Hi

Just a query about insulation. We are trying to retrofit a 1976 bungalow. Cavity wall with loft. We are going to insulate the cavity and externally and clad in timber. We are intending to remove part of the loft./ceiling to create an open plan kitchen/living room. We are raising the ceiling over the kitchen just to give some visual interest to the room. The roof above the kitchen is an east facing pitch roof with a bitumen felt beneath Burlington slates. We will install some roof lights, taking the ceiling up to the rafters.

The bungalow roof has had three large purlins installed, each 350 mm deep, 5600mm long. These were installed because of a failed trussed purlin before we moved into the house. The new purlins are vertically installed and variably at approximately 900-1000 mm apart measured along the rafter slope. The purlins consist of two pieces of timber glued and bolted together and appeared to be doing their job satisfactory, although I don't know why they needed so many given the small spans between the rafters.

We are hoping to put as much insulation in as possible. We are intending to insulate under the rafters to allow ventilation to the full depth of rafters, which are 100 mm deep. We were then going to insulate between the purlins. Sorry, can't seem to attach a drawing.

The question I have is what type of insulation to use. One of the cheapest and possibly easiest is to use PIR. After angle cutting the PIR to ensure the sheets would fit accurately between the vertical purlins I would add Gapotape to ensure a snug fit, though would probably needing some support at the lower end where the angle wilth the purlin is obtuse. We were intending to install 260 mm of PIR in whole 2.4m boards, staggering the joints between layers. We would then tape the edges to the purlins and between each board to form a continuous VCL and achieve air tightness.

However, we have been spooked by the potential condensation issues and whether we should go for a more breathable/vapour permeable solution. Given this is above a kitchen generating lots of moisture, we are wondering which way to go. We are intending in any case to install a ventilation system, hopefully an MVHR, to manage the moisture levels in the house. The purlins are not fixed via brackets to the gable so at least no embedded timber to worry about and the cavity and exterior would be insulated.

The other options is to use flexible wood fibre or Thermafleece batts. These cost quite a bit more as we needs 280-300 mm, and don't achieve quite the same level of thermal performance (albeit I am not convinced about the claimed performance of PIR). But they are at least vapour permeable and more sustainable products. Mineral wool batts are another option but it is actually quite expensive at the moment and because of the spacing between the purlins (900-1000mm), it would involve lots of cutting, bearing in mind this material is horrible for lungs and skin.

The other problem with batts is that they are quite tricky to install as I would probably have to install lots of regularly spaced "I" beams or Larsen trusses to achieve a friction fit. Annoyingly, the rafters are variably spaced at around 400 mm (the some are 450mm!) so it is not simply a case of hanging these directly from them. I would have to screw the larsen truss ends into the purlins first then fit the plywood inbetween for the friction fit. I would need approximately 30 of these and then cut every 1200mm batt, so pretty tedious to do.

If I put the batts in horizontally I could put a horizontal "I" /larsen truss beam in between the purlins, though I would probably have to hang these vertically so that the width between is consistent for the depth of insulation. So a bit of a phaff. Also, the width of bats is inconvenient so would have to be cut down.

To add to the difficulty, batts would require a membrane above to minimise wind washing from the ventilation above and also an air tightness membrane below the insulation. This all adds to the trickiness of the installation and the cost.

Thermafleece would probably be the way to go rather than wood fibre as the sharp angles with the purlins may be easier to fill with the softer wool. Hemp is another option

The risk with PIR is obviously condensation on the purlins. Moreover with the purlins being composed of two pieces of wood glued and clamped, there is a risk of condensation if there are any gaps inbetween (so would need to seal these). But at least there would be plenty of ventilation above the insulation, with the ventilation in the rafters linking to the loft space which is cross ventilated at either end of the gables.

The big sheets of PIR would also minimise the joints through which moisture could enter the roof space and also avoid cold bridging (though appreciate moisture has a way of finding its way in however well this is installed).

I guess there is also the option of loose fill...

I would really appreciate some advice on which way I should go on this. Many thanks.

Trussed purlin. Basically a standard purlin with timber hangers to a ceiling joist below. One end into gable, the other into masonry cross wall. Anyway it's being removed so not relevant. No other trusses just gables and two masonry cross walls with purlins in 3 bays

There is quite a small limit on file upload size, so as Tom suggested, try reducing the size of your images.

Are the cross walls loadbearing, or do the purlins just pass through the pockets?

I think it is important to decide definitely for or against an MVHR. I would strongly recommend one, as it means you can ignore a lot of condensation issues, quite apart from the benefit of fresh air! Ducting is usually the most difficult part.

