Insulating my home

[31K]

Hi all, this is my first post and I hope it's in the relevant section. If not please let me know where it should be and I'll repost in the correct place.

 

I'm hoping to get some advice on insulating my home which in many places feels like a shed. The other day after returning from a few days away (heating was turned off, maybe I should have left it on low instead) the bedroom was 7 degrees! So, I think I better insulate the areas I can considering the cost of living crisis and need some opinions. I will try to break it down into the sections as I plan to insulate them.

 

1) Pitched roof at the top of the house

 

This area is approx 7m X 3m and not tall enough to stand upright in (I have to crouch a little). I am planning on creating a cold roof. I am looking to use the following as an initial layer:

 

https://www.wickes.co.uk/Knauf-Insulation-Space-Bottom-Layer-100mm-Loft-Roll---8-3m²/p/166877

 

I am looking to then lay the following at 90 degrees (on top of the initial layer) so that there's a total of 300mm of insulation:

 

https://www.wickes.co.uk/Knauf-Insulation-Super-Top-Up-200mm-Loft-Roll---5-61m²/p/109451

 

I would like to board approx half of the loft space and will use 210mm loft stilts/legs (roof joists are 120mm) to allow for the 300mm of insulation underneath. I know this should not be compressed, however will there need to be an air gap between the insulation and the bottom of the boards?

 

Considering the loft space is already limited, is there a better alternative?

 

By better I mean, something that has lower profile with a similar u value (I think this is the correct term) maybe PIR board like Celotex or Kingspan, possibly removing the need for loft stilts/legs. This will both save money (which can be put towards the potentially more expensive insulation) and space. 

 

2) Flat roof at the front and back of the house (same level as the pitched roof in the point above)

 

This currently has no insulation and is above the upstairs bedrooms/bathrooms.

 

The area is approx 7m X 2.5m at the front and the same at the back. I am planning to create a cold deck of insulation. I have a little over 120mm (height of joist) of space between the bedroom/bathroom plasterboard ceilings and the ply (assumed). This will be done without removing the internal ceiling or the flat roof externally as I have found a way to access the flat roof from inside the pitched roof (lucky I know, at least I hope anyway until someone tells me otherwise).

 

I am looking to use the following as it will give me some flexibility (when working around a water pipe feeding a overhead shower) and I will use a telescopic rod or poles to push it down towards the eaves (I know these should be kept clear, I have measured to the end and will set the rod/pole to a slightly shorter length) I have also selected 50mm insulation as this will give me more than the minimum 50mm of ventilation gap needed between the insulation and the ply. Is this correct and could I push this to 75mm instead?

 

https://ukflatroofing.com/mineral-wool-1/knauf-rocksilk-rs45-knauf-30mm-600mm-1200mm-k-rs45-30?utm_term=&utm_source=google&utm_medium=cpc&gclid=Cj0KCQiAkMGcBhCSARIsAIW6d0DrKJpeJXMCVM8jY3iRuhdKmCyMNrH0eJyPunkaVd_qXWtvZpu9HdsaArRcEALw_wcB

 

Some of the rooms have spotlights, I was looking to pull these out, push the insulation in and cut it away from inside the room before re inserting the spotlights, will this be ok?

 

Also I believe there is a vapour barrier on the plasterboard which looks like some type of foil, is there any way to confirm this?

 

It is worth knowing that I can only see the end of the eaves on every other void in the flat roof as every other void features a noggin between the joist (I will insulate up to the noggin in this case as some insulation is better than nothing).

 

3) The walls upstairs are timber frame with a half height pitched roof externally (flashing is just below the window) and hanging vertical tiles either side of the windows. I will be looking to cut a hole inside the room big enough to crawl through (only need to cut one hole at the front and one at the back as the voids go all the way across) to create a cold roof in this space. Will the Knauf 100mm and 200mm insulation be suitable for this? I will then use PIR board on the walls up to where the half height pitched roof meets the wall. I hope that makes sense.

 

I am then looking to remove the external hanging vertical tiles, install the PIR board on the areas I am not able to reach from inside the half height pitched roof voids and install cladding.

 

Appreciate this is quite a large post, but any advice would be appreciated.

 

I will add a picture of a diagram to help visualise things and post pictures where I can.

 

Thanks in advance! 

[SteamyTea]

28 minutes ago, 31K said:

the bedroom was 7 degrees

Did you leave a window open? my house seems to never go below external temperature + 3°C when empty and unheated.

