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Roof vs Loft insulation - top tips for improving your home's energy efficiency

  • 23 November 2021
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Roof vs Loft insulation - top tips for improving your home's energy efficiency
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 Roof insulation vs loft insulation - which is best?

 

Our Content team have been busy bees creating some great guides on house insulation. They’ve made a guide on loft insulation here, and one on roof insulation here.  

 

As always our online community has offered its members some first hand experience. One of our resident home builders, @Transparent, has outlined some tips to minimizing the first risk of electricity cables with house insulation, here:

 

 

And they posted about house ventilation, here:

 

 

Now we want to go a step further and leverage our wider community of home improvers, to answer some questions around house insulation. This will help someone looking to save energy and reduce their bills and carbon emissions by improving their home’s insulation. So, are you up for sharing your knowledge on this? 

 

Question 1: What’s the actual difference between loft insulation and roof insulation, and which is better?

 

 

Question 2: What is the expected return on investment of loft/roof insulation? Should I do other improvements first?

 

 

Question 3: Should I look to make these improvements myself, reducing costs by only buying the materials? 

 

Have you got first hand experience? We want to hear from you in the comments below! 


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I think I ought to expand on the difference between two different types of “roof insulation”.

 

A: The type described in the Guide from the OVO Team is where the insulation layer is positioned between the rafters.

The outer/upper edge of the rafters are therefore at exterior temperature, which can be below freezing, whilst the lower edge faces into the attic. This creates a large temperature differential across the wooden rafter which can lead to condensation forming. That’s why the Guide emphasis that you still need airflow within the attic to prevent the build-up of rot/fungus.

Here’s the 2021 rebuild of my front porch in which I’m able to install Celotex PIR boards (Polyisocyanurate) between the rafters from the outside, before adding the battens and slates

 

B: The second type is called a Warm Roof Construction. This can only be achieved whilst the roof is being renovated, and the outer water-proof layer is absent.

The base layer is as just described, but then a second layer of insulation is fixed on top, spanning both the rafters and the insulation between them.

In this case there is no condensation problem with the rafters because they are entirely within the warmed envelope of the house. They remain dry and won’t succumb to rot.

The Counter-battens must have air running between them, but this has no effect on the ventilation of the attic space itself.

The main roof of my house has been re-constructed using this Warm Roof principle, which then looks no different on the outside from the more usual ‘cold-roof’ system.

The upper roof uses 50mm Celotex boards for the outer layer, held in place with 50x25mm counter-battens.

The smaller roof below the glazed section uses Actis Triso Super-10 multi-foil insulation for the outer layer. This behaves differently, reflecting more radiated heat back into the building, but having a lower response to convection and conduction.

Triso Super-10 is no longer certified for use in the UK, but Actis have other multi-foils available. The British company, Superfoil also offer a range of suitable thin multi-foil products.

 

On new-build houses, it is becoming more common to have the entire sloping roof made from a Structural Insulated Panels (SIPs), thus avoiding the use of rafters completely. This is the natural extension of the Warm Roof principle using modern techniques.

British firm Kingspan have pioneered SIP building and have extensive information on their website.

If your house requires major insulation upgrades in order to meet the requirements for a Heat pump, then a complete re-roofing project with SIPs should be considered.

A Heat pump requires a SAP energy-efficiency score above 80 in order to be viable/effective.

An entire SIP-roof can be craned into position in one day, making the house weather-proof and air-tight. The outer layer of OSB3 (Stirling-board) can withstand British weather for at least a month whilst a team of roofers completes the required finish in tiles or slates.

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@Transparent Looking at your final paragraph, could that external finish be solar PV tiles? 

That seems to be a better option than either standard tiles + panels on top, or (for many of us) removing existing PV panels, reapplying standard tiles and re-fixing the original panels.  Which leads to the side issue of whether there is a market for second hand panels, otherwise unnecessary waste is created.

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Yes, you’re absolutely right @EverythingNeedsAUserName 

Putting PV Solar Panels directly onto a SIP roof is very easy because it’s a completely flat layer of OSB board!

