Breather gap needed for full fill cellulose insulation in rafters???

KingPuck · 29-09-2010 09:05AM #1

Hi,

We are at roof stage with our dormer bungalow at the moment. I am considering pumping rafters with cellulose. I have 7 x 2 rafters with an airtek breathable felt on top. I plan on putting in a Siga Majpell membrane inside for both air tightness and because we need a membrane anyway to keep cellulose in. My questions is can we full fill the rafter with cellulose or do we need a breather gap under the felt? The roof is already latted and felted, single lat.

heinbloed · 29-09-2010 12:50PM

Several points should be looked at.
The technical point of the installation would be: Is the outer layer of membrane tight? Would the cellulose be blown through gaps, would gaps in this membrane allow moisture or wind to blow into the insulant?

Then the dew point problematic: Is the dynamic dew point calculated, would there be a risk of condenstion in the finished structure?

There are many more issues to be thought about, but if these Two can not be answered then the proposed attempt could be risky.

Poor Uncle Tom · 29-09-2010 02:31PM

This might be of help. Page 28 even has a diagramme.

caesarthechimp · 29-09-2010 11:24PM

7 inches (down to 5 with the 2 inches ventilation) cellulose will not be enough insulation. You will end up using some polyurethane inside the rafters as well, or possibly just in between with no cellulose.

KingPuck · 30-09-2010 12:14AM

Thanks Uncle Tom I'll have a look through it.

Enrate I had hoped to fill the full 7" with cellulose but my question is can I do this without counter battening the roof as this is no longer an option?

Any ideas?

caesarthechimp · 30-09-2010 10:45PM

You should have 50mm ventilation plus some vent tiles along the ridge.
Maybe you can now pack out the rafters on the inside with battens if you have enough headroom to spare, giving more space for insulation. The reason people use polyurethane is because you can get around twice the insulation value into the same space. Anyhow, read the link provided by uncle tom above.

Breather gap needed for full fill cellulose insulation in rafters???

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