Sprayed insulation between roof rafters

Don't waste your money insulating at rafter level. A total depth of 350mm uncompressed insulation at ceiling level will be far more effective and cheaper. After all why would you be heating your attic when it's not part of the habitable area of the house. Don't forget to seal around any pipes and wires going into the attic and make sure your attic hatch is draught stripped and insulated.

Why not both and have usable space for storage or to convert into.

You should insulate and airtighten the attic trapdoor. How to do it depends very much on the type of trapdoor.

Yes, adding insulated boards designed for the purpose is far better than rafter insulation. Be sure to fill the full space between the joists first but don't overfill (i.e. if the gap is 100mm, then only put 100mm of insulation and not 150mm and compress back to 100mm. And remember not to compromise the attic ventilation.

abbey

All materials on building sites must be fitted in accordance with the manufactures instructions. Many workers on building sites never read these instructions. 

Slating and Tiling instructions state that the roofing membrane must be fitted well sagged between the rafters 

The purpose of the roofing membrane is to prevent any ingress of rainwater blown in between the tiles or slates during very strong winds, from getting in to the attic area. 

The membrane is sagged to form troughs between the rafters, to enable any of this rainwater to flow down in to the gutters. 

If it is not well sagged, then the membrane will be up tight against the lower surface of the battens which will prevent the rainwater from escaping from the membrane. 

Rainwater may build up and soak in to the battens and may cause Wet Rot. 

if there is cracked or damaged slates or tiles allowing ingress of rainwater it could cause excessive damage. 

If spray foam is sprayed under the membrane, it will expand and will push up the sagging in the membrane, up against the roofing batters.

I’m sure that many lay people here do not need to purchase or read any detail recommendations.

Many just want simple advice - simply explained.

There is nothing wrong with the advice I have given.

Hi. I have similar question to Abbey, re insulating the attic trapdoor. My house has a Staire. In particular, what type of Airtightness tape would be best to put around the frame?

Thanks C Eastwood for the advice.

In regards to the spray foam does the fitting of a vent card lessen the pushing up of the membrane?

Having read some of your and others advice, I think we would like to avoid putting in spray foam.

We had someone out to the house who was suggesting multi-foil insulation as an alternative to spray foam. It's not something that is common in this country. Why is that?

A topic I have been looking into myself.

I have heard that another approach being taken is to slab and skim (or tape joint) existing ceilings with 50mm insulated plasterboard.

Benefit being claimed is that eliminates the joist cold bridging.

Any thoughts on this approach?

Am I mixing up rafters and joists?

I mean slabbing over existing ceilings and not considering any changes to rafters.

No, I am. This thread is about rafters and I didn't read your post properly to spot the difference!

Why are you considering insulating your ceilings? What's above them?

Hi, it is bungalow with attic and 200mm rockwool.

abbey

Apologies for the late reply.

Timber needs ventilation. The attic needs ventilation. The roofing membrane should be sagged between the rafters to allow any ingress of rainwater to flow down unimpeded In to the gutters.

Spray foam interferes with all of the above.

This is unbiased advice.

The person advising you to fit Spray foam or multi foil insulation is a sales person - possibly on commission and looking for a few thousand euros from you.

Furthermore, I do not understand why anyone wishes to heat an uninhabitable void in the Attic or - the voids above and at both sides of Attic rooms.

Example - an attic with a 2.7 m rise of a pitched roof - in a house 9 m x 14 m gives a volume of space in the Attic of approx 9 x 2.7 / 2 x 14 = 170 m3 volume of air in the Attic.

Heating this unused 170 m3 of space is ludicrous. - Just because a sales person advised insulating between the rafters. They are fantastic salespersons.

200 mm of insulation on top of the plasterboard ceilings will give an approx U value of 0.10 watts/m2/degreeC/hr.

In the above example a house with a floor to ceiling height of 2.4 m will have a volume of air of 14 x 9 x 2.4 = 300 m3 of air which must be heated to approx 20 degree C, which is expensive to do for many months of the year- and every year

WHY heat up the 170 m3 of uninhibited attic space, which is approx half the volume of air in the house which must be heated and Will increase the heating costs by 33%. €€€€€€€€€€€€€

dathi

I‘m at a total loss as to what you are referring to here, or what you wish me to state.

I would not under any circumstances alter the advice I have given to abbey B, with the information given.

On the 19 April Andrea B stated as follows:-

• I mean slabbing over existing ceilings and not considering any changes to rafters.

AND

• Hi, it is bungalow with attic and 200mm rockwool.

I do not have any further details of their house

I do not know when the house was constructed or if it was constructed before or after the Building Regulations of June 1992, or if the house was constructed before or after the Draft Building Regulations.

Therefore if the house was constructed in 1978, the ceilings did not have to be insulated.

Building Regulations do not have to be complied with retrospectively.

Therefore it is axiomatic that we do not know what Regulations, if any, which are applicable to the said house.

Here are the current BR, for works to existing building, and which states “as far as is practicable “.-

The only logical reason that I would instruct a Builder to remove perfect Attic Insulation, would be for Health and Safety reasons - when the plasterboard slabs of a ceiling are being removed.

andrea B has 200 mm of Rockwool insulation at ceiling level.

The Building Regulations state inter alia, that insulation material at ceiling level Must be non-combustible. - Rockwool meets this criteria.

We have not been given the density of the Rockwool, so we cannot ascertain it’s k-value ( thermal conductivity) . Without the k Value the U value cannot be calculated.

It would be totally ridiculous and expensive for any person to remove perfectly good insulation material from an attic, in order to install new insulation, when they only have to add more insulation on top of the existing Rockwool.

You requested dathi that I should advise Andrea B, to remove the 200 mm of Rockwool. I would not advise any person to do that under any circumstances as it would constitute an act of gross incompetence and negligence.