Improving insulation to suit an Air Source Heat Pump - what are you doing?

As far as I’m aware, 270mm is probably as much as you’re likely to want unless you’ve got a house which is really bad at leaking absolutely every bit of heat and is a constant iceberg as a result!

It’s quite common advice to consider blocking up or removing fireplaces that you’re no longer using. I’d also agree with this, as long as it’s done properly - but you’d probably want to have the chimney swept out first because you won’t be able to later. Given that fireplaces have said chimneys which can let heat escape with no real control, it’s not worth keeping it open if you’ve decided to stop using the fireplace.

Please make sure you’re certain first though! Undoing something like that is not easy or cheap.

As with everything, it’s often down to what works best for you. If the cavity wall insulation is in good shape and you’ve already got 270mm loft insulation and you’ve got double-glazing on the doors/windows… You’re already doing pretty well!

Thanks. I think we’re pretty safe having the fireplace blocked up, we’ve lived here 18 years and never used it!  Our energy rating from RetrofitWorks is 53E - the biggest culprit being our gas fired warm air heating. I’d say the house may well be a constant iceberg unless that’s running . 

Hi @juliamc - sorry I missed this. If you start a new Topic and you want to hear from me, then please make sure you “tag” me so I get an email alert

If we’re talking about glass-fibre insulation then you probably won’t gain anything by going deeper than 250mm. The crucial bit is how it gets fitted where it dips down into the eaves. Have look at this diagram, helpfully supplied by BRE:

Notice how the insulation roll doesn’t stop at the batten holding the ceiling. It continues right over the wall-plate and down to meet the cavity-fill (if present).

The diagram isn’t completely accurate because it’s the wall plate (my red label) which supports the rafters rather than there being a gap as shown above.

Nevertheless, you will also note the critical gap for air to pass into the attic space and prevent condensation gathering on the cold roof timbers. Without this they will rot!

I have never seen a professional insulation company get these details correct. They normally use the most junior team member to crawl into the narrow angle and prod the insulation with a piece of wood. That’s not effective, and you will end up with cold corners on the bedroom ceilings which eventually grow black mildew.

I wouldn’t get rid of any warm-air ducted system. Just decommission the gas input to it.

I actually installed a warm-air ventilation system when I refurbished our 1937 farmhouse. It takes stale air from bathrooms and kitchen, passes it through a heat exchanger and pushes fresh warmed air back into living spaces.

I supplement the heat by having a small “radiator” to further raise the temperature of the air before it gets piped to bedrooms and sitting-room. You could do the same trick by taking heat from the Heat pump. I’ll explain in more detail later if you want.

My house now has a warm-roof construction.

We took off the slates, added 50mm phenolic insulation board and then used counter-battens to position a new slate roof on top.

Following the outside part of the job, I used 75mm-thick phenolic boards between the rafters, giving us 125mm overall.

It’s called “warm roof” because the rafters, purlins and trusses are now within the insulation envelope of the house. They don’t get cold, so no condensation forms and thus they can’t rot.

Large quantities of B-grade phenolic board insulation are available from Seconds & Co who will deliver by lorry when they have other orders in your area. Some sheets have damaged edges which can be cut off, and some are of uneven thickness. But they are very much cheaper than you could get elsewhere.

I am very much a hands on renovator, and don’t like employing contractors if I can possibly avoid it. So any suggestions I make are from real first-hand experience.

Hi @Transparent thanks for your reply - I didn’t know about tagging !

I was that junior team member years ago when I installed the insulation  and I think I did poke it into place with a stick… claustrophobia will now prevent me going back there but everything seems okay regarding condensation and mildew. I’m replacing the insulation in the middle part of the loft having fixed a LoftZone platform so that I can easily access the new equipment. I’m not convinced the ASHP will give us enough heat over the coldest winter days so hanging onto the old gas system. I assume the rules of the trial mean I’m not allowed to run it but I don’t want anyone to decommission it - for one thing I’ll get it serviced during the summer and obv it’ll have to work for that! I did think about having the ducts sealed by ‘Aeroseal’ to improve its efficiency. Expensive, unless I can find neighbours who’d like that done too. I like the idea of the ventilation system with heat recovery so I’d definitely want to keep it as intact as possible until I decide what’s required.

This shows the area needed for access to the new equipment now. The cold water cistern was previously high up on a platform beyond the carpeted area, the original cement asbestos tanks are now sitting on a celotex insulated platform out of the way. LoftZone beams ready for boards.

That all looks good @juliamc 

Seeing those deep rafters makes it very tempting to go for a warm roof construction!

The parts aren’t expensive. But the real costs are the scaffolding and labour to put an external layer of insulation under the tiles/slates.

Your hot-air ducting looks well insulated already. You could add to that, but I’d hesitate about getting in contractors to do a “professional job”.

I’m not tempted !!

The silver wrapped ducts are the ones I did some years ago, the less accessible ones and the diagonal one in the distance are just covered in the original glass fibre and paper wrap with my extra layer of 100 mm loft insulation. I intend to get some Foil faced Ductwrap to redo the diagonal one. The Aeroseal would be to seal any air gaps, I’m sure there must be gaps which I’m not going to search out, some of the furthest registers hardly have any heat coming out of them. But of course if the ASHP works okay I don’t need to worry!

Even if the ASHP works OK, I can’t see how the configuration being installed is financially viable. You’re going to need that insulation to be top-notch if you want to get the COP above 4.0

When I added warm air ducts most of the pipes were run in the attic, which is within the insulation envelope of the house. So I bought lightweight/cheap plastic components from BES and taped them together. I applied a foil bubble-wrap (400mm wide) around the outside and held it place with aluminium tape.

If there were any minor leaks in the plastic pipe joints, they had little effect because the taped insulation layer also prevented escapes.

If I were to tackle the same task in your “cold” attic, I’d employ the same approach, but use a much higher grade foil-insulation, such as Actis’ Triso Super-10. That’s equivalent to about 100mm of conventional mineral-wool insulation.