Wall Insulation Question?

[blackeyes]

I'm currently trying to insulation the dwarf walls in our dormer bedrooms, the plasterboard is foil backed (no insulation behind) the studs are 100mm in depth & 380mm apart. I'd like to try and get at least 200mm of insulation behind them, would it be ok to add strips of wood say 4 x 2's to the existing studs and then fit the 200mm rockwool between?

Also would I need to add anything else once this was done to keep the stuff from slumping over time? My budget just doesnt stretch to semi rigid insulation and access is limited in height and width.

 

[Sandals]

Id spent all my money on insulation. We put the fleece stuff in our sunroom roof and although my husband did nail it in to the rafters we're assuming it slumped as room get chilly in winter.

A family member just put up cosyboard, its plaster board with huge amount of rigid insulation and foil. Looks the business.

 

[Leo]

Rockwool will sag over time, applying it to vertical walls is a waste of time and money I think. As soon as it sags even slightly you'll have cold bridging.

Ideally, wait until you can afford to use a rigid insulation cut to the size of the joists, then apply an insulation backed plasterboard over that.

 

[blackeyes]

Many thanks for your responses. I'm looking at 100mm rockwool flexi as opposed to rolls now and it might just be a goer with the budget. Once inbetween the joists should I add something for airtightness, plastic or such does anyone know?

 

[lowCO2design]

Many thanks for your responses. I'm looking at 100mm rockwool flexi as opposed to rolls now and it might just be a goer with the budget. Once inbetween the joists should I add something for airtightness, plastic or such does anyone know?

a holistic retro-fit of air-tightness to the eaves/ cavity closer location is really required. this layer should ideally be continuous through-out the building envolpe, such as layer is notoriously difficult to achieve from the inside of the crawl space. air-tightness of these areas really requires removal of the plasterboard, to start with!

please note

• the air-tightness membrane, which serves the same purpose as a Vapour control layer, should always be on the inside (warm side/internal side) of the insulation.
• you need ventilation into the crawl space,
• and you probably dont have air-tightness continuity at the first floor joists junction, so the knee high wall air-tightness is second to the floors.

My budget just doesnt stretch to semi rigid insulation and access is limited in height and width.

if your budget is that tight, what are you really hoping to achieve here? as air-tightness membranes and tapes are expensive and require reasonably invasive works in crawl spaces areas to achieve a reasonable level of air-tightness.
with what you have outlined here and especially considering your budget - get a few tubes of canned foam, silicone and door draft stoppers and go around with your hand as a guide to determine the main drafts. best of luck