The Panels which fit within the roof-line are different to those which fix to rails above tiles/slates. Here’s a link to the Solfit system, as supplied by Midsummer. When fitted with the required flashings, it results in a completely weather-tight array and a 15-year guarantee. The animation video on YouTube shows how the parts are screwed directly to battens:

 

Other suppliers of integrated roof-line Panels are Viridian Solar and  GB Sol.

Better still, many Local Planning Authorities will allow Roof-line Solar Panels to be installed in designated Conservation Areas.

 

Solar Panels rarely ‘die’ but their energy conversion rate declines over time.

I don’t know of any recognised second-hand market route. However they could surely be gifted to schools and horticultural establishments where they could be put to good use closer to ground-level.

If a Panel really is dead, then it’s substantially recyclable. The aluminium frame and the glass both have existing routes to recycle operators. Even the silicon wafer itself is chemically very similar to glass!

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Wow such a good comment, @Transparent. Lots of info there I want to ask about. 

 

Tis interesting hearing about new build and new roof insulation approaches, and the advantage they have vs the retro fitting option. With so many houses (although I’m sure one might argue not enough) built in the last century, the retro fitting route will be popular. Unless most of the time insulation is already present. Do most houses have loft / roof insulation already? Tricky to answer I’m sure. 

 

Anyway back to my list of questions, assuming we’re looking at a house that’s already built, with the occupant not wanting to renovate the whole roof, @Transparent and @EverythingNeedsAUserName is loft vs roof insulation an actual debate or is there always a clear winner? Does the answer include ‘it depends on the roof and the loft’? 

 

Taking it back to ‘do most houses have loft insulation’, this ties into that ROI. If the costs of the work and the materials of insulating your loft/roof are easily made back with savings in heating costs, isn’t it reasonable to assume that a lot of houses would have this already? Well, no. Renters such as myself, low income households may not have the option. Any thoughts on ROI for loft/roof insulation? 

 

And as @Transparent mentions, then there’s heat pumps. As members like @Gingernut49 @SandraAA @juliamc @James_N @Amarritt have found out, the temperature of the loft makes a different when there’s heating equipment kept there. But isn’t ventilation equally as important, to protect against high temperatures? Is that from summer heat or from the tanks and pipes themselves? 

 

What do we think trialists? 

 

Question 1: What’s the actual difference between loft insulation and roof insulation, and which is better?
 

 

Question 2: What is the expected return on investment of loft/roof insulation? Should I do other improvements first?
 

 

Question 3: Should I look to make these improvements myself, reducing costs by only buying the materials? 

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Comfort rather than payback.

First prevent draughts - even simple things like gaps round doors, between skirting board and floor, between floorboards. As part of that we've always had windows replaced with double glazing. IT YOU HAVE GAS APPLIANCES YOU MUST ENSURE THAT YOU HAVE VENTILATION. 

Our house was built around 1980. It had loft insulation between the rafters but it had settled a bit. I bought rolls of insulation from B&Q (other DIY stores are available) and put them on top at 90°. The end depth is about 12 inches or 30cm. There is an area in the centre that I had boarded with old wardrobe doors where I store things such as Christmas decorations - I'd like to put down insulated boards but would need shorter legs to be able to subsequently use that area. The job was easy enough so long as you tread on the joists and don't mind a lot of tight spaces and wearing goggles, a mask and gloves - depending on the type. You can't go right to the edges because the loft needs ventilation. I'm lucky to have had a Skoda Octavia at the time with a cavernous boot so that I could collect the insulation when it was on offer, but you'd be able to get it delivered easily. You're saving the cost of the workers but perhaps paying slightly more than them for the materials. 

Warm Roof insulation to be done properly is a professional job and would be as @Transparent said, for renovation not payback. It needs to have that insulation above and below the rafters. If we ever needed a loft conversion that would be the time to do it.

