View Full Version : Heatloss calcs

21-08-2012, 06:50 PM
Following on from a couple other workshop threads on here, I'm really wanting to sort out some insulation and heating ready for this winter.

The current workshop is aprox. 10.5m long x 3.5 wide x 2.2 high, and is a lean to onto an existing single brick unheated building, with the other walls being single thickness breezeblock, and one end having pretty much full width 19mm ply doors (one of those temp jobs that has remained more permanent!). The roof is coated corrugated metal, with a layer of foil bubblewrap insulation underneath. There is a single width doorway into the other building, which currently has no door.

I can't afford to insulate it all, however I want to figure out where I need to concentrate priorties, so using Heat Loss through Building Elements due to Transmission (http://www.engineeringtoolbox.com/heat-loss-transmission-d_748.html) as suggested by irving, here's the heat loss calcs.

Wall area
long wall = 10.5 x 2.2 = 23.1sq/m
short wall = 2.2 x 3.5 = 7.7sq/m

So ignoring the door opening, total wall area is 53.9sq/m.

Now taking a R-value of 0.2per inch (a quick bit googling puts brick and breeze blocks around the same value), gives a U value of 1.25.
So taking the equation Ht = U x A x Dt = 5 x 53.9 x 20 (seems to be a common standard for calcs) = 1347W

Roof area
Area = 10.5 x 3.3 = 36.75sq/m

Now taking the corrugated metal (R=8.5), and the insulation (R=0.125), U = 1 /(8.5 + 0.125) = 0.116
So heat loss = 36.75 x 0.116 x 20 = 85W
(this doesn't seem right, however now I think about it, having the space heater running didn't really affect the snow lying on the roof...)

Main Door
Area = 3 x 2 (aprox. sizes) = 6sq/m

R value of 19mm ply is 0.93, so the U is 1.07.
Heatloss = 6 x 1.07 x 20 = 128W

Now some of those results seem a bit wrong to me (anybody feel free to correct them!), however it really means I need to find an internal door, and sort the draughts, followed by getting the walls lined.
I was going to do the roof first, but those calcs show it really isn't the main problem.

21-08-2012, 06:59 PM
Those calcs assume the door is close fitting and there are no draughts which will seriously affect things. As to how good the calcs are, a 2kW heater should keep it warm in there... so how was it last winter and whats the output of the space heater? The open door will permit significant heat exchange through air movement, so much depends on the temperature of the other building. I get the impression this is also unheated/uninsulated?

You didn't assess the floor losses?

21-08-2012, 10:30 PM
Totally forgot the floor!

It's a 6" slab, so
Area = 36.75sq/m
So allowing for U value of 2, and assuming ground temp of 10deg (I've knocked a couple degrees of for being further north than you!)
Heat loss = 36.75 x 2 x 10 = 735W

So total heat loss through conduction is 2295W

The other building is unheated and uninsulated, and it's a single brick construction with corrugated fibre roof, with two set of badly fitting double doors, so is as good as being open to the elements!
As for the space heater, it's somewhere in the 15kW range. Get's the air upto temperature quickly, but smokes the place out andgiven the amount of cold metal in the shed, you're frozen again within minutes of turning it of.

I know drafts are a major issue, but it was interesting to see just how little of the heat is actually lost through the surfaces, so number one priority is getting a door fitted between the two buildings, followed by some draught free doors to the outside. Then I'll work on getting the walls lined and insulated, as the plan from day one was to fix 3x2's to the external walls and have insulation sandwiched behind some 1/2" or 3/4" ply.

21-08-2012, 10:33 PM
And for reference, while googling I found this page with lots of R-values for various materials - R-value (http://www.sizes.com/units/rvalue.htm)

22-08-2012, 12:02 AM
And for reference, while googling I found this page with lots of R-values for various materials - R-value (http://www.sizes.com/units/rvalue.htm)

Got to be careful with the U <-> R conversions. A lot of non-US sites give 'European' R values, i.e. 1/U, where R is (Kelvin m^2/W) per 25mm thickness. (mental note to self to go back and check my own calcs!)

So loss = U x A x Dt. For several layers, of materials having R-Values R1 - Rn of thicknesses t1 - tn, U = 1/(R1t1 + R2t2 + ... + Rntn) If all the R-values are US ones, then U = 5.682/(R1t1+R2t2+... +Rntn)

I used several sites, including this one: http://www.stronggreen.com/docs/R-Values%20of%20Common%20Building%20Materials.pdf for R-values

That give R value for 8" concrete as 2, so 6" has R of 1.5 or U of 5.682/1.5 = 3.8, not the 2 you have used... so loss through the slab is higher at 36.75 * 3.8 * 10 = 1400W approx.

However this article (http://www.theconcreteproducer.com/industry-news.asp?sectionID=1423&articleID=250647)from the concrete industry confuses it all by talking about U = 1/R when R is in the BTU form! Their figure gives a U factor, in SI units, of between 4.7 and 9.4!!!

This page was useful too: http://www.the-flat-roof.co.uk/Appendices.pdf

Just revisited your walls calculation. R = 0.2 per inch, so a 4" brick = 0.8, so U in SI units = 5.682/.8 = 7.1, so your heat loss through the walls is 7.1 * 53.9 * 20 = 7.6kW. I note your calc was 5 * 53.9 * 20 = 5.4kW, yet you wrote = 1347W ???

22-08-2012, 03:09 PM
SO, playing around with different insulation types... there's expanded polystyrene, the white stuff used for packaging, also known as beadboard because its made up of, and breaks up into, little beads... (U value = 0.35/100mm, .7/50mm) and Polyurethane sheet, the more rigid and sometimes structural (U = 0.21/100mm, .42/50mm)
100mm Polystyrene works out about 5/sqm on eBay, Polyurethane (rigid insulation board) 12/sq m
50mm Polystyrene works out about 2.6/sqm on eBay, Polyurethane (rigid insulation board) 6.5/sq m

So which to use?

In my workshop, total wall area = 42sq m. (50mm insulation), roof area = 24sq m (100mm insulation)

So Polystyrene, heat loss = (42 * .7 + 24 * .35) * 20 = 756w, Polyurethane, heatloss = (42 * .42 + 24 * .21) * 20 = 453w

Cost of electricity = 12p per Kw/hr
therfore cost per hour difference = (.756 - .453) * 0.12 = 3.6p/hr

Cost of insulation, polystyrene 2.6 * 42 + 5 * 24 = 229, polyurethane 6.5 * 42 + 12 * 24 = 561

Polyurethane pays for itself after (561 - 229)/0.036 hours = 9222 hours, or in reality, slightly over 5 years assuming only gets that cold 20% of the time (and I'm not in there 24 x 7!)

Since I'm working to a small and diminishing budget, polystyrene it is for the workshop....

Obviously for a new house build polyurethane is the better choice... (its more stable, U value doesn't change with time, and the additional cost is a small proportion of the total build cost)

22-08-2012, 04:22 PM
Just revisited your walls calculation. R = 0.2 per inch, so a 4" brick = 0.8, so U in SI units = 5.682/.8 = 7.1, so your heat loss through the walls is 7.1 * 53.9 * 20 = 7.6kW. I note your calc was 5 * 53.9 * 20 = 5.4kW, yet you wrote = 1347W ???

I honestly don't know where I got that one!
But those figures certainly sound more plausible.

I'll do some calcs later to figure out how the different thicknesses of insulation will affect things, as I may swap the 3x2's for 4x2's, to get that bit extra insulation.

08-09-2012, 01:27 PM
Just got around to doing some insulation calcs.

This is all for walls, just using idealistic values.

Thickness Polys Polyu
50mm 754W 452W
75mm 503W 301W
100mm 377W 226W

I suspect 100mm will be too much in terms of costs/space loss.
I'm currently thinking 3x2 tanalised timber mounted vertically between battons with ties onto the wall in the middle for support (the walls aren't straight enough to mount straight onto!), have an air gap between the brick wall and 50mm insulation, then face it all of with OSB3.

Ebay cost of enough 50mm rigid insulation panels to do all the walls is 384, which isn't too bad.
And I could always add 25mm polystyrene to space them from the wall, for not much additional cost.

08-09-2012, 06:40 PM
I hope I'm not being seen to be paranoid but I'd prefer to use an insulation medium with far greater fire resistance than polystyrene; even if enclosed in a wall space. IMHO there are many other materials that aren't significantly more expensive with a far greater fire resistance.