While insulating the loft properly is a very good idea, start checking for air leaks, this is often easier to do before you insulate.

[Radian]

52 minutes ago, 31K said:

I would like to board approx half of the loft space and will use 210mm loft stilts/legs (roof joists are 120mm) to allow for the 300mm of insulation underneath. I know this should not be compressed, however will there need to be an air gap between the insulation and the bottom of the boards?

I'll just take this question - yes you certainly do need a gap between the insulation and boarding. In a cold  loft the board will be at outside temperature. The insulation is vapor permeable as is your ceiling regardless of how it is constructed (unless the builder was as meticulous as the specialised types who construct passive houses). This means there's a load of water working its way towards your cold board and when it meets, it condenses and drips back down. The essence of a cold loft is having plenty of airflow to transport vapour unhindered to the outside world.

 

The temptation to use lofts for storage is great and I understand that perfectly. However, it creates numerous problems and should be resisted. If you must have somewhere for the xmas tree, I do have one left-field suggestion though: get a big cargo net and suspend it between rafters hammock style. This adds minimal extra weight and promotes plenty of airflow around the space.

[31K]

47 minutes ago, SteamyTea said:

Did you leave a window open? my house seems to never go below external temperature + 3°C when empty and unheated.

While insulating the loft properly is a very good idea, start checking for air leaks, this is often easier to do before you insulate.

No windows were left open. When checking for air leaks I assume you mean l should draught proof right?

[31K]

16 minutes ago, Radian said:

I'll just take this question - yes you certainly do need a gap between the insulation and boarding. In a cold  loft the board will be at outside temperature. The insulation is vapor permeable as is your ceiling regardless of how it is constructed (unless the builder was as meticulous as the specialised types who construct passive houses). This means there's a load of water working its way towards your cold board and when it meets, it condenses and drips back down. The essence of a cold loft is having plenty of airflow to transport vapour unhindered to the outside world.

 

The temptation to use lofts for storage is great and I understand that perfectly. However, it creates numerous problems and should be resisted. If you must have somewhere for the xmas tree, I do have one left-field suggestion though: get a big cargo net and suspend it between rafters hammock style. This adds minimal extra weight and promotes plenty of airflow around the space.

Thanks for confirming and explaining the purpose of an air gap. Will 30mm be sufficient? 

 

It's likely I will board the roof for storage as with the already limited height in the roof I can't imagine crawling underneath a suspended cargo net to store things. Although it is a thinking outside of the box solution.

[Olf]

1 hour ago, 31K said:

not tall enough to stand upright in (I have to crouch a little)

 

1 hour ago, 31K said:

total of 300mm of insulation

So after adding insulation you would be crawling...

Would what you can reach from the hatch be enough?

 

 

1 hour ago, 31K said:

maybe PIR board like Celotex or Kingspan, possibly removing the need for loft stilts/legs

Rigid boards are ok to transfer load (compressive strength), but too fragile to support. You'd need to board under (over the joists, filling between the joists), so the boards are what holds you standing on PIR.

Wool (Knauf) is going to be cheaper to buy and easier to lay (PIR is messy to cut), but in your case all begins with storage, so you need to sort that.

 

1 hour ago, 31K said:

I have also selected 50mm insulation

Here I'd put PIR instead of Rocksilk: twice teh performance at the same thickness and zero risk of compressing when pushed.

 

1 hour ago, 31K said:

Some of the rooms have spotlights, I was looking to pull these out, push the insulation in and cut it away from inside the room before re inserting the spotlights, will this be ok?

Check if the spotlights are fire rated, what type of bulb is installed (halogen or LED)? When wrapped in insulation they'll heat more. Also when refitting think about how to seal gaps around, as you've just mentioned that your ceiling is a sieve leaking warm air..

[Tims]

I'd bite the bullet on the flat roof and get well fitted (tight and no gaps) PIR insulation up in between all the gaps in the rafters, plaster board is reasonably cheap and a quick job for a plasterer to plaster a small ceiling.

 

In our loft I installed an air barrier at ceiling level then a layer of 50mm PIR that I salvage from the demolition of our old conservatory (floor) and old extension topped with 200mm of uncompressed rockwool and again some reclaimed timber to hold a deck above it for storage I did have some nice big bits of steel up there to support stuff off though. But PIR is a much better insulator than rockwool just not cheap and not as safe. 

