Roof insulation and building code

Hi,

I recently bought a brand new townhouse and I don't believe the roof insulation is correct to the building code so the insulation contractor is coming out to look at it and explain but before they come I want to clarify one thing.

When for example the building code says the roof insulation should be greater than 4, does putting R4 batts down between the timber beams count as getting an overall R value of 4?
I would have thought that the wooden beams will have an R value less than 4 so really they need to be covered by insulation as well. Is my thinking correct?

To meet the code should there be insulation over the wooden beams? Has anyone else argued this with a contractor before?

Thanks

that would count as R4
not common to cover the trusses

Are you talking about ceiling insulation ( between or resting on the ceiling joists ) or roof insulation ( foil or foil blanket ) laid over the roof rafters/trusses before the roof tiles or Colorbond roof sheeting gets installed? Bear in mind that R4.0 batts are around 200mm thick.

Stewie

I meant ceiling insulation

At the moment the insulation is only slightly higher than the wooden beams which leads me to believe it is less than R4 before taking the wooden beams into account

Most bottom chords of trusses over medium spans are 75mm or so. Larger spans 100mm so if your insulation is only slightly higher than the bottom chord then I'd suggest your ceiling insulation is around the R2.5 - 3.0 at most. The roof insulation ( foil blanket ) may also have been taken into consideration as part of the calculation. It depends how they have worked it out. So you may have R2.5 on the ceiling and R1.5 over the trusses giving you total of R4.0. What is under your roof sheeting/tiled roof? Just foil sarking or a foil blanket?

Stewie

The roof is part flat roof and part sloping, the sloping part is metal and there is nothing there, no foil or sarking or blanket, absolutely zilch!

The only insulation is above the ceiling between the trusses, and like you say I think it is about R2.5

Good luck and let us know how you go with the insulation guy.

Stewie

Hi,

Firstly I'm not 100% sure about the mandatory requirement for sarking, as I still see some homes getting built without it.

But in terms of your batt insulation, depending on what brand it is it should be around 200mm in terms of thickness.

One of the more common brands used is Bradford Gold, and their spec for R4.1 is 215mm:



If we assume you have metal battens installed for your ceiling plaster, then you would expect around 110mm from the plasterboard to the top of your truss bottom chords, so it should sit above your plates in theory.

The reality is many factors would potentially lead to the batts being compressed, including not being 'activated' properly during installation, settlement over time and trades stuffing around with them over time. [Edit] I see you have bought this as new, but they can still settle a bit. We recently installed R3.5 batts into a reno house and they did sit above the bottom chords but only just...and I activated them all properly during the install.

Honestly my advice would be to contact a local 'blown-in' or cellulose insulation installer and for a few hundred dollars you can get heaps of 'fluffy stuff' blown into all the gaps and to whatever height you want.

...oh another thing I would do is find out what brand/type they used and buy yourself a bag. Typically only $30-$40 per bag. Then you can directly compare the batts to proper R4

I contacted our builder after handover, as I went around the roof and measured the depth of the blown-in cellulose and it was not to R5 spec depth everywhere.

Anyhow installer turned up, both of us in the roof and he was measuring in spots that were clearly packed higher against trusses. So I showed him the shallow sections. He gave up arguing at that point and just went and loaded up the hopper to blow in another 20 or so bags.

Depends on your builder and installer but they will do their best to get out of doing anything else for you. If all else fails, save yourself the stress and you can very easily get cellulose pumped in to get your roof to a proper R5.

This was our roof after the initial install (see below). We installed R4 + R2 batts along the entire perimeter to give the cellulose something to back up to.

....oh and our house in a cold Ballarat winter doesn't really drop below 17deg overnight even when it's like 0 deg outside. So something is working. Most nights we don't need the heater as the living areas sit around 20-21 deg if there has been some sun out during the day.

The insulation contractor came out along with the rep from the insulation manufacturer. The rep agreed its not the correct height and something has gone wrong so hes gone back to talk to his boss but he thinks that they will lay another layer of R1.5 or R2 over the top, Does this seem like an ok thing to do? Is R4 that is compressed to R2 thickness still effectively R2?

No one can still give me a definite answer regarding whether I am correct about it should be R5 due to the dark roof so I will keep going with that.
Incidentally I was up in the roof last week on a cold day but the sun was out and the inside of the metal roof was hot to the touch and the air was hot as well, it must have been over 30C up there even though it was probably about 15C outside

Thanks

ahhh, definitely not to spec if compressed. When you install batts, manufacturer guidelines require you to 'activate' the batt...which is basically getting it out and tapping it on all sides to get it to fluff up. If you compress it it loses air gaps and therefore R value.

But having said that, I placed 2 layers of batts on our perimeter (R4 + R2) and the overall heights were fairly close to full spec when you add both together so I don't think if done carefully it is too bad.

The main thing to be honest is the quality and care of the installation. Avoiding and gaps is important as studies have shown that even 5-10% gaps can reduce the effectiveness by up to 50%!

If it is no cost to you I would get them to install the batts, paying particular attention to the outside perimeter area, as these zones typically leak the most heat/cold. If you still want to top it up maybe later down the track then I would suggest getting someone in to blow in cellulose over the top and it does a great job of filling in any gaps as well.

**Anyhow back to your originaly question one way to check your install is to look up the specs for 'thickness (mm)' for your batt type and you can then easily measure the overall to see how compacted it is.

They are going to put an extra R2 layer on top, the original stuff is only around R2.5. Afterwards I think I will do as you say and get some loose fill stuff put it. Is it possible for me to buy bags of that and do it myself? I've been looking but could only find people who come and do it for you.
Thanks for your help

Why not to go with the roll/blanket?

Because there are still many little gaps around the air con, vents, joists etc etc

When you apply roll, you generally would have to seal with polyutherane the gaps around penetrations. Small hard-to-seal type of gaps are still okay, they won't be really adding significantly to the overall thermal loss/gain (may be up to 5% tops).

You will certainly lose much more than that (30%-50%) by placing batts between joists due to thermal bridging effect.

I'm a little confused...
So where does the roll/blanket go? over the joists?

I had wondered about the joists not having any insulation over them at all, I was thinking ideally batts up to the height of the joists and then some more insulation, batts or whatever options there are, over the top.

Over the joists, correct. Insulation in batts will certainly add extra points. But what about downlights, electrical cabling, etc.? Make sure you do not create fire hazard. I would be only running electrical cables in conduits and would need to cover downlights with special domes in that case.

OK right, so is the roll/blanket the same as batts just not cut up into pieces that fit between the joists or made of different materials?

Similar or same materials. It is continuous insulation, hence, reduces thermal bridging. You can also run 2 of them on top of each other for better insulation.

https://pricewiseinsulation.com.au/prod ... il-facing/