Context : Observations of edge-on galaxies allow us to investigate the vertical extent and properties of dust , gas and stellar distributions .
NGC 891 has been studied for decades and represents one of the best studied cases of an edge-on galaxy .

Aims : We use deep PACS data together with IRAC , MIPS and SPIRE data to study the vertical extent of dust emission around NGC 891 .
We also test the presence of a more extended , thick dust component .

Methods : By performing a convolution of an intrinsic vertical profile emission with each instrument PSF and comparing it with observations we derived the scaleheight of a thin and thick dust disc component .

Results : For all wavelengths considered the emission is best fit with the sum of a thin and a thick dust component .
The scaleheight of both dust components shows a gradient passing from 70 \mu m to 250 \mu m. This could be due to a drop in dust heating ( and thus dust temperature ) with the distance from the plane , or to a sizable contribution ( \sim 15 - 80 \% ) of an unresolved thin disc of hotter dust to the observed surface brightness at shorter wavelengths .
The scaleheight of the thick dust component , using observations from 70 \mu m to 250 \mu m has been estimated to be ( 1.44 \pm 0.12 ) kpc , consistent with previous estimates ( extinction and scattering in optical bands and MIR emission ) .
The amount of dust mass at distances larger than \sim 2 kpc from the midplane represents 2 - 3.3 % of the total galactic dust mass and the relative abundance of small grains with respect to large grains is almost halved comparing to that in the midplane .

Conclusions : The paucity of small grains high above the midplane might indicate that dust is hit by interstellar shocks or galactic fountains and entrained together with gas .
The halo dust component is likely to be embedded in an atomic / molecular gas and heated by a thick stellar disc .