Discussion of the benefits of either solution, and why you may choose one option, compared to another situation.

Which will make my home warmest in winter and coolest in summer, and save the most energy?

In theory, either solution will create the same benefit, if they are the same R-Rating (thermal resistance). However, if batts cannot be laid neatly between the joists on bare ceilings, they may not perform correctly. When a home has poor access, due to a low roof, multiple A/C ducts, or other obtructions, batts may not be possible to fit correctly. In this situation, blow-in insulation will be able to get into the nooks and crannies, and will actually achieve it's correct performance.

Does Blow-in insulation move around in the ceiling?

There are 2 types of material blown into ceilings. Rockwool (melted rocks such as basalt and dolomite) is denser and will remain the same for the life of the building. Cellulose is recycled paper with Borax and Boraxic acid added to it for fire and insect protection) is hammer-milled and a finer product, and may be succeptible to some movement in very open windy roofs. However, it is not something we have found to be an issue in most roof spaces. Rockwool is denser and holds together better, preventing any possibility of movement.

My electrician says that batts are better than blow-in. Why would he say that?

Batts are easier to lift or shift if new wiring is needed. Most tradespeopel will prefer batts if they are working the roof. However, you need to consider which product will give you the lowest power bill, and keep your home the most comfortable in the long term. 

Pros and Cons of Batts


  1. Easy to move
  2. Simple to work around
  3. Wont move around
  4. Timbers readily visible
  5. Guaranteed for the life of the building
  6. Can be fitted close to all downlight covers


  1. May not be able to get them to corners of low roof
  2. Every 1% gap can cause up to 5% thermal loss, so you could easily loose 30-40% of the benefits if poorly fitted
  3. If timber joists are non-standard sizes, there may be gaps left between batts and timbers
  4. May need to remove much of roof to get access into low/tight roof spaces
  5. Ducts may prevent batts being laid underneath them
  6. Batts cannot be fitted into existing walls without taking all the plasterboard off
  7. Steel frame ceilings are often the incorrect spacing for batts
  8. Old batts/material needs to be removed before new insulation is installed

Pros and Cons of Blow-in insulation


  1. It will ensure a complete coverage, meaning no gaps anywhere
  2. Cavity wall rockwool is the only product licensed by the CSIRO for pumping inot existing walls
  3. It can be pumped over existing material, making it cheaper than removing and replacing
  4. It can be pumped into raked and flat roofs with little roof disturbance


  1. Can be harder to work in the ceiling if it is above the joists
  2. Harder to find loose wiring if not lifted up when installing insulation
  3. Must totally protect downlights from the insulation

How do I decide what is best for my roof space?

If you have a high roof, with no obstructions or old material in the roof, and standard joist widths, batts will work very well for you, and are cheaper than blow-in materials. As soon as the roof is awkward to access, low pitched with ducting, or has old insulation, you may want to consider blow-in. However, the majority of homes can be insulated with batts, especially if you are removing the old material

It is best to call a company that can offer you all the possible alternatives, so that you can get some un-biased advice. Ask the salesperson to show you what is the benefit in your roof space, and what alternatives their company can offer you. If they only offer one type of product, then they will generally tell you that is the best solution, as they may not have an effective alternative!!

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