Blown cellulose or blown EPS beads? (pitched roof)

[DannyG]

Hi folks

 

We were planning a masonry build (200mm cavity) with SIPs roof (warm loft/vaulted), but the cost of the roof is potentially going to be an issue.

Our builder has proposed metal web posi-rafters with blown cellulose as a cheaper alternative - which seems like a good combination. However, as we were planning on blown EPS beads in the wall cavities, it seems a bit silly to have two different teams on site doing essentially the same thing.

 

So, what are the thoughts on either blown cellulose in the cavity or blown beads in the roof? Is there any reason this use-case is not really mentioned on most of the manufacturer websites offering these products?

 

Thanks.

[Temp]

The roof would need to be designed for blown insulation but i don't think it's difficult. Contact suppliers of blown insulation to see if they have any specific requirements. Perhaps also check if your warranty provider is happy.

 

If you are insulating between the rafters this is known as a "cold roof construction" (not to be confused with a warm or cold loft/roof space). In the past a cold roof construction needed a 50mm ventilated void below the roof membrane to avoid interstitial condensation and rot. You can't do this if you plan to totally fill that void with blown insulation.

 

To avoid the need for a ventilated void you can/must use a vapour permeable membrane and tiles that allow good air circulation under them (eg plain clay tiles rather than very flat artificial slate or sheet metal). So don't let the builder use the cheapest membrane he can find - it must be a vapour permeable type.

 

Then there is another issue.. The insulation must not be allowed to press the membrane onto the underside of the tile battens. Rain water or snow blown under tiles can collect above the batten and cause batten rot. The solution is to fit counter battens to lift the tile battens up so there is a gap even when the blown insulation causes the membrane to bulge out a bit. Better still fit osb (or sarking boards), then the membrane, then counter battens, tile battens and tiles. 

 

Some people are concerned about voids being left unfilled or the insulation settling. I think settling is less of an issue with the coated eps bead system.

 

You will probably need to get a cross sectional drawing done and approved by building control. I think I would do this even if your builder recommends going down the Building Notice route rather that the Full Plans route for getting Building Control Approval.

 

Avoid spray foam. Im sure it's fine if done right but some lenders appear to be imposing a blanket ban on lending on houses that have spray foam in the roof because it interferes with ventilation and can cause rot 8f not done correctly. Some people have had to get it taken out. I just don't think you need the possible aggravation.

 

Edited October 17, 2022 by Temp