Edited December 8, 2022 by Tims

[31K]

10 hours ago, Olf said:

 

So after adding insulation you would be crawling...

Would what you can reach from the hatch be enough?

 

 

Rigid boards are ok to transfer load (compressive strength), but too fragile to support. You'd need to board under (over the joists, filling between the joists), so the boards are what holds you standing on PIR.

Wool (Knauf) is going to be cheaper to buy and easier to lay (PIR is messy to cut), but in your case all begins with storage, so you need to sort that.

 

Here I'd put PIR instead of Rocksilk: twice teh performance at the same thickness and zero risk of compressing when pushed.

 

Check if the spotlights are fire rated, what type of bulb is installed (halogen or LED)? When wrapped in insulation they'll heat more. Also when refitting think about how to seal gaps around, as you've just mentioned that your ceiling is a sieve leaking warm air..

I would probably need a bit more than what's reachable from the hatch, although I should get rid of more than I keep! 

 

Im not sure what you mean when you say: 'Rigid boards are ok to transfer load (compressive strength), but too fragile to support. You'd need to board under (over the joists, filling between the joists), so the boards are what holds you standing on PIR.'

 

If I decide to use PIR in the pitched roof instead the 300mm of Knauf will 100mm be sufficient? The joists are 120mm in height and I can board on top of that without the need for stilts or will I need to lay another layer of PIR on top (over the joists) to ensure the coverage and depth of insulation is sufficient? 

 

Regarding the flat roof, I was thinking of using something more flexible so that it fills the width between the joists incase the are not of an equal distance apart all the way along and use something to grip and push from the far end (closest to the eaves) to avoid compression instead of pushing from the closer end (closest to where the flat roof meets the pitched roof). If I used PIR here does it matter if it's close to spotlights and if there are small gaps near the joists? I'll try to cut the board a few mm more than the width though. If I use multiple PIR cuttings, do they need to be taped with aluminium tape? 

 

I will check the spotlights, the majority of them are LED with a few exceptions. How do I check if they are fire rated? Quick Google search of a model number? When you say sealing the gaps, what should I use? 

 

I will add some photos and diagrams later.

[31K]

10 hours ago, Tims said:

I'd bite the bullet on the flat roof and get well fitted (tight and no gaps) PIR insulation up in between all the gaps in the rafters, plaster board is reasonably cheap and a quick job for a plasterer to plaster a small ceiling.

 

In our loft I installed an air barrier at ceiling level then a layer of 50mm PIR that I salvage from the demolition of our old conservatory (floor) and old extension topped with 200mm of uncompressed rockwool and again some reclaimed timber to hold a deck above it for storage I did have some nice big bits of steel up there to support stuff off though. But PIR is a much better insulator than rockwool just not cheap and not as safe. 

Thanks for the pictures, the issue is I cannot afford (time wise) to do such an extensive amount of work by bringing the ceilings down.

 

I can however install the PIR in the pitched roof area, similar to how you have done. When you say air barrier is that the first picture with the blue tape joining it? What is the purpose of this?

[Radian]

11 hours ago, 31K said:

Thanks for confirming and explaining the purpose of an air gap. Will 30mm be sufficient? 

No. If the air gap was vertical then maybe, like when you put PUR between between rafters and leave a 50mm air gap to the roof membrane. But air is able to circulate by convection in that case. When the layer of air is horizontal it needs to be much bigger to allow for dispersal. Your dewpoint would occur on the underside of the boarding and the dew would just drip down.

 

I think people massively underestimate the issues with using non-breathable insulation in cold roofs.

[31K]

25 minutes ago, Radian said:

No. If the air gap was vertical then maybe, like when you put PUR between between rafters and leave a 50mm air gap to the roof membrane. But air is able to circulate by convection in that case. When the layer of air is horizontal it needs to be much bigger to allow for dispersal. Your dewpoint would occur on the underside of the boarding and the dew would just drip down.

 

I think people massively underestimate the issues with using non-breathable insulation in cold roofs.

So if I go down the mineral wool option and have it at 270mm rather than 300mm that will give me a total of 60mm between insulation and the bottom of the board (considering joist is 120mm and stilts are 210). Will that be enough? 

 

Or is it better to use PIR, not sure what depth I would need to be the same as 270mm of mineral wool, and leave a much bigger gap but still use stilts. For example:

 

140mm of PIR (assuming this is the same as 270mm of mineral wool).

