External insulation of an unplastered house extension

Red Hare

We recently got an extension built that has yet to be plastered on the outside .(The builder said to wait until March to get it plastered because of the weather. ) The extension is insulated internally with thick board that has been skimmed and painted. it looses heat very quickly compared to the rest of the house.Would it be a good idea to have external insulation put on the exterior wall at this stage or would this cause issues?

10-10-20

Hi, how was the extension constructed, using cavity blocks or a cavity wall? How was the roof constructed and insulated, and was the building made air-tight using membranes and sealing products?

Red Hare

I think it was cavity blocks. It is a flat fiberglass roof.that was insulated with insulation board the sqme as the walks . The roof was supposed to be sprayed with insulation but the person booked to do it went missing do the builder used insulstion board between the timbers- i dont think they used anything to make it air tight

10-10-20

https://www.boards.ie/discussion/comment/124132589#Comment_124132589

What you have described is probably 90% of the problem. Cavity blocks don't allow for an airtight or well insulated envelope (as they have a cavity which cannot be filled), and unless the installer was fastidious with his finishing then the ceiling/roof is likely leaking heat through the gaps in the PIR boards in the roof. Spraying was never going to be the correct solution for a flat roof anyway, it would have resulted in damp, rot and mould within the unventilated space, so you dodged that bullet.

I'm sorry to report that if the paint is already on the walls (ie, you're not in a position to perform much remedial work) then it's not going to get much better unless there is a glaring oversight in the insulation (such as open gaps between the wall insulation and the cavity blocks).

10-10-20

…and to answer the question which was asked by you - no - external insulation on a cavity block build will not have a significant gain in thermal performance as the cavity will remain open, leading to 'thermal cycling' in the cavity itself and also due to ambiguous dew-points within the fabric of the wall.

Red Hare

what would be the best way to get the extension to retain heat?

Red Hare

I am not 100 percent about the cavity blocks. I must check with the builder

10-10-20

https://www.boards.ie/discussion/comment/124132974#Comment_124132974

It's a very open-ended question to answer. It all depends on how it was finished behind the insulation and whether there is thermal mass (to retain and release heat over a period) or whether it's just a matter of looking at better methods of heating the space. A first step might be to employ a heat-loss consultant / energy auditor to determine the heat-loss factors at play.

Red Hare

thanks for the reply- i will do as you suggest a look up a consultant. Everyone is saying to me that extensions are always colder than the house , would you know if this is good or bad - i had the heating on last night equallu warm in main house and the extension. - when i got up this morning at 5am the sitting room in original house was 17 degree and the kitcken extension was only 14 degrees , at this stage after 3pm afternnoon the extension has dropped to 13 but the sitting room held at 17 degrees. It is as if the sitting room of original house feels warmer that it did before the extension was built,

standardg60

https://www.boards.ie/discussion/comment/124132974#Comment_124132974

Lined cavity block construction tends to heat and cool sooner as opposed to cavity wall which utilises thermal mass so bear that in mind. Assume you have a suitable heat source installed in the extension?

Plastering the exterior will help as it will (should) seal the block against moisture ingress which is important with cavity block. Not surprising it's losing heat quickly as it is. Have they covered the block against the weather on the outside at all? Not sure I'd have the inside lined, skimmed and painted before the outside was sealed if it is cavity block.

10-10-20

https://www.boards.ie/discussion/comment/124133120#Comment_124133120

Factor into account the way that extensions have three external walls and a roof, where a room in a house only has one or two walls and may have an internal ceiling rather than a roof. That equates to a significant heat-loss in any eventuality.

Red Hare

thanks for the advice - it is a mid terrace corporation house so we are used to being snug, yes hopefully plastering will help 🙄- also as it sheeted down rain as the extension was being built - it still has a lot of drying out to do - a good spring and summer might help

10-10-20

https://www.boards.ie/discussion/comment/124133216#Comment_124133216

Well damp walls will not help when trying to heat it. In that case I'd suggest doing as standardg60 suggested and waiting until it's plastered before you decide on next steps. But do bring the potential mentioned issues up with the builder, all of the same.

autumnalcore

How thick is thick and what type of insulation was used, there a massive difference between the types of insulations, to me thick is 80 to 100mm of PIR while a lot of builders install 50mm slabs which is only 38mm insulation.

Red Hare

I have measured the insulation board through the holes in the wall where therebare electric cables sticking out ( we are waiting for the electician to do 2nd fittings) and it is 70mm , the board in the roof looked twice as thick, i do not know what type but the builder bought the boards in chadwicks and i think he said there is wool in it- when i held it in my hand and it heated up.

I am hoping that when the electrician comes he can improve the heat loss?

Would this type of thermal plaster help? https://www.conservationtech.ie/product-page/ecocork-insulating-plaster

Also the radiators in the extension have thermostats ( gas mains boiler)- i am hoping that if I apply for seai thermostat grant for the radiators in the original house - that this would help to control heat in the new space. I don't turn gas on during day because the original house stays warm, and as i have no thermostats in the original house then i cant get the benefit of having the thermostats in the new space.

10-10-20

Is there an air-draw from the holes around the cables or sockets (if you take one out)?

Red Hare

The cables are really thick, jut out of the holes and can't be removed. There is a hole the size and shape of a door letter slot where the fuse board is supposed to go. They wall area around the all holes are colder than the rest of walls and several of the holes have a breeze . One hole the size of a 2 euro coin is very breezy and seems to go all the way through - I think that is where the switch for the outside light is to go.The wall there is particularly cold and the hole temperature is 6 degrees in comparison to the room temperature which is 16 degrees ( I had the heat on for a while) . I can't see anything that looks like a socket - but where the sockets are supposed to go there are just small holes - I am afraid to go poking at them - but they are much smaller than the holes with the cables protruding. I hope this explains the heat loss, and that the electrician can fill these gaps.

Red Hare

Further to plugging the holes, and in circumstances where the weather temeperature has improved , the extension this morning is 17 degrees and the original house is 18 degrees. the heating was turned off 10 hours ago - big improvement !

Grassy Knoll

https://www.boards.ie/discussion/comment/124132997#Comment_124132997

did you ever get this clarified? Thanks

Red Hare

I am going to wait to next week to go over to him over to him ( He has just started a new job this week around the corner where he is under time pressure cause the owners have to move out - I want to give him a chance to let settle in there first. I will post here again, thanks