Hello

This is a topical post - getting the xmas stuff out of the attic reminded me about upgrading our loft insulation! Here's my embryonic plan ....

Our home is a timber framed house with brick outer leaf built in 2002. There is usually a small amount of condensation on the underside of the roof membrane in winter but on v cold days that drips causing small puddles on items stored up there.

Existing insulation is 200mm rockwool between 100mm high bottom chord of the rafters, ie a gap along the top of each chord.

The central third of the attic is boarded with T&G chipboard on the 100mm timbers so the 200mm insulation here is compressed to 100mm.

The roof has truss rafters at 450 cc with W shape infilled diagonals. Overall plan is 10m x 10m.

I'd like to boost the insulation and also try to improve vapour control.

1 - Vapour control & air tightness

We have plain plasterboard throughout the house, ie no foil backing. The rafters sit on the stud walls hence the top of the stud walls are visible which are obvious air leakage routes.

There is polythene behind the external wall plasterboard but nothing on the ceilings. The edge of the external wall poly is visible from the attic so I could tape along the wall header to join a ceiling vapour barrier to that.

Ignoring the boarded central third, I could pull back the existing insulation on the outer thirds to run a vapour barrier on top of the ceiling plasterboard and over the rafter chords. There would be difficult details where the inner diagonal infill meets the bottom chord. Also at the eaves where the bottom chord oversails the wall to form the eaves.

Is condensation a risk where I will get an inevitable air leakage where the diagonals meet the bottom chord, and where the bottom chord oversails the wall at the eaves?

2 - Extra insulation

Is it worth getting a higher density insulation to provide a bit of a diurnal thermal buffer? This is going to be most noticeable on hot summer nights. If so, I think it more effective to have that under the existing rockwool?

If I don’t go for a vapour barrier how thick can the insulation be before i need to worry about a dew point inside the insulation?

3 - Boarded area

The central third of the W truss is boarded with 22mm T&G chipboard. I suspect this has very low vapour permeability and I could glue 100mm PIR on top then glue 12mm board (OSB for price) on top of that without concern for interstitial condensation?

I'm thinking of the VB because the top of the stud walls are so leaky with gaps between the studs and wall plasterboard shown in the pic. All those specks in the pic are silver birch seeds, they get everywhwre!

My thinking is that if I'm lifting the existing insulation then I might as well pop down a membrane to cover all those cracks.

There was no condensation when I took that photo just now with +5C outside. It was dripping at -7C last week. Our neighbours tolerate it as an occasional thing just as we have for many years. A few in the street have had ridge vents installed in addition to the existing continuous eaves vents. For me it illustrates how the ceiling is very leaky and if that was fixed the vapour transfer would slow down enough for the ventilation to be satisfactory ... as well as dealing with heat loss & comfort (ie draughts) though all those leaks.

I'd mentioned a thermal buffer to moderate diurnal variations. I had cellulose or woodfibre boards in mind. It occurred to me that these materials can also provide humidity buffering which is sadly lacking in a modern timber framed house.

Cellulose is said to be good for cutting down draughts and has a decent thermal capacity which will be useful for summer decremental delay. There is some info online that it doesn't need a VB which would enable it to act as a humidity buffer too. Some articles suggest a VB is needed in wet climates, they are typically based in the US but I guess the UK is classed as wet in that context?

Does that all seems too good too be true? Humidity buffer, thermal buffer, air tightness all in one?

ps In this scenario I'd VB over the stud wall head but leave areas of plain plasterboard without VB.

Links for cellulose in attics and VB:
https://passivehouseplus.co.uk/news/marketplace/choose-cellulose-to-protect-against-damp-in-attics-ecocel
https://greenatticinsulation.com/education/airvapor-barrier/
https://hvacseer.com/does-cellulose-insulation-need-vapor-barrier/
https://www.greenbuildingadvisor.com/question/cellulose-insulation-vapor-barriers

Thank you everyone for your feedback.

I think I'll settle for airtight(ish) breathable ceiling. Either using 200mm tape over the stud walls or a vapour permeable membrane over the whole lot? I'm thinking a full membrane is likely to be diminishing returns bearing in mind the rest of the house is standard construction. Also sealing a membrane around every truss diagonal isn't going to be easy and probably leaky? So I'm minded to just tape the exposed wall studs and cable holes then trust the plasterboard ceilings to be reasonably airtight for my need/intent.

Breathability means I can couple a dense insulation layer with the internal climate as a humidity buffer. That appeals to me having worked on a couple of straw bale buildings, also my parents lived in a cob house. I realise 100m of fibre board its not going to be like a straw bale or cob house but I'll know there's a notional humidity buffer up there.

Regarding insulation material - Decremental delay is a biggy for me to try to deal with summer heat. I realise that isn't applicable in winter. Woodfibre is well known for decremental delay and cellulose looks to have good properties too. Both of them might have an acoustic benefit too but we're in quietish suburbia. Woodfibre is pricier then cellulose (£1500 v £750) so I'll look at the decremental numbers before choosing.

https://www.greenspec.co.uk/building-design/decrement-delay/
https://www.ecomerchant.co.uk/news/are-you-getting-too-many-hot-sticky-nights/

Greenpaddy - thanks for the tip about lifting the existing boarded area. I think 6x2 fixed to the trusses would be easier than something thinner with props onto the bottom chord. Also fewer obstructions for sorting out the insulation and fewer thermal bridges. I had wanted to side swerve lifting the existing boards but I really ought to do the job properly!

ps - Re airtightness for cellulose noted in my earlier posts, I think that is for stabilised cellulose which seems to be installed slightly wet so it glues together as a contiguous layer? I think the cellulose insulation used in the UK is dry and loose?

Cylinder of frothpak style spray foam and spray all the wall tops, wool back over?

Posted By: CoverleyRe airtightness for cellulose noted in my earlier posts, I think that is for stabilised cellulose which seems to be installed slightly wet so it glues together as a contiguous layer? I think the cellulose insulation used in the UK is dry and loose?

It depends which you buy. Loose bagged for DIY loft fill is dry and loose. Spray injected into cavities is bonded. Doesn't matter which country you're in.