 

Total height to board of 330mm (120mm joist + 210mm stilt)

 

Leaving a gap of 190mm.

[Radian]

You've got to get your head around the fundamental issues. I can tell you haven't completely because if you were to use a closed-cell foam insulator like celotex then very little vapour would get past to require ventilation. You would have, in effect, a locally warm room. But to create a warm roof you need to have a full vapour barrier on the warm side.

 

With breathable insulation like mineral wool, then the sort of gap you need to guarantee no problems with condensation are kind of 'loft sized'.

[31K]

5 minutes ago, Radian said:

You've got to get your head around the fundamental issues. I can tell you haven't completely because if you were to use a closed-cell foam insulator like celotex then very little vapour would get past to require ventilation. You would have, in effect, a locally warm room. But to create a warm roof you need to have a full vapour barrier on the warm side.

 

With breathable insulation like mineral wool, then the sort of gap you need to guarantee no problems with condensation are kind of 'loft sized'.

Indeed, this is why I joined the forum as it's not my area of expertise in the slightest!

 

To confirm my understanding, if I use PIR for a cold roof (in the pitched roof) I would have little issues with vapour (rising from the rooms upstairs and past the plasterboard on the ceiling) which would require less ventilation. Bear in mind I do not want to create a warm roof.

 

If I go down the mineral wool route I should leave as much of an air gap as possible right?

 

 

[Adsibob]

1 hour ago, 31K said:

I will check the spotlights, the majority of them are LED with a few exceptions. How do I check if they are fire rated?

You need to install a fire shield around each spot light. Something like this:

https://www.nationallighting.co.uk/heatguard-small-round-fire-hood-60min-fibreglass?vat=1&gclid=CjwKCAiAs8acBhA1EiwAgRFdw10MRNYsGAbvoreW2mwdX2DumPVNckVKWpTL_XAaw0SFeZTTodR54hoCwLcQAvD_BwE

[Radian]

38 minutes ago, 31K said:

Indeed, this is why I joined the forum as it's not my area of expertise in the slightest!

 

No problem. Keep reading.

 

38 minutes ago, 31K said:

To confirm my understanding, if I use PIR for a cold roof (in the pitched roof) I would have little issues with vapour (rising from the rooms upstairs and past the plasterboard on the ceiling) which would require less ventilation. Bear in mind I do not want to create a warm roof.

 

 

No. Without an effective vapour barrier and airtight seal, you might have big issues. You keep saying you don't want to create a warm roof but that's almost what you're doing - the sort that cowboy builders make and home owners discover, years later, is growing mushrooms under the decks. I suppose you could argue that the PIR is just like having thicker mineral wool, although the later provides free passage to vapour, but you intend to board over the insulation and this is where the real problems lie. The vapour needs a good amount of air circulation to transport it to the outside.

 

Even in a heated and ventilated room, looking behind furniture next to walls you may find signs of mildew simply because of restricted ventilation in those areas. The outside temperatures you'll find in lofts amplify this effect enormously. It's not uncommon to go up into a loft space and almost immediately have drips of condensation fall from the underside of the roof as the warm air from the house rises up to it.

 

If your detailing is not correct then this effect is taking place 24/7 throughout the heating season and it's the cumulative effect that will begin to cause problems.

 

I related elsewhere how I had put a large sheet of 100mm celotex up into our loft in preparation for cutting and fitting to the back of the loft hatch. I put it up there laid flat across the top of the fiberglass insulation. After it had been there a while and I finally got around to the job, I was amazed at how much mildew had developed on the underside of the celotex and what a swampy mess the fiberglass was in below. Vapour passing through the plasterboard ceiling could not escape to the air circulating in the loft so condensed on the foil surface of the insulation.

[31K]

1 hour ago, Radian said:

 

No problem. Keep reading.

 

 

No. Without an effective vapour barrier and airtight seal, you might have big issues. You keep saying you don't want to create a warm roof but that's almost what you're doing - the sort that cowboy builders make and home owners discover, years later, is growing mushrooms under the decks. I suppose you could argue that the PIR is just like having thicker mineral wool, although the later provides free passage to vapour, but you intend to board over the insulation and this is where the real problems lie. The vapour needs a good amount of air circulation to transport it to the outside.

