roof insulation: heat transfer from roof sheeting?

[lolobau]

Hi

I was wondering, this can't be right what most contractors are doing.
They build up the roof structure, provide on top of the purlins the insulation foil and then screw the roof sheeting into it.
Obviously the heated up roof sheeting (corrugated iron) is transferring the heat into the foil (which is somehow aluminum coated)

should this foil not have an air gap above?

but the funny thing is that even the books are showing the detail also the wrong way. See attachment

or am I somehow wrong?

lolobau

 

[Ron]

This is a common process for "pre-engineered metal buildings" (PEMB). Is it good? No. Is it acceptable by building code? Apparently, yes.

 

[SlideRuleEra]

lolobau - This is a "radiant barrier foil", and you are right that it does almost nothing for conductive and convective heat transfer. As well all know, "heat rises" so the roof sheeting and the foil itself are hot, but that (convective) heat stays at roof level - it does little to warm the interior. The foil keeps the radiant heat from the hot roof from heating the building floor, walls, contents, occupants, etc.

I looked into radiant barrier foil about 20 years ago for residential use... and installed it, in addition to existing adequate (fiberglass) ceiling insulation - the foil does work very well... in a low-latitude location (lots of solar exposure to the roof).

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[lolobau]

I think you would achieve much much more if you would do it this way...

here the "radiant barrier foil" does NOT touch the hot roof sheeting. This is the perfect fixing, or?

 

[lolobau]

see my attached sketch

option A and B would this not be the perfect solution?

lolobau

 

[EngineeringEric]

Insulator blocks are used on your better quality PEMB.

is not great what is done but it is done and industry wide. Everyone knows it isn't great but it is the cheapest and does the job, sort of.

 

[lolobau]

Hallo Eric

what do you mean with "....Insulator blocks are used on your better quality PEMB..."
what is it?

yes I mean it is always done this why but why should I not improve and old detail?

 

[Brad805]

The Pre-eng industry strives on being cost effective. The typical rolled out batt roofing system is typically specified for a low end building. Adding details to this roof type is not a great idea unless your client has specifically requested to do so. In cases where the owner requires an improved thermal efficiency, we have used insulated panels or a standing seam roof plus a liner. Pre-eng suppliers can provide either options. In our area, the improved roof/wall systems leads to an increase in cost around $15/sqft of cladding area, so it needs to agreed to by the owner before indicating. In Canada, the new energy code prevents the typical pre-eng roof system you are indicating, but we still see people using this. I work in a cold climate, and the typical pre-eng roof leads to significant ice formation.

The blocks Eric is referring to are basically 1" tall (or taller) hockey pucks that are fastened to the roof girts. The idea behind them is to reduce the amount the batt insulation is compressed over the girts. It is not great, but it is a cheap addition.

 

[Archie264]

This is worth the hour it takes to watch it:

https://www.youtube.com/watch?v=rkfAcWpOYAA

One thing Joseph Lstiburek points out is that insulating between light-gauge metal studs (or purlins, in this case) is nigh on a fool's errand.

Years ago I was tangentially involved in a warehouse project where condensation had accumulated in the roof sheathing/insulation to the point of forming pools of water visible from below. The building owner had to resort to using an air rifle to drain them.

 

[EngineeringEric]

I was not saying you should not improve on it. I am just saying that it doesn't mean the contractor is doing it wrong. I try to improve my design every time i do it. I commend you for trying!

This link is for a Butler Roof detail showing the thermal block. The idea is that this 'helps' reduce the thermal short and stops the insulation from compressing 100%. I think the latter part is the most critical.

http://www.butlermfg.com/y856m1WUA8opoM/reroof/Y007.jpg

Brad's comment is spot on. PEMB is cheap and does what it does well. Sure it can be improved, but for a cost. Think like cars, you can make a cheap base model amazing or start with a base luxary. both do the same and cost the same but one has way more extra features.


PEMB - Pre-Engineered Metal Building. Sorry if never defined that!

 

[SlideRuleEra]

lolobau - There are three conversations about three different type of insulations.

1. Your original question is about "double sided reflective foil laminate", this is what my answer concerns.

2. Your next post is about a product called "RafterFit"... which is a different product from #1.

3. Much of the other advice you are receiving is about thermal blankets or batts. This is different from both #1 and #2.

Which do you want to talk about? #1, #2, or #3?

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[lolobau]

Hi guys

sorry for confusion
I am actually not refering to the indicated products. This are just images from the internet
I was refering more to the method of fixing of the "radiant barrier foil"?
Shall the foil be fixed above the purlins or rather suspended down
see attachment with the three versions which i actually wanted to discuss

lolobau

 

[lolobau]

@Archie

thanks for the link, I am busy downloading the video and will watch it later

 

[SlideRuleEra]

lolobau - Ok. The way radiant barrier foil works, the air space is not really important. The insulator blocks are great from heat transfer by conduction but are not important for radiant heat transfer. A reasonable size air space is good for minimizing convective heat transfer, but the thin metal foil would conduct heat from the air space to the building interior almost immediately.

The foil does not even have to pulled tight, it can sag between supports as shown in your OP. The only thing that is important for a radiant barrier is that location is between the (hot) roof and the contents of the building. For example, no one would do this, but if the foil was hung, say 2 feet below the roof it would work almost as well.

Finally, the way radiant heat transfer works, the foil helps only when the building interior needs to be kept cool. When the building interior needs to be heated, the foil will not help at all, and would limit any help sunlight on the building roof would contribute. I agree with what Ron and Brad805 have said, for a radiant barrier, just go with the cost effective installation detail.

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