J Manual and self-calculated U-values

[Drazen]

I make precise calculations of U-values for (European) construction elements, but as values significantly differ from those in tables of J Manual (though actual construction elements are similar), I dare not using them in J Manual calculations as I have idea that U-values in J Manual are intended to be used only "coupled" with reference middle temperature differences.

Does anyone have experience in that, in using any U-values calculated "outside" Manual tables?!

 

[EnergyProfessional]

I could imagine differences are because Manual J takes into account wall compositions (like studs, filled with insulation in between). OR more wood around windows and doors, corners.

Much of the load is because of infiltration too, that is hard to estimate.

European elements can be different. For example there are very sophisticated brick and concrete blocks with small air pockets and insulating mortar that have pretty good R-values. You won't find those int he US and the same thickness cinter block has very low R-value. Only use data of material you actually can get here.

 

[Drazen]

My biggest problem right now seems to be extreme differences that I cannot understand.

Composite U-values for windows range from ca. 3 to 6, but mine are around 1,5.

Slab on grade is even worse . That is calculation not directly (proportionaly) related to U - value, but what I have from J Manual is 4-6, and what I calculate is bellow 1

(SI unit is W/m2K)

Is there some direct link to site with typical U-values known?

 

[Zesti]

"Composite U-values for windows range from ca. 3 to 6, but mine are around 1,5."

U=6 W/m2K is the right value for a single sheet of glass.

U=3 W/m2K is the right value for double sheet glass (6mm glass-12mm gas-6mm glass)

U=1,5 W/m2K is the right value for double sheet glass with special heat reflective coating

I am not familiar with "manual J" (don't even know what it is). If it is some manual that lists "standard" building materials and their U-values, you should be absolutely sure that you are comparing the right materials, as already mentioned by HerrKaleun (yeah, good movie, "Das Boot"!).

But, why don't you trust your own calculations of your U-values?

 

[Zesti]

Going completely off topic here:

I just notice that "Kaleun" is short for "Kapitänleutnant".
I didn't know that and thought that "Her Kaleun" was a specific character in the movie "Das Boot".

Learning all the time...

Still, it might be interesting to know some background to the names people have chosen on this forum.
I'll start a topic and see if it gets some following...

 

[EnergyProfessional]

there is a difference between just building material, and the building component, be careful. for a window for example you have the glass U-value, but that doesn't take into account the frame. So you need the overall U-value. For a stud wall you need to take into account the studs (metal, wood) and the insulation in between. Our code has some calculation procedure how to account for that. Surprisingly your R-19 insulation maybe gives you an R-11 wall, or so. If you have bricks, there is a brick and the mortar joint. with highly-insulating bricks that can make a huge difference to have those thermal bridges.

now I don't know what manual J gives you, that is fr residential calculations. There should be an explanation in the manual. Maybe manually calculate the R-value of a composite wall, and compare it to Manual J? then you know how they did it. Possible Manual J gives some slack for other thermal bridges, in corners, around openings etc.

As said before, air infiltration can be the dominant heat load anyway.

Finally people know the most awesome movie ever. Just imagine early 80's before computers and those special affects. there is a sequel book (die Festung - the fortress) where the war correspondent is in France after D-day. Actually all a true story by Lothar Günther Buchheim, the war correspondent.