 

Even in a heated and ventilated room, looking behind furniture next to walls you may find signs of mildew simply because of restricted ventilation in those areas. The outside temperatures you'll find in lofts amplify this effect enormously. It's not uncommon to go up into a loft space and almost immediately have drips of condensation fall from the underside of the roof as the warm air from the house rises up to it.

 

If your detailing is not correct then this effect is taking place 24/7 throughout the heating season and it's the cumulative effect that will begin to cause problems.

 

I related elsewhere how I had put a large sheet of 100mm celotex up into our loft in preparation for cutting and fitting to the back of the loft hatch. I put it up there laid flat across the top of the fiberglass insulation. After it had been there a while and I finally got around to the job, I was amazed at how much mildew had developed on the underside of the celotex and what a swampy mess the fiberglass was in below. Vapour passing through the plasterboard ceiling could not escape to the air circulating in the loft so condensed on the foil surface of the insulation.

Thanks, appreciate the advice.

 

The same principle applies to the flat roof which is essentially 'boarded' by the ply above?

 

This issue can be resolved in the pitched roof by not boarding the loft or limiting the amount of boarding/using cargo nets as advised and leaving as much gap as possible in the flat roof.

 

Will any of the solutions I proposed in my initial post be suitable in this case?

[Radian]

2 minutes ago, 31K said:

The same principle applies to the flat roof which is essentially 'boarded' by the ply above?

 

 

Yes. A good airflow above the insulation is essential - cross ventilation is the only way to ensure this. Keeping moisture from the inside from getting into the roof space is also highly desirable. All this makes it very difficult to achieve using keyhole surgery like you were discussing. Have a read through this article by tlxinsulation for some more info about interstitial condensation.

 

9 minutes ago, 31K said:

This issue can be resolved in the pitched roof by not boarding the loft or limiting the amount of boarding/using cargo nets as advised and leaving as much gap as possible in the flat roof.

 

Will any of the solutions I proposed in my initial post be suitable in this case?

 

You set out your most cost effective solution using loft roll. That still stands. If you avoid covering it with boxes or boards and the loft is well ventilated, you are onto a winner. Don't forget the loft hatch!

[Tims]

10 hours ago, 31K said:

When you say air barrier is that the first picture with the blue tape joining it? What is the purpose of this?

For me what it has is a dual purpose, its wrapped under my steel and joined to the vapour control layer on the large warm flat roof that's connected to the other side of it as I, A) don't want condensation on the warm side of the steel and B) want to get my house as airtight as possible for an MVHR system install and don't want drafts from all of the holes through the ceiling plaster board to the open air ventilated loft.

 

As Radian says the most cost effective and time proven solution for you loft is lots of un compressed mineral wool.

 

On your flat roof there are lots of detail drawing on the web of how to do a successful cold roof. If you leave lots of uninsulated gaps and have ventilation open to the outside as it should be the insulation you have put in it will get bypassed by all of the leaks and massive cold bridges where you've left it out. Also you don't want the ventilation paths above the insulation (to the outside air) being blocked by noggins (you should maintain your 50mm ventilation gap above your noggins). My BCO was happy to see that I'd put a warm roof on our extensions as he saw lots of issues with cold roofs that builders have bodged and blocked the ventilation with cross members for roof lights etc.

[31K]

19 hours ago, Radian said:

 

Yes. A good airflow above the insulation is essential - cross ventilation is the only way to ensure this. Keeping moisture from the inside from getting into the roof space is also highly desirable. All this makes it very difficult to achieve using keyhole surgery like you were discussing. Have a read through this article by tlxinsulation for some more info about interstitial condensation.

 

 

You set out your most cost effective solution using loft roll. That still stands. If you avoid covering it with boxes or boards and the loft is well ventilated, you are onto a winner. Don't forget the loft hatch!

Thanks for the link to the article, has answered some of the questions I had. Also managed to find a good video on YouTube explaining cross/through ventilation and its importance.

 

I think limited boarding and cargo netting will save me a lot of head ache down the line.

 

I will try to seal gaps around extractor fans and down lighters where possible to reduce the amount of moisture entering the roof space. Fortunately not all of the rooms have down lighters! 

 

I will post a plan and some pictures later as a final before commencing the work incase there are any bits that I have missed and give some updates as I carry out the work.

 

Thanks again.

