Is R everything?

[Garald]

Let's talk about roof insulation in particular. Is R the only thing that matters? There's also airtightness, sure - you don't want outside air getting through the tiles and circulating through the insulation. Does thermal inertia also matter? How much?

 

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Real-world example: As I've said a couple of times, it gets rather hot in my attic in the summer, and the situation in winter isn't fantastic either. The insulation there is still that put in by previous owners some 15 years ago - 15cm of fiberglass, I believe, and not in great condition (some was replaced by rockwool recently, when we had skylights installed).

 

I can't really lower the ceiling any further (it's an inhabited attic - I'm typing from it right now - and I don't like hitting my head). Thus, unless I insulate from the outside (expensive!) or have the entire roof 'raised', i.e., redone at a greater height with changes in structure (even more expensive, though a worthwhile long-term project), I'm stuck with R=4 or so at best. What I could do better is (a) have some sort of airtight membrane installed, (b) replace the fiberglass by something else entirely.

 

Wood fibre and cork are tempting, in part because of their sound insulation properties. But can I expect them to insulate better than fiberglass, particularly in summer, in spite of having very similar Rs for given thickness?

Edited March 25 by Garald

[jayc89]

Only really echoing your own findings, but for roofs I'd argue decrement delay is as big, if not a bigger factor. My friend has a new build, built a couple of years ago so should have achieved somewhere around 0.15W/m²K as a minimum. It's a warm roof and they have a room in their loft which is still too warm in the summer and pretty cool in the winter. It was insulated using PIR - so it ticks the boxes but in real world terms it's still not the most comfortable. He's since moved his home office out of there and into a spare bedroom "downstairs". 

[RobLe]

Shade is fantastic for keeping lofts cooler - a free unsung advantage of PV!  Roof tiles can get to 50C+ quite easily in the sun otherwise, but will just be at ambient air temp if shaded.  I totally agree about the decrement delay - so a better roof construction in available space would be be:  tiles with well ventilated air gap behind (ridge and soffit vents), celotex type insulation with good air tightness, three layers of plasterboard.  

[Garald]

 

1 hour ago, RobLe said:

Shade is fantastic for keeping lofts cooler - a free unsung advantage of PV!  Roof tiles can get to 50C+ quite easily in the sun otherwise, but will just be at ambient air temp if shaded.  I totally agree about the decrement delay - so a better roof construction in available space would be be:  tiles with well ventilated air gap behind (ridge and soffit vents), celotex type insulation with good air tightness, three layers of plasterboard.  

 

That sounds very good.  I'll ask a couple of insulation companies, but I'll be lucky if I find one that likes technical challenges more than money.

 

If I am not lucky, I guess what I should do is:

(a) keep saving money, try to get to know some good local engineers, and, in 5-10 years, if I don't move, redo the entire structure of the roof to get much more standing space, and then do things optimally,

(b) cope and see what I can realistically do in terms of DIY.

 

In (b): airtightness is the easy part. I've already sealed with acrylic and silicone everything I could seal.

My hardwood guy suggests: paste a 1cm layer of cork on the inside yourself. (As in: don't open the ceiling - paste cork on the ceiling, using glue made for cork.) I may like the aesthetics (... even if my stepcat's cohuman does not), but in terms of R the improvement would be piddling. Would it help noticeable with decrement delay, though?

 

 

Edited March 26 by Garald

[Mike]

On 25/03/2025 at 23:34, Garald said:

Wood fibre and cork are tempting, in part because of their sound insulation properties. But can I expect them to insulate better than fiberglass, particularly in summer, in spite of having very similar Rs for given thickness?

They will do better than fibreglass, but won't make a big difference unless you have space for multiples of 100mm. R (thermal resistivity) can tell you how much heat you retain / loose over a period of time, in a steady state. However it takes time for that product to heat up - that is, for the heat to travel from one face of the product to the other - before it stats getting lost at that rate R. The 'travel time' is the decrement delay (déphasage thermique in French) mentioned by others above.

 

In general, the heavier and denser the material, the more the decrement delay will increase. If you're tight on space then adding solid timber boarding or an additional layer of plasterboard will likely make more difference (but still not that much) rather than adding any kind of insulation. Which is why churches, with their thick walls, are slow to react to temperature change.

 

If you want to experiment, the Concrete Centre has a free calculator that you download, from https://www.concretecentre.com/Publications-Software/Publications/Dynamic-Thermal-Properties-Calculator.aspx, if you subscribe to their newsletter.

 

Edited March 27 by Mike