Had we still been in our Sheffield house (we left 30 years ago) I'd have considered it. It was a 1920s semi and had what had probably been an inch of insulation that had flattened to zero. The tiles had had cement underneath to stop draughts or keep them more secure, and much of it had fallen off. We got a local firm to put in cavity wall insulation and boost the loft insulation with blown Rock wool. That and UPVC double glazing made an enormous difference to the comfort. It was a lovely house but, too soon, work moved us away from the region. 

Back in around 1964 our family moved to a Victorian semi. My dad took out the old lead-clad electricity cables and sold them to buy rolls of loft insulation! He was an electrician so competent at the rewiring. 

If it snows you can often see the empty roofs where the escaping heat has melted it enough to slide off. Those are the ones I'd target if I was an installer or a government. 

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I topped up our loft insulation myself and wished I'd paid someone. We have a roof with multiple slopes so you can only stand up in the middle of the loft. That meant crawling out to each side to lay the new insulation which was exhausting work. The protective clothing made it hotter and even more difficult.

 

We had insulation that originally came just above the joists but had flattened a bit so it was mostly level with the joists. I laid an additional 150mm over it all.

The middle is boarded so I lifted them and used solid slabs of insulation (Kingspan or similar). That provides the equivalent insulation but without having to raise the boards. I also attached a slab to the loft hatch and put a foam strip around the edge.

 

I have no way of measuring any improvement. It possibly feels a bit warmer but we've changed our heating system since then so it's hard to compare. I am considering getting an assessment of the house with a thermal camera which should show up any areas to improve. If anyone can recommend a company who could do this please let me know.

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@M.isterW wrote:

I have no way of measuring any improvement. It possibly feels a bit warmer

I really glad you wrote that. It gives a chance to correct an ‘urban myth’.

Unless the house is suffering from a tremendous amount of draughts, it is very unlikely that an occupant would feel warmer in their house after increasing insulation levels.

If you think about it, almost all houses have thermostats which control the room temperatures. Adding more insulation doesn’t alter the thermostat settings.

The only difference which most people will notice is that the house uses less energy to achieve the set-temperatures. So that gets reflected in the bill, not the comfort level!

If a salesman tries to make you commit to a package of insulation upgrades by telling you that you’ll feel warmer afterwards, then you know that the proposal is a suitable candidate for BBC Rogue Traders. Show him the door… and then close it firmly once he’s outside. :relaxed:

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@EverythingNeedsAUserName  wrote:

I bought rolls of insulation from B&Q (other DIY stores are available)

Let me help to narrow the choices a bit!

As a self-builder over many years, I’ve learned how the UK insulation market works and where the better deals are likely to be achieved.

 

1: Almost any local Builders’ Merchants will be happy to price-match insulation products from your nearest B&Q store. Just give them the chance and they’ll probably also deliver for free :slight_smile:

 

2: The Travis Perkins Group own CCF, one of the big national insulation companies. Even if you want a type of insulation which your local TP branch doesn’t usually stock, ask them if they can obtain it from CCF for you.

 

3: The other large national insulation suppliers is Sheffield Insulation Group (SIG) who have their own fleet of lorries with fork-lift trucks hung off the back!

Individual customers won’t be able to open a trading account, but SIG sell online under the name Insulation Express. Order from there and you may well find one of their big red lorries off-loading your pallet a couple of days later.

 

4: If you want a quantity of PIR boards such as those made by Celotex, Kingspan or Recticell, then contact Seconds&Co. They stock ‘seconds’ of these products at their Welsh warehouse. They combine orders for an area over several days and then deliver using their own lorries.

Reject boards may be slightly narrower where the edge has been damaged and then cut off. Or the thickness may be uneven. Ask what they’ve got available which roughly matches what you’re trying to achieve. Prices are roughly two-thirds of the 1st quality product.

 

5: To cut insulation rolls and boards, use a saw with no teeth!

I have a wavy-edge saw made by Bahco which produces no dust when I make the cut. :mask:

 

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What about the house walls?

If you have a house with solid walls or with insufficient gap to fill with adequate insulation, then it’s tricky to justify putting enough insulation into the roof to facilitate having a Heat Pump, for example.