[31K]

22 hours ago, Adsibob said:

You need to install a fire shield around each spot light. Something like this:

https://www.nationallighting.co.uk/heatguard-small-round-fire-hood-60min-fibreglass?vat=1&gclid=CjwKCAiAs8acBhA1EiwAgRFdw10MRNYsGAbvoreW2mwdX2DumPVNckVKWpTL_XAaw0SFeZTTodR54hoCwLcQAvD_BwE

Looks like a good idea as it appears to be flexible so I should be able to put this into the hole from inside the room. 

[31K]

13 hours ago, Tims said:

For me what it has is a dual purpose, its wrapped under my steel and joined to the vapour control layer on the large warm flat roof that's connected to the other side of it as I, A) don't want condensation on the warm side of the steel and B) want to get my house as airtight as possible for an MVHR system install and don't want drafts from all of the holes through the ceiling plaster board to the open air ventilated loft.

 

As Radian says the most cost effective and time proven solution for you loft is lots of un compressed mineral wool.

 

On your flat roof there are lots of detail drawing on the web of how to do a successful cold roof. If you leave lots of uninsulated gaps and have ventilation open to the outside as it should be the insulation you have put in it will get bypassed by all of the leaks and massive cold bridges where you've left it out. Also you don't want the ventilation paths above the insulation (to the outside air) being blocked by noggins (you should maintain your 50mm ventilation gap above your noggins). My BCO was happy to see that I'd put a warm roof on our extensions as he saw lots of issues with cold roofs that builders have bodged and blocked the ventilation with cross members for roof lights etc.

Thanks, from inside the pitched roof I can only insulate up to the noggins without taking the roof or plasterboard off and I don't want to go down that route. I appreciate that there will be uninsulated spots if every other gap between the joists is only insulated up to the noggin, but surely some insulation is better than nothing? I will have to see what amount of clearance there is between the top of the noggin and the roof, hopefully it's sufficient. If not I won't insulate these areas, only insulating the voids where I can reach up to the eaves.

[31K]

On 08/12/2022 at 14:35, Radian said:

 

Yes. A good airflow above the insulation is essential - cross ventilation is the only way to ensure this. Keeping moisture from the inside from getting into the roof space is also highly desirable. All this makes it very difficult to achieve using keyhole surgery like you were discussing. Have a read through this article by tlxinsulation for some more info about interstitial condensation.

 

 

You set out your most cost effective solution using loft roll. That still stands. If you avoid covering it with boxes or boards and the loft is well ventilated, you are onto a winner. Don't forget the loft hatch!

For the flat roof, which one of the following would you use?

 

https://www.knaufinsulation.co.uk/products/rocksilk-rs45

 

Or

 

https://www.knaufinsulation.co.uk/products/omnifit-slab-35

 

After speaking to Knauf they confirmed that the RockSilk is water repellant (not waterproof) will this be a potential issue? Could this trap vapour (which has penetrated through the ceiling) between the plaster and insulation and prevent it from going through the insulation into the air gap for it to be ventilated out? The water vapour resistivity appears to be the same for both products.

 

 

 

 

[31K]

So, bit of an issue when inspecting the existing ventilation on the flat roof. It doesn't appear to have any!

 

The soffit most definitely does not have any, but there may be something where the top of the fascia board meets the roof overhang.

 

Is there an easy way to confirm this? Possibly shining a light or something to see if it can be seen from inside the roof void?

 

 

[Tims]

3 minutes ago, 31K said:

So, bit of an issue when inspecting the existing ventilation on the flat roof. It doesn't appear to have any!

 

The soffit most definitely does not have any, but there may be something where the top of the fascia board meets the roof overhang.

 

Is there an easy way to confirm this? Possibly shining a light or something to see if it can be seen from inside the roof void?

 

 

 I wouldn't be surprised for it to be unventilated if they haven't insulated it. If you can see into the flat roof from the roof void as long as you have your loft lights off I'd expect you to be able to see  some daylight coming in from the fascia or soffit if it has vents.

[31K]

5 hours ago, Tims said:

 I wouldn't be surprised for it to be unventilated if they haven't insulated it. If you can see into the flat roof from the roof void as long as you have your loft lights off I'd expect you to be able to see  some daylight coming in from the fascia or soffit if it has vents.

Thanks Tims, I'll pop my head up there in a bit to see if I can see anything.

 

If there are no vents, is there a cost effective DIY solution to add vents into the fascia? I say fascia as the soffit is very narrow. I will also consider alternatives. Don't want any ventilation issues