Hello all, first post! I have bought a 1950's house, and it has been basically untouched since the 1970's (in terms of wiring, insulation, décor and heating). we are in the process of improving things, especially before winter kicks in (the house is freezing). The house has cavity walls, which to my knowledge are unfilled (seems no evidence of drilling). We are considering external wall insulation to improve things, but I cant seem to get a clear answer on whether or not you can apply EWI to an unfilled cavity. In my mind, the heat would be lost through the cavity before it even hits the external wall. The company who have quoted us, believe it is fine to apply without the cavity being filled. again in my mind, the cavity should be filled before EWI is applied. The other thing we are considering is just increasing the room insulation to 100mm (from nothing).

what are peoples thoughts?

thanks :D

I would fill the cavities, otherwise there will be air currents in them (convection) and through them (draughts) which will carry heat from where it should be. It would be extremely difficult to make the cavity airtight, and even harder to keep it so! Cavity insulation is also a cheap and well-proven way to add insulation value of the wall.

(Edit: you might get it done free, under Energy Company Obligation or subsidised under Green Homes Grant Scheme)

Also think about how you can connect up the wall insulation to the roof insulation so you don't get a cold area if they don't meet.

Good also to think about controlled ventilation.

Good luck, it will make a big improvement to your house!

If you decide one CWI only do it if you are not in an exposed situation. Our 1st house built in the 1930's we did cavity fill that was in about 1973 and it worked very well but we were in a sheltered position. Where I live now close to west coast it is exposed and would not entertain filling a cavity and there have been many horror stories of houses having severe trouble with damp. There are cowboys about in this industry so would choose carefully.

If you have very large rooms you could consider internal insulation you get the best of 2 worlds very efficient insulation with the benefit of a cavity to keep the house dry. It would of course be very disruptive to do.The benefits of a cavity is often overlooked.

Posted By: revorWhere I live now close to west coast it is exposed and would not entertain filling a cavity and there have been many horror stories of houses having severe trouble with damp.

Is there still a risk of damp in a cavity if EWI is put on outside the walls? If so, then a rainscreen on the outside of the EWI ought to remove any risk at all.

Posted By: Thorvaia(the house is freezing).

Does it have a basement and/or a crawlspace ?

gg

Posted By: WillInAberdeen>>>>"The rendering if correctly done should stop all air flow into the property .”

Not sure this is the whole story though... Cavities are often open at the top into the roof void, and at the bottom round the ends of the ground floor joists (if present), and through gaps in the pointing of the inner leaf of the gables into the loft, and gaps round cables and pipes and under window sills etc etc.

+1
Any cavity I have had dealings with has had a gale blowing through it! (which is why I prefer solid wall with EWI). The EPS EWI with thin coat render should stop any chance of rain ingress through the outer skin.

Thorvaia - before you shell out the (considerable) money on EWI make sure all the draughts are fixed. If the windows are sound then frames can be improved with better sealing and often double glazed units can be fitted to replace single glazing which is usually cheaper than new windows.
You haven't mentioned the loft - that is often a quick cheap fix. If the place hasn't been touched since the 70s then there is probably only 100mm of sagged out insulation up there. Target between 200mm - 300mm.

Tony - would it be effective just to stuff some fibreglass quilt material tightly into the top of the filled cavity (i.e. filled with EPS beads or blown fibres) to prevent air moving up through it?

jms452 - so stuffing quilt tightly into the top of the cavity would do the job do you think? I only ask because this would be a very simple thing for me to do with my cavity walls which are filled with Rockwool blown-in fibres.

Posted By: Jeff Bjms452 - so stuffing quilt tightly into the top of the cavity would do the job do you think? I only ask because this would be a very simple thing for me to do with my cavity walls which are filled with Rockwool blown-in fibres.

I dont think stuffing compressed quilt would such a good idea as even squashed its not air tight and squashing it tight would remove much of its insulation properties and give a degree of cold bridging. If the cavity is pretty well sealed aside from the top I personally dont think theres likely to be much air movement through blown cavity insulation. If you want to be sure then its probably best to use a squirty foam bead to close the top of the cavity and then extend the loft insulation to sit on top of the foam.

Weve got blown CWI in part of the original house and all weve done is levelled the blown fibres with the top of the outer brick leaf then cut pieces of loft roll to fill from the top of the blown fibres to level with the top of the wall plate cutting to fill under and around the rafter ends without leaving gaps and avoiding squashing the insulation too much. Then weve run a short length of 75mm roll over the top of the wall plate to join with the loft insulation. Cant get any more than 75mm without block the ventilation gaps. That level of detail has made a big difference to one of the bedrooms below where there had been black mould on some of the edges of the ceiling/top of the wall due to cold spots. Same usage of the room and theres been no recurrence of the mould since😊

Hi everyone, thank you all for the comments, its nice to get some more info on EWI. After discussing with another company, they matched what has been said here, that the cavity should be filled (and the loft insulated to "seal" the cavity) before EWI is applied. We are now planning on installing internal insulation, after discovering all i have in the walls is 10mm of polystyrene.... I would rather stop the heat (or limit the heat) entering the cavity. We are looking at between 100-150mm of kinspan now, funds dependent. The loft has 300mm of mineral wool, but planning on boosting that to 400mm. We have rooms in the loft, and there is little insulation in the walls or on the roof itself, so another area to look at.

I have also been discussing getting the cavity filled, and it seems to be a no-no up here in Orkney, due to the driving rain (the council no longer recommends CWI when renovating their properties). I will see what the estimated cost of internal insulation will be

philedge and jms452 - thanks for your further comments. I have tried to ensure that the loft insulation extends over the wall plate and cavity and taking care to leave a gap above for air to circulate. Maybe that's sufficient then?

Thorvaia - what do you mean by "all I have in the walls is 10mm of polystyrene"?

Posted By: ThorvaiaI have also been discussing getting the cavity filled, and it seems to be a no-no up here in Orkney, due to the driving rain (the council no longer recommends CWI when renovating their properties).

Orkney - see p34 here - unsurprisingly it looks like you are rather exposed:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/431943/BR_PDF_AD_C_2013.pdf

This is well worth a read (there may be a more recent version?) as the answers to many of your questions are location dependent.

We did CWI and EWI (separately on different bits of the house) but are about as sheltered as you get.

Posted By: jms452Orkney - see p34 here - unsurprisingly it looks like you are rather exposed:
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/431943/BR_PDF_AD_C_2013.pdf

This is well worth a read (there may be a more recent version?) as the answers to many of your questions are location dependent.

I think that is the latest version, and it is definitely helpful, but note that it says on the front cover "For use in England". I think that https://www.gov.scot/publications/building-standards-2017-domestic/3-environment/310-precipitation/ may be more relevant for Orkney.

The Scottish document doesn't discuss EWI on cavity construction, but I think that may be because it is focussed on new construction. I expect there are good documents on retrofitting that our Scottish members can point to. The document does mention cladding the outside of EWI as an alternatvie to render, and I would argue that if EWI over a solid wall is safe, then EWI over a cavity-filled wall must be a fortiori. But I can see why comments like "This construction is only recommended for sheltered conditions" with regard to insulated cavities might be putting people off if they don't consider the whole context.

<blockquote><cite>Posted By: Jeff B</cite>philedge and jms452 - thanks for your further comments. I have tried to ensure that the loft insulation extends over the wall plate and cavity and taking care to leave a gap above for air to circulate. Maybe that's sufficient then?

Thorvaia - what do you mean by "all I have in the walls is 10mm of polystyrene"?</blockquote>

Hi, my internal walls (well the internal walls on the external walls if that makes sense), consists of a 1inch stud, with 10mm of polystyrene sheet then the plaster board...thats it.....

If you have the time to read and understand the information from

http://www.sensiblehouse.org/nrg_heatloss.htm
which I have not yet done and as I do not have the knowledge to know how correct it is .

It may help on your decision

What I do know is that If you have the ability to have the roof adequately cover the EWI and can minimise and successfully seal all intrusions through the EWI there should be no penetration of air or water in any of it's forms through the EWI.

All movements of air and moisture within the envelope should then be entirely under your control.



I believe you will be surprised as to how little energy you will need to input

My entire energy import for the year in a 3 bed semi is less than 1000 kWh with the aid of 7.5 kW of PV panels and a 20kW battery.