Personally, I would never use PIR because of its behaviour in a fire. I'd likely go for phenolic if I had to. But I prefer graphite EPS. I don't think either woodfibre or thermafleece is irritating to work with, and even rockwool is not too bad. It's glasswool that is the nightmare. Warmcel is also a good fill insulation. Boards or cloth on the surface of the rafters and joined to the purlins on one side, and boards or cloth fastened on the lower corners of the purlins on the other side.

I'd think a separate VCL would be better and easier than trying to use the PIR etc as a VCL. And a service cavity if you're planning on ceiling lights.

Photo of loft

So " The purlins are not fixed via brackets to the gable" was a mistake?! They're 5.6m long, supported in hangers off one (two?) gable end walls - plus
built into intermediate cross-walls (plural?), so average span 1.85m? Can't be - 350h @ 1m c/cs should span the whole 5.6m.

The rafters look like 150x50s, @ variable 400-450c/cs, beneath non-breatheable undertiling felt. You'll need 35mm av ht through-ventilation zone, between the rafters, under the felt, in via 25mm continuous vent inlet like Glidevale FV250 at one eave, clear under the ridge board, out via ditto vent at the other eave. Here's a spec I've just written for similar:

To create a 150mm extra insulation volume hanging below the extg rafters’ soffit: Cheap offcuts, 2.1m or less, of (min 400h) timb I-joists, cut lengthwise to form one ‘web’ with one ‘flange’ (as nailing-strip). Or DIY prefab ladder frames consisting of apx 175 squares of 9mm ply or OSB3 glued (as ‘web’) at intervals tight into grooved 35x50 sawn regularised nailing strip (as ‘flange’) to detail. Then, the ‘flange’ nailed to side of each extg rafter.
Blown-in cellulose insulation e.g. Warmcel or equiv, between the rafters + ladder-frames. Fix the p’b’d only after, as the blowing pressure may push the p’b’d off its screws/nails. Instead, use the method of installer-fixed ‘scrim’/netting staple/tensioned across the inboard face of the ladder-frames before blowing-in, thus allowing visual check of filling completeness, rollering-back if nec, before p’b’d is fixed unpressured. Many Youtubes and articles.
Warmcel installer to also fit a second layer of the ‘scrim’/netting, reaching deep up between the extg rafters, to form a vented cavity, above the insulation, beneath the extg underslating felt, as an unobstructed through-ventilation layer, nom 50mm high, min 15mm clear not touching the felt, incl careful shaping to below the eave inlet, clear under the ridge board, to below the opposite eave outlet.
Extg fascia boards replaced with smaller painted ditto with 25mm-equiv continuous vent inlet/outlet Glidevale FV250 or equiv. The extg underslating felt drains out above the vent; the ‘scrim’/netting tucks beneath it.

Blown-in Warmcel avoids all your worries about sizing/fixing batts, as well as being 100% breatheable, and not requiring a VCL at all, according to my WUFI study - but get your own second opinion on that controversial conclusion.

Based upon the assumption of a well ventilated 100mm rafter cavity, and MVHR, I would design your roof as follows...

Although my preference for insulation is always Knauf glass wool, which from my experience of installing over a couple of decades is the best insulation to work with from a comfort/mess point of view, the structure arrangement doesn't really lend itself to a non-rigid insulation. Therefore I would keep with your initial strategy of rigid insulation boards between the purlins, forming the 100mm ventilated cavity as you suggest. Consider the cost and capabiltiy of the labour doing the work.

Cut the boards 5mm shy all around, peg in place with wedges, and foam fill all the edges, ensuring full depth foaming. Remove the wedges after 30 mins and fill any gaps or missed bits. Two layers of 100mm PUR/PIR would get you to around 0.1W/m2K (quick calc). You can add a "gunk" of your choice at the purlin to board junctions for improved air tightness, though 2 layers of foamed in boards will be pretty good, certainly compared to the rest of the house. Even tape over the board joints if that's what building control need to see to proove a VCL, though again 2 staggered joint layers of foil faced boarding will not have an issue at the board joints, rather at gable/wall head butts or butts with timber.

Strap over the surface with 25x100 timbers (use say 270mm woodscrews fixed into the 50x100 rafters) to give the service cavity/fixings for plasterboard.

Any vapour moving into the ventilated cavity will be ventilated. The MVHR will take care of the internal vapour. Now focus on walls, windows surrounds, wall to floor junctions (any junction is always a problem to solve), under floor insulation, electricians and plumbers shoving screwdrivers and drills through nicely sealed layers etc.

There is always more than one way to go about any project or part thereof. Look at your available time/money/contractors ability/ultimate objectives - then choose.

Thanks GreenPaddy. Yes there is some sense to using PIR. Wish phenolic was cheaper. The fire risk is off putting.

Yes, that’s fair comment. The installation with boards is so much easier. Anyway, managed to buy phenolic cheaply, so fire risk lessened. Intending to use gapotape as not confident about foaming tbh. Eye watering price though but hopefully will give a snug fit.