Expanding Insulation for Rafters

YourAverageJoe

I have to insulate my dorner roof on my new build.

My rafters are 150 mm deep, so I had planned on screaing 2x2 on the underside of rafters to allow me use 150mm of insulation while keeping my 50mm air gap.

I had intended using 150mm rafter**** which is 150 mm thick pir insulation board which you can squeeze between rafters and then it expands and locks in.

my query is do all of the grooves in the board, which allows it to squeeze in and out, actually reduce the effectiveness of the boards as they are effectively thermal bridges allowing cold air passes into the centre of the pir boards? By right should these grooves not be sealed somehow after installations.

I also intend screwing 2x1 on side of rafters up against felt which will keep my 50 mm gap, so an alternative is to use floor pir boards and cut them to fit, sealing the sides with foam. No cuts in board so more effective?

beyondpassive

How are you achieving airtightness?,
how are you insulating where the dormers project
Have you put the battens on the outside yet?
how are you insulating the bridging effect of the timber?
The PIR is conductivity 0.025 the timber is 0.133, the timber represents about 15% of the roof fabric, up to 35% at the dormer windows, your heat loss with be through the timber.


I generally recommend softboard to the outside of the rafters, then a 75mm rail to the inside lined with OSB and the 225 fill between with celloulose where cost and space allow.

PIR is technically a good product, though the conductivities the manufacturers claim are a little good. While your detail to space the PIR 50mm off the felt and foam between timber and board is excellent. Problems arrise in installation, where the insulation is compromised at the wall plate and air escapes through uncontrolled openings in the fabric caused by installing wiring and lights. The biggest issue with room in the roof is summer overheating. Cellulose and softboard attenuate fluctuations in indoor temperature between day and night resulting in a steady temperature.

The cellulose and softboard option with airtightness is expensive, but for thermal comfort in the upper floor and reduction in drafts, coupled with reduced heating costs, its an option worth considering.

PIR and Phenolic is great for insulating cold stores or refridgerated storage warehouses, but its not really appropriate for modern buildings. The here goal is thermal comfort, humidity management and air quality. In my opinion the natural insulation materials do this job far better.

YourAverageJoe

Thanks for the length reply, it is much appreciated.

My exact situation is;
150mm rafters in place.
Breathable membrane on outside.
Lats and slates fitted.

How are you achieving airtightness? - I intended placing a breather membrane on the under side of the rafters covering insulation and rafters and stapling it to rafters and sticking it to plaster below.

how are you insulating where the dormers project? - I have only one but had intending doing a simailar detail as in the roof.

Have you put the battens on the outside yet? Yes

how are you insulating the bridging effect of the timber? - Was intending to apply 50mm of insulation combined with 12.5 mm plaster board on underside of rafters after membrane has been installed.

Above the flat portion of the ceiling i intend using fiberglass rolled out in attic to allow some breathability.

beyondpassive

Ya thats good attention to detail. I'd continue the PIR and 50mm insulation from eaves to ridge, changing insulation at the knee board or at the flat section near the apex usually compromised the continuity of insulation and airtightness.

one way to look at insulation, is to keep an eye on the end game. If you want your rooms to maintain 20 degrees your surface should always stay over about 16 degrees. If it is zero on the oyter face of the insulation, will the inside face of the wall stay over 16 degrees. Is the insulation sufficiently dense and thick at all points to maintain surface temperature. Maintaining the integrity of insulation is similar to the approach to weathertighness. You wouldn't tile a roof, leaving out the ridge tiles and say its a job done, yet contractors lob insulation between most rafters, plaster board it over and say job well done.

Your on the right track Joe.

YourAverageJoe

Great Passive, thanks for your input.


so does nobody else have an opinion on the effect of the slots cut in the boards?

YourAverageJoe

well?

Rafterloc or ordinary floor insulation cut to suit (even if it is a pain in the arse to do)

sydthebeat

100mm rafterloc works out about 16.90 per sq m
100mm thermafloor works out about 11.00 per sq m.

even of you friction fit standard cut boards, theres no guarantee that in years down the line the timbers wont shrink, or the insulation may also shrink, createing gaps between boards and timber.

to be honest the most important insulation is the thermal linings which close the thermal bridge.

YourAverageJoe

Rhanks Syd,

One thing i was considering doing was foaming between the boards and rafters, but would this possibly cause rotting of the timbers as i will be in effect sealing the sides of the timber

sinnerboy

YourAverageJoe wrote: »

so does nobody else have an opinion on the effect of the slots cut in the boards?

Yes .

PIR insulations depend on entrapping penthane gas . It does not do this forever . The penthane will entirely exhaust given time .

The thermal conductivity ( lambda ) of PIR boards will typically leave the factory floor with a lambda of 0.18 W/m.K . Typically Irish and British certifications will state "a 25 year average" value of 0.23 W/m.K . This is the design value that one must reference when designing to comply with b regs part L .

So question- what is the 25 year aged value ? Answer ( 0.23 X 3 ) -
( 0.18 + 0.23) = 0.28 W/m.K . Eventually the penthane will exhaust entirely and the PIR will then perform at a similar lambda to fibreglass , rockwool , cellulose sheepwool and insulations like these .

How does the PIR board entrap penthane - by virtue of the foil facing .

What happens when that facing is cut .....