One possibility is to extend the building with new super-insulated rooms!

If you’ve never considered an extension made from Structural Insulated Panels, then have a look at this video from the Lowe Group, which has manufacturing bases in Cornwall and Buckinghamshire. It shows a 45-minute sequence speeded up to less than 5-mins. Enjoy!

 

That video showed a single-storey commercial extension. If I was doing this for a domestic dwelling then I’d have made a number of changes:

  • 150mm thick panels instead of 125mm
  • Insulated foundation blocks and DPC (damp-proof course) extended to seal into the floor area
  • join the panels using splines fixed with PU-adhesive (like Gorilla glue)
  • screw/nail panels to the bottom sole-plate

But I hope the video shows the simplicity and speed of construction, and may inspire you to explore SIPs further.

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@M.isterWwrote:

I have no way of measuring any improvement. It possibly feels a bit warmer

I really glad you wrote that. It gives a chance to correct an ‘urban myth’.

Unless the house is suffering from a tremendous amount of draughts, it is very unlikely that an occupant would feel warmer in their house after increasing insulation levels.

If you think about it, almost all houses have thermostats which control the room temperatures. Adding more insulation doesn’t alter the thermostat settings.

The only difference which most people will notice is that the house uses less energy to achieve the set-temperatures. So that gets reflected in the bill, not the comfort level!

If a salesman tries to make you commit to a package of insulation upgrades by telling you that you’ll feel warmer afterwards, then you know that the proposal is a suitable candidate for BBC Rogue Traders. Show him the door… and then close it firmly once he’s outside. :relaxed:

Draughts are the enemy. Whether from gaps or from warm air rising, then cooling on cold surfaces, falling and hitting the back of your neck. We did feel the house more comfortable from double glazing - perhaps because better insulation lessens that cold air circulation? We certainly have warmer feet since we put underlay with a higher tog rating under our new carpet. For the record, I've never sold insulation nor directly or indirectly worked for any company that did. I disbelieve double glazing salespeople on principle. 

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Our house was built around 1980. It had loft insulation between the rafters but it had settled a bit.

 

Interesting to think of insulation ‘settling’, @EverythingNeedsAUserName  - I’m guessing this is less of an issue with newer insulation materials or is there still an anticipated lifespan for how long insulation will be effective? 

 

How would you check whether existing insulation would need updating? (other than waiting for the next snowfall! :snowflake: )

 

 

We had insulation that originally came just above the joists but had flattened a bit so it was mostly level with the joists. I laid an additional 150mm over it all.

 

Surprised to hear that newer insulation can be added on top of an existing insulation layer too, @M.isterW - although I’m guessing there’s no such thing as too much when it comes to insulating layers!

 

Excuse what might be a bit of an off-tangent question but are materials of these insulating layers changing too (feel like I’ve heard of sheep’s wool being a good bio-degradable alternative?) - Ready for @Transparent to step in with some fact-checking for that one! 

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Well I’d first like to comment on the issue of insulation “settling”.

Yes it happens slowly with (horizontal) loft insulation, especially if dust and debris can fall on the top surface and start weighing it down. You can speed it up significantly by throwing boxes of Christmas decorations on top each year! :face_palm_tone1:

As for settling within a cavity wall…. this can be checked generally by using a thermal gun and then evaluated with a borescope after drilling a small hole.

Bosch GIC-120

Scam alert:

There are con-artists who telephone home-owners and offer a free survey to see if the cavity wall insulation has settled. Do not let them anywhere near your house!

The answer is always that it requires their specialist attention… but if you agree to it immediately then “you get a 50% discount from their workforce who just happen to be finishing a job at a house nearby!”

