U value calculations

I went to a firm of heating engineers who charged me a bit to prepare a report that said that yes, I could meet standards with single glazing by stuffing more rockwool and or celotex everywhere.

Do Rockwool given any number for a different thickness?

Could you extrapolate accordingly?

Any use?

Probably. If I didn't find mathematics challenging:-)

Umm.. As TNP says, for the time spent, it might be better to use a commercial site.

I am mixing PIR foam with Rockwool which complicates things. As Tim Watts knows, attic dormers have lots of extraneous timber and hard to reach corners over soffits.

I don't think this construction and *airtight* go together unless the builder is onside from the outset.

I'll have a closer look over the weekend.

Current job is stapling up plastic sheet for vapour barrier.

We have fitted current standard double glazed Argon filled units throughout.

One trick for the Wiki.. use a sheet of Perspex or polycarbonate to mark out shapes for cutting foam to fit angled/non parallel bits of studwork. Felt tip pen and then rub off the marks for the next sheet.

Building inspectors wan to see a proper report that covers their arses signed by someone who appears to be an expert.

The truth is not enough.

... has got indemnity insurance.

LOL!

That made me chuckle...

U-Value calcs are pretty simple. Do you want to do them yourself? In which case much help will be forthcoming here (from me for a start as I've done a lot - the only thing I cannot do is floors, but that sounds like it's not a problem for you).

Rule of thumb based on my experience:

I "got away" with the BCO approving 100mm celotex on my roof, with 75mm around the periphery where 100mm was not practical - but that was an exiting dormer conversion and I was upgrading the insulation from the

However, for a new dormer, they will want it to be upto current standards. Without checking, I suspect that's more like 125-150mm celotex or equivalent - BUT they will usually accept less if you can upgrade other parts of the building.

Do you have any guidance as to what your BCO wants?

Wonder if they'll start insisting on VIP instead of PIR for dormers and other places where there's not much space available, rather than letting you get away with it?

It seems to depend greatly on the BCO. I found the one I have had a couple of times was quite content to take your word for it, once he felt comfortable that you knew what you were doing. Although you may recall tales of Mr B, the BCO and the cooker circuit which went completely the other way.

Find someone with a civil engineering degree to sign off your calculation. He/she doesn't have to have any real experience!

That bit is actually quite easy. What you need is the "k" value - this is a measure of the intrinsic heat conducting property of the material itself, per watt and a standardised thickness. The U value is simply the K value divided by the depth.

So say you have a spec for the material that says its U value is 0.4, and its 100mm thick, that tells you that :

So you can work out that K must be 0.04

Once you have that for your chosen insulation material, you simply divide that by the new thickness to get the revised U value.

So for the above material, the U value of 125mm thickness would be

Celotex for one have a free online calculator on their site where you can select various wall/roof/floor options with varying construction types either for new build or retrofit with options for joist/rafter spacing and various brick/block manufacturers.

The end result is a pdf with the specifications and all the calculations, unfortunately it can't accommodate existing insulation from other manufacturers but you'd probably be ripping that out in any major refurb.

They also have whole raft of application datasheets

There should be no reason why you couldn't substitute your 150mm of Rockwool for an equvalent (lower) thickness of PIR in the calcs and end up with a usable result. Cut and paste and then edit the pdf for something buiilding control would accept.

You can extrapolate.

U-Value of 1 means (in SI) 1 watt is passed for a section 1m2 in area,

Double the area, double the power. Double the temperature differential, double the power.

Now, for thickness variances: this is a little harder - like putting resistors in parallel - it's a reciprocal formula, because U-values are a measure of thermal *conductivity" (the reciprocal of resistivity which is the more usual physics unit of thermal property).

R-Values are additive for additional thicknesses, even of dissimilar materials.

So if you switch to R-values, you can multiply the quoted R value according to the thickness.

eg if they quote for 100mm rockwool, and you need 150mm, you can multiply by 1.5.

You can further add the R-values for other layers, such as the 9.5mm plasterboard facing and so on.

There's a trick for dealing with parallel paths too (eg rafters bridging the insulation) - but I'll save that for now.

If you want to break your

If that is a practical material, then probably - in the same way stuffing 4" of wool in where there used to be 3" is both a bit pointless and the BCO would probably not have agreed it.

Have you got any links to VIP pricing?

Sorry ^^^ should read "of the quoted thickness" (eg 50mm or 100mm).

I was mentally jumping towards K-values.

I was under the impression that VIP panels could not be shaped or cut, would that not be difficult in many retrofit applications where you need to fill irregular spaces?