Breathable roofing felts

sas

Just looking for a little clarification. If you felt a roof with a product such as Solitex Plus does that literally mean that you can then run insulation out across the wall plate i.e. not leave a room for cross ventilation in an attic.

I kind of assume thats the whole point of the product but I wasn't 100% sure.

SAS

construct06

one for Viking i'd say!

greenboy

Ventilation between a non breathable roofing underlay and the insulation is recommeded to minimise the risks associated with condensation. Typially this ventilation space should be continuous and about 50mm from eaves to ridge.

The disadvantage with this is that ias insualtion requirements increas we need deeper rafters and because we needed top maintain that 50mm ventilation space, this added again to even deeper rafters.

Also this 50mm space is very susceptable to insect infestation. And finally, this continuous ventialiton layer above the insulation dramatically reduces the thermal performance of the insualtion due to convection.

A vapour permeable or breathable underlay, such as Solitex PLUS allows vapour to dry out from the inside, dramatically reducing condensation risk, while acting as an effective rain and water resistant layer on the outside.
Because it is breathable, this allows the insulation to be installed against the underlay, without the standard 50mm ventilation. However when the insulation is forced against the underlay it may come into contact with the tiling batten. Therefore, a counterbatten should be installed on the outside to create a continuous drainage path for moisture to run to the gutter on the outside. THis space also allows for ventialtion to occur between the slate and the underlay. Ventilation may be required between the tile and the underlay if the slates are sealed or very tight together, (i.e. fibre cement).

To minimise condesation risk and to maximise thermal performance a continous sealed vapour control should be installed on the warm side of the insulation also.

Hope this Helps....

ardara1

Each breather membrane has differing vapour ermeability. The theory is that the act like your gortex jacket letting water vapout thru but prevents liquid from outside.

Check the BBA/IAB certificate - some require to be supported by a substate others unsupported.

BRE Good Practice guides on warm roof recommend that ALL breathers are provided with 50mm ventilation ABOVE the felt AND a sound vapour control layer the underside.

Ongoing debate between TYVEK (who manufacters TYVEK and NILVENT) and BBA and Building COntrol UK - i've seen Tyveks presentation and have no worries with product. Leaving a 25mm void under felt allows for drape for run off and adds to thermal value of slope.

sas

All good stuff, thanks SAS

Viking House

Well explained Greenboy only one comment to make. Solitex Plus is for the outside of the roof, the product for the inside of the roof is Intello.
You can fill up the area between the two membranes with insulation like Greenboy said and cross batten on top of the roof to allow an airflow being extra careful to tape all the joints.

One more thing! when the air is blowing between the slates and the Solitex it robs heat from the insulation below. In Scandinavia they use an extra dense/stubborn insulation in this area like a layer of dense Paroc or Softboard.
This is why we put 50mm of Softboard on top of the rafters, then crossbattens then slates.