Insulating plasterboard thoughts on 1930s house

We have 1930s house that is having cavity wall installation (EPS beads) on 50mm cavity. This is supposed to bring the U-value on walls from 1.603 to 0.498

As plaster is off inside, I am wondering if its worth having insulating plasterboard on external walls to further insulate? With just wet plaster and skim, the U-value will go down to about 0.479. Thats the cheap(er) option

If however I add 20mm/12.5mm K17 insulating board, the U-value will drop down to around 0.342. This is kind of close to requirement for new build and sounds quite good on paper. Its also going to cost around 2k extra.

What I am wondering if its practically going to be worth it or is it going to create more problems than it solves. Specifically, are there any advantages to taking insulation on walls that far when the house is a typical draughty 1930s house - suspended timber floors, draughty window openings (windows are double glazed)), open fireplace, etc. Is it likely to only create cold spots or is it going to make actual difference. Also, how does U-value of 0.479 work in practice on 1930s house - are we going to be freezing or is it actually pretty good?

Many thanks for any opinions/thoughts on this.

Moving air i.e. drafts are much worse for comfort tham radiation through a poorly insulated wall.


Just going on your comments, I would say the best area to tackle would be your floor. It's a big area, cold air around your lower limbs is particularly uncomfortable and the answers can simple but disruptive.

Some posters have put down a thin layer of hardboard to stop the drafts only - less disruptive. A more in-depth solution is to lift the boards and insulate between the joists. Use whichever material you like, but mineral-wool is cheapest and will conform to any irregularities : you will however need to fix some kind of netting under the joists to hold it in place.

Cost and effort much less than doing the walls, and , I guess, bringing more comfort.

After that tackle the windows. Don't know what kind of gaps you have so cannot talk about methods to tackle them.

Fireplace. Whether you use it occasionally or not , you could use a chimney-balloon( search if not clear) to seal it , with a small cardboard tube up the side to maintain ventilation .

By the way , it is pointless to calculate U values to three decimal places as this is only a mathematical exercise.

Insulation is not a precision engineered product and so will only be made to general dimensional tolerances and I suspect the same of the insulating value due to changes in the chemical constitution. I guesstimate the difference could add up to 5%, so only believe one decimal place reasonable.

EDIT

Check out all electrical socket as these can sometimes be open to underfloor void and let in cold air.

Personally if budget allows, I'd forget about the cavity wall insulation and fit thicker insulated plasterboard on the inside. As mentioned already floor insulation is well worth as obviously is insulation at ceiling (roof) level.

Its too late - cavity got filled this morning. The only question left is whether to install insulating plasterboard or not.
With regards to comment about insulating under floor boards, I was under the impression they have to ventilate from below and to do something that will stop that will result in timber rot. Also, I am not sure who I could get to do that, are there any companies that specialise?

jesteh said:

What I am wondering if its practically going to be worth it or is it going to create more problems than it solves. Specifically, are there any advantages to taking insulation on walls that far when the house is a typical draughty 1930s house - suspended timber floors, draughty window openings (windows are double glazed)), open fireplace, etc. Is it likely to only create cold spots or is it going to make actual difference. Also, how does U-value of 0.479 work in practice on 1930s house - are we going to be freezing or is it actually pretty good?

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A U-value less than 0.5 is good. The value of 0.3 is probably a response to the Government's agreement on greenhouse gases. As you say, other factors (huge 1930s windows, floor and draughts) are going to be much more important. Adding insulating plasterboard to the inside is unlikely to show a worthwhile return on £2k (maybe 3%-5% of old heating bill).

As an aside, you could dot'n'dab ordinary plasterboard to the external wall bricks as a cheaper alternative to wet plaster. You don't need a skim. A couple of coats of drywall sealer will give you a vapour barrier and allow you to use a steamer for stripping wallpaper.

Insulating a suspended floor generally involves taking up the floorboards, stapling a net over the joists and filling between the joists with mineral wool up to the floorboards. There is no vapour barrier as liquid spills must be allowed to drain through the insulation. Also fill gaps around the joists with mastic where they go into the inner leaf. This stops one source of draughts and possibly reduces condensation on the joist ends. You could put down new T&G floorboards, which can be sanded and varnished or Danish oil, so you get a floor and flooring in one. You might need to add some extra air bricks front and back and clear any rubble to ensure good ventilation to keep your joists dry.

Are there any companies that install underfloor insulation? I am not workshy and it doesn't sound complex but I have too much on as it is.

jesteh said:

Are there any companies that install underfloor insulation? I am not workshy and it doesn't sound complex but I have too much on as it is.

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The job is really labour intensive and so I would recommend you only consider doing it yourself, if at all.

If you have sufficient space to crawl under your suspended floor then insulating from below into the spaces between the joists is possible. It is hard work and you need to be fit. However it does mean you can still do it as a later option when time allows.

Blagard said:

The job is really labour intensive and so I would recommend you only consider doing it yourself, if at all.

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I'd go along with that. If you have fitted carpet you will be comfortable. Hardboard 'rough side up' under carpet will make the floor flatter and reduce draughts. If you want wooden flooring then underfloor insulation will probably make a noticeable difference.

Hi!

Without going into the details, if you went for a 37.5mm k17 board you would get an approximate end U value of 0.51W/m2C, or for an additional 15mm of insulation (52.5 board thickness) you would end up with an approximate end U value of 0.27W/m2C.

These figures include your cavity insulation.

Take the above figures multiply by the external wall area and this will give you your heat loss in Watts/deg.C where C is the temperature differential between the inside and external of the building. Take a temperature differential of say 16C and also do a worst case and use 20C, multiply these figures into your W/C and you will get your energy losses. Then multiply by 1000 to convert energy to kW/s (crept the s in, but thats as far as I am going.)

Regards