Topping up loft insulation

Hello everyone - the loft we inherited needs some extra insulation.

The existing insulation is the standard glass fibre roll (~100mm deep). The last renovation was ~15 years ago so I guess it is original per then.

What’s going to give me the best bang for my buck whilst being easy/safe to handle and not damaging to the planet?

I’ve done a few searches but find the array of possible solutions overwhelming.

I’d be interested to know what other CC members have used in their lofts. Thanks.

We contacted our gas supplier and they came and put about 30cms of insulation on top of what we had. It was covered by a grant at the time so the cost was nil.

Although the current grant programme has been deemed a failure I think it’s still in effect. So you’d probably pay much less than the cost. I would start with your gas supplier as there are fairly strict guidelines as to who can install under the current programme.

If their quote is too high you can still DIY. By then you will know what depth of insulation is recommended.

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Thanks Paul. We’re all-electric (no gas in our village). I’ll drop a note to our leccy supplier and ask the question though.

The funding programme for loft insulation has in the past been ECO - Energy Company Obligation. In the mid 2010s the eligibility was massively cut back and rates of insulation across the UK dropped off a cliff. Unless you are receiving a benefit (JSA, Universal Credit, etc etc) you won’t get ECO. Child Benefit is eligible for ‘dual measures’ - which I think is usually an insulation PLUS a new boiler under certain conditions. There are companies working on behalf of energy companies so you can simply google it to find companies. However beware the work can be sloppy - to get it signed off and get it recorded on a EPC there only needs to be insulation for 1 metre surrounding the loft hatch.

The top quality job for the loft is to incorporate air tightness and vapour control. Removing all insulation, putting heat/fire proof and air tight protection over downlighters and sealing around cable penetrations, and adding an insulated air tight loft hatch. Then you lay a vapour barrier membrane across and between the joists, and taping up the overlaps. Then after laying the insulatoin you put an airtight membrane over the top of the insulation to prevent air movement getting in or behind the insulation. Also ideally you would design wall insulation to be continuous with loft insulation. Example: Roof eaves: insulation & airtightness for a typical W. Yorkshire terraced house - YouTube
Most people don’t bother with this - I won’t in my current house - but this would be necessary for an AECB or Passivehouse standard retrofit.

Anyway the main step is adding up to 400mm of quilted insulation - mineral wool, glass wool and sheeps wool all have similar thermal properties. Sheeps wool and, I understand, some of the more expensive glass fibre, is nicer to handle. But the stuff permanently on special offer in B&Q is thermally as good as anything else. For all these materials you need to ensure it is not compressed, for example by objects resting on top. Building Reg in England requires a depth of 270mm, but the more the merrier. Joists are typically 6" deep = 150mm, so conventionally you put 1 150mm roll between the joists, and another 150mm roll across the joists. But you will get thermal benefit and good ROI on throwing an additional layer on top.
I experimented with rigid insulation but it is extremely difficult to get a airtight fit between the joists - avoid!
Important to make sure electrical cables have enough slack to rest on top of the insulation, otherwise you risk them overheating.
A detail I struggled with is at the walls where rafters meet joists and there isn’t 300mm of space so you risk a thermal bridge (cold spot) in the ceiling below by the wall, and you risk blocking ventilation which needs to be maintained around the rafters. See my comment about this here How to insulate a loft at the eaves
I have gone with these simple plastic inserts which maintain airflow while allowing you to push in more insulation than is otherwise possible. I’m planning to install on Monday so I’ll let you know if it doesn’t work out! Hambleside Danelaw HDURFT Universal Refurb Loft Tray Vent - Pack of 25 only £24.71

All this is from interest as a DIYer and AECB Carbonlite Retrofit course so professionals might provide more reliable advice.


That’s very helpful, thanks Chris.

Hi, I would suggest you do it as a DIY job unless you are very allergic to the stuff that is currently in place and I am going to disagree a little bit with the suggestions before.
Leave the current insulation in place unless you must plug holes for cables and downlighters.
Top up your current 100mm with as thick as you can pack in to just above the top of the joists. Cover the joists with a breather membrane (I spent £1/m2 for good tear strength and you can spend a lot more-). Staple down the breather membrane to joists and tape the overlaps for good measure. Breather membrane function is to act as a barrier to stop windwashing and convection currents that suck heat from low density insulation i.e. mineral/ glass/ sheep wooll. You then put another layer of insulation across the joists e.g. 200mm. Doing it this way increases insulation efficiency considerably and maintains breathability of the materials used.
When buying insulation check the coverage per pack/ retailer and convert to £/m2 as I found the same brand varied per retailer.
For the loft opening get a piece of solid foam board 100mm thick (lots of offcuts in Skips) and combine with Gapotape to fit behind loft panel – because it is friction fitted it can be removed and replaced for access many times.

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Excellent stuff, thanks. Gapotape is a good tip - I have a door to insulate too and that will work a treat.