And in case you’re wondering, yes there are ways to make cavity wall insulation slump by pouring the relevant chemical into the hole in the wall through which they inserted the camera! :scream:

 

Sheeps wool insulation. If you can afford a bit extra to have it, then do so. It’s wonderful stuff with good insulation properties and no itchy fibres to get into your eyes during installation. There are different types and thicknesses. Get it from a supplier who knows what they’re talking about (not Amazon!)

www.celticsustainables.co.uk/thermafleece-cosywool-sheeps-wool-insulation

Sheeps wool insulation also works well as an acoustic absorber. Put it under a teenager’s floor before they’re old enough to stay up all night playing video-games.

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@Transparent I’d love to use wool for insulation, but we have carpet moths here eating our wool carpets under and behind furniture, so I’d fully expect them to eat the insulation too unfortunately… 

Meanwhile I am indeed armed with a thermal gun to try and track down cold spots and slumped cavity wall insulation, now it’s cold enough to show up. 

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What about contacting the main UK manufacturers, Thermafleece Ltd and ask them about carpet moth, @juliamc

If they have a remedy which is applied to their products, then ask if you could also use it on your carpets.

Alternatively, I notice they offer insulation made from Hemp. The moths still eat it, but they quickly get stoned and lose the will to live. :wink:

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Haha.. is Surrey ready for hemp insulation ? I have stuff that kills the moths but it has to be re-applied (apparently every three months !)   I also have put diatomaceaous earth under the thick wool underlay under our sitting room rug, as advised by the pest control people. 

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What about this little fella?

Trichogramma evanescens

Obtainable as a cluster of 2400 eggs on a card from Dragonfli  :relaxed:

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What about this little fella?

Trichogramma evanescens

Obtainable as a cluster of 2400 eggs on a card from Dragonfli  :relaxed:

In all the time I’ve spent on this forum, I think I might just have seen the single most useful link…

Cheers, @Transparent ...

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@Simon1D - keep us posted… this could be a thread all of it’s own !

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@Simon1D - keep us posted… this could be a thread all of it’s own !

I hope not.

It’s not that we moved house to escape them, honest.

But the thought that some of the little devils might have moved with us would be troubling, in a house that, like the previous one, has an awful lot of high %age wool carpet in it.

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Wow, who’d have thought you can get parasitic wasps delivered these days? Quite the novel approach to tackling insulation-eating moths which could well be deserving of it’s own thread if anyone is tempted to put those moth-eaters to the test. :wink:

 

I’m guessing the man-made stuff is the only fully moth-proof insulation option then if you’re not willing to release the wasps? :bee:

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[Warning - discussion going temporarily off-tangent]

 

I’m intrigued @Jess_OVO -

Why might someone not be ‘willing to release the wasps’?

 

Suppose I had a problem with flies in my house

 

So I raise the issue on the Forum and receive the reply that I need some of these

Do I hesitate to introduce spiders?

Do I feel happier with flies?

Or is it that I just don’t like ordering things on the web? :spider_web:

 

What if I take the argument a step further and publish a post here about a virus?

Should I be cautious about releasing the mRNA which gets suggested?!

Would we be happier if the remedy was less….    biological?!

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so what happens to the wasps once they’ve eaten all the moth eggs/larvae ?

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:coffin:

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so what happens to the wasps once they’ve eaten all the moth eggs/larvae ?

 

This was my concern, @Transparent! And from the sounds of it these ‘parasitic wasps’ come at a bit of a cost if it’s a re-occuring issue. Likely to impact the ROI on this form of insulation perhaps? :shrug_tone2:

 

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Ah… now I get it. :slight_smile:

The beauty of biological treatments is that the parasitic species doesn’t just infect the prey’s larvae/eggs once. It creates a new generation of adults which then seek out further lepidoptera eggs within the home.

The question about how often the wasp treatment needs repeating should be asked of the ‘manufacturer’s, DragonFli. It depends on what proportion of eggs survive the plague of wasps and go on to hatch into adult moths. This in turn depends on the wasps having good access to the egg locations, such as drawers and wardrobes with woolen clothes in them.

DragonFli must have sampling data to know the answer to this issue.

2400 wasp eggs costs £12 and ‘treats’ 20sq.m. The more you buy in a batch the cheaper it is.

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