older home r-value estimates

chavis

Wondering how people are estimating r-values on older homes for heat loss calculations? walls, ceiling, windows, etc...

Unknown

depends. How is it insulated (if at all)? What kind of windows? 2x4, or even older than that?

hvacfreak

assume the worst

For older construction , I always assume the worst. If you are sizing for a hydronic application , I can't see where this would be a problem if all other construction data is accurate. If I think there may be improvements , I will " arbitrarily" step down on capacity ( especially on condensing gas air or water ). Nice older homes in nice neighborhoods will probrobly get better windows , I try to think about things like that when I do residential work . - M

Brad White_9

Puts the \"Arrrrrgh!\" in R-Value doesn't it?

I calculate mine using the ASHRAE method, layer by layer. Generally I think in terms of "u-factors" (the inverse of R) but here are some samples of what I have calculated for older ininsulated structures, say balloon framed Victorians or later:

Full 2x4 walls, 7/8" spruce sheathing, crumbling felt, clapboards or shingles outside, wood lath and plaster inside;
No insulation (just a 4" airspace) and framing at 20% aggregated across the wall:

R=5.4

u=0.185

Same but with 4" blown-in cellulose in stud cavity (R4 per inch, NuWool dense-pack):

R=17.435

u=0.58

Brick wall, common brick, 8" thick, plaster inside, air space between strapping:

R=3.524

u=0.284

Roof-ceiling assemblies (strapping below joists or rafters, sheathing etc.) are similar to walls; more air cavity but more air movement within. If insulated above, that is quite variable.

In all of this, the airtightness (or lack thereof) is far more critical than the insulation value of the building materials. Give me an inch of rigid insulation that is tight rather than a foot of widely gapped, loose-fill fiberglass any day. Rounding up to the next second decimal point is a good idea in old construction (e.g: a u of 0.75 becomes 0.8).

Brick/masonry is a real heat-sucker, but the mass helps. Also the wall construction tends to be more homogenous and less air infiltration through the membrane of the building.

But it underscores the importance of insulating first, does it not?

Hope this helps.

Brad