What happens if I have gaps in my insulation?
like electricity tends to follow the path of least resistance.
From how insulation works:-
- The total power of heat through the insulation and the gaps in W is given
- P = (Ti - Te)·(ai/Ri + ag/Rg).
- The thermal resistance of the ceiling in K·m²/W is given
- Therefore the effective thermal resistance in K·m²/W is:
- R = a/(ai/Ri + ag/Rg).
- P is total heat flux in W.
- (Ti - Te) is the temperature difference in K.
- a is the total area in m².
- ag is the gap area in m². (ag = 0.01 a)
- Rg is the gap thermal resistance in K·m²/W. (Rg = 0.30 [AIRAH])
- ai is the insulated area in m². (ai = 0.99 a)
- Ri is the insulated thermal resistance in K·m²/W. (Ri = 2.50 +
0.30 = 2.80)
- Effective thermal resistance for R 2.5 with 1% gaps is:
- R = 1/(0.99/2.80 + 0.01/0.30) = 2.58 K·m²/W.
- Effective thermal resistance for R 3.5 with 10% gaps is:
- R = 1/(0.90/3.80 + 0.10/0.30) = 1.75 K·m²/W.
So R 2.5
insulation with 1 % gaps will only add R - Rg = 2.58 - 0.30 = 2.28 to the total
R value. (This is a reduction of 8.8 %.)
So R 3.5 insulation with 10 % gaps will only add R - Rg = 1.75 - 0.30 = 1.45
to the total R value. (This is a reduction of 58 %.)
Cellulose fibre is a loose fill insulation which means it leaves NO
This 'parallel paths method' is valid for heat flow through the ceiling.
Other methods may be required for heat flow along a surface such as a concrete
slab. For more information see James M. Fricker 
and H. A. Trethowen .
Cellulose - Naturally Better Insulation