Velux and a window on opposite sides of each room that lets you run a draft in summer. For extra comfort, install external roller shutters on the veluxes. Expensive, but keeps the heat out when you are not in the room. 

[SteamyTea]

I am a bit bored, so thought I would try and calculate what sort of U-value you would need to be equal to something really shiny so that the power transfers are equal.

 

Shiny has an ε of 0.03, with a roof tile temperature of 70°C and a room at 22°C, 11 W.m^-2 would be the power delivery.

For the same temperature difference, insulation with a U-Value of 0.2 W.m^-2.K^-1, would deliver 10 W.m^-2.

 

So if you have 0.18m of k 0.035 insulation, you will be better off.

 

(it is actually the same radiative process going on in insulation, but because of the reflective angles, and interference, less power is transmitted though the material)

Edited August 7, 2022 by SteamyTea