We have a small porch extension on our solid wall stone cottage, the porch is cavity wall construction, breeze-block internal wall and brick external. The internal walls on the porch get very cold, as does its (single-storey) ceiling. It's a real heat-loss zone, and we intend to fix it asap.

I strongly suspect that the muppet who built it around 12 years didn't insulate the walls or the roof-space. So, I'm going to pull down the ceiling, stuff the void with insulation and then replace the standard plasterboard with 45mm thermal board.

On to the walls...money and access are a real issue, so the options of cavity insulating or external insulating the walls are not an option for us. So, we're going to dry-line it with thermal board. Here's the question...would the advice be to:

a) glue the thermal board directly to the internal wall (once the nasty woodchip has been striped off)?

b) attach battens to the wall and then thermal board over the battens, leaving an air gap?

c) same as b), but stuff insulation between the battens before putting up the thermal board?

Re. the battening option...would it help/not help to attach horizontal battens as well, in effect to create "zoned areas" of battens beneath the thermal board, to minimise/stop air circulation underneath the board?

Everywhere I look, I see different opinions. Guess here will be no different...

I'm guessing one advantage of options b) and c) are that I could batten straight over the woodchip?

Thanks.

Is there any heating in this porch or is it just a draught lobby?

Why not keep it as a porch unheated and then you could save the cost of insulating it. And the time too.

It is not a porch then!

What will you be using the "porch" space for ?. If just for boot/coat storage do you have the option of adding a door to the kitchen, this would save the cost of insulation etc.

Option A without a doubt. Dead easy on a flat wall. I would use PUR laminated plasterboards stuck with very wet adhesive and also mechanical fixings to be sure it stays there. Lots more here http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1070&page=2#Item_2

I would use battens - including horizontals at floor and ceiling level and all around any door and window openings - for the reasons you have stated yourself - see page 31 of this document

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,16557,en.pdf

I would then fix kingspan boards , foil faced both sides and foil tape the joints - to maximise air and vapor resistance

Then platerboard and skim over

I would not leave the woodchip in place . It would support the development of mould growth behind the insulation

Agree option A. Type 'CE184' into google, and take the EST publication it offers you. Very good.

Sinnerboy, have you seen the PUR laminated plasterboards? Why would you fix insulation and then plasterboard when you can fix the two in one go?

I agree about the difficulty of using ribbons on uneven walls, but on flat walls [like this case] its just like sticking on a giant wall tile - easy peasy. Just use the adhesive nice and wet. Also the mechanical fixings used to complement the adhesive are failsafe really.

I've been thinking about this.. I can see some of the advantages, but also some downfalls as I see them:

1. If you put CH pipes in the air space on the cold side of the insulation you are introducing unnecessary heat losses to the