Calculating U-factor

RadPro

I'm digging for the U-value cheat sheet I had made up in excell spreadsheet some years ago and I'll post when i find it. As you know in excell do the first couple and it will do the math on all the rest in a nice little chart Paul

[Deleted User]

Calculating U-factor

Hi All & Happy Hoildays

I am working on a small spreadsheet and (if I am starting to understand this correctly) am looking for a formula to determine the BTU loss through a sq.ft. of a material with a given R-value. I think I know that U=1/R ... and from there I am a little lost. I do know that the temp diff (F) needs to be factored in and I think the energy transferred needs to be converted from Watts to BTU's multiplied by the sq.footage in question.

I am hoping that someone has a straight forward formula or reference for me

Thanks in advance

Mike

[Deleted User]

Thank you

Thats what I am looking for .... and pretty much what I am working on. I use Elite RHVAC for detailed stuff and I was using the Slant Fin program for quicker stuff but want more flexibilty .. especially with some of the different R-values, so being able to figure the U-value and the loss per sq.ft. would be important


Thanks again

Mike

steve b_9

The information that you are seeking about heat loss calculations can be found at:

http://www2.fe.psu.edu/~dxm15/aet121/Ch10HeatLoss.htm

[Deleted User]

Why go with U values...

The ONLY insulation factor I see expressed in U values is the window glazings. Everything else is expressed in R values.

Everything has to be based on a common denominator, either R or U.

I use the formula A/R X Delta T. A being area, R being the R value and Delta T being the air temperature differential between the two sides. No converting from watts to btus, just simple math.

Different means to the same end.

ME

GMcD

Needs more than just the \"delta T\"

When calculating heat(energy) flow through an assembly, you cannot ignore the surface temperatures, air films, and radiant effects. For example- on a sunny day in the middle of winter, the south side of the house walls will have a surface temperature of say 70F, while the ambient air temperature outside may be 30F. Now, do your sums through the wall, and the inside surface temperature of the wall might be at some value higher or lower than the ambient air temperature inside the house. Air films also form small boundary layers against the surfaces and these vary with wind speed/air velocities. And then don't get me going with the thermal bridging de-ratings....The simplified Q= A x U x Delta T is really just an "estimate", and the actual heat flows can be off by a factor of 50%-100% depending on orientation, colour of the outside walls, glass tints, etc.

OK what about at night? The actual surface temperatures of walls, glass, and roof may be up to 4F to 7F cooler than the ambient air due to night sky radiation on a clear night. The "canned" programs are at best a rough estimate, and unless you know what algorithms are being used to calculate the heat flows, you are the mercy of GIGO, and his friend Murphy.

[Deleted User]

That is a big help

and I appreciate it, Mark. I'm sure it is a tried and true formula .. and seems to plug in quite nicely here!

Thanks again

[Deleted User]

Incredible

I understand there is always much more than meets the eye and apparently about triple what I can comprehend!

Thanks!

Brad White_11

Although,

as my mentors once told me, you design heat loss for the cold of the night or at least an overcast day. (Any solar gain is just a bonus.) And the gaps in insulation, etc. are taken up with the proverbial 10 percent safety factor. (Big IF).

After years in this business (on the engineering side, granted), I would pass this on to my students, after they had taken the long route of breaking down the method of collective R-values, de-rated them for percentages of framing, inverting, etc.:
In a framed wall, generally speaking, all non-insulation components (sheathing, siding, GWB, air films, etc.) amount to an R-value of 2.5 to 3.0. And when diminished for framing, the framing cancels these out more or less, such that the adjusted R-value of the wall assembly is about equal to the net R-value of the insulation component itself.

It would be safe to assume for quick estimation purposes that the insulation alone defined the wall R-value. Even if a bit high, installation is never "laboratory hot-box perfect"..

In other words, R-13 batt insulation in a 2x4 wall with 25% framing has an at-insulation value of about R-16 and an at-stud insulation value of about R-8.

(R-8 x 0.25) + (R-16 x 0.75)= 14.

Just another way of looking at things, in short-hand.

Weezbo

It is important to stress these minor technicalities....

as in deed there are a lot more than these you have mentioned,...we are moving into an era of development where they too will come to light.some of these variables are Transient some may be more persistent...

Young people coming up need something to do with their college educations that cost a fortune there are fun things to practise their Physics still available however man fluidly adapts to his environment....and one day our heat loss calculations will look like some sort of dot to dot picture to the ladds and lassies . the good thing is they will have a lot of information and examples that will be invaluable in making more precise determinations. it really isn't the time to throw the baby out with the bath water as we change for the better.

Weezbo

Well:) during my reply...

was a call from the building inspection office ,a drive by from an electrician buddy for some clarifications on control wiring of some components and a call from me Mum*~/:)
coupled with the time it takes me to type anything ..:)

the need to plug in my van, make two more cups of coffee in the interim , me reply only off by about a few days cant rush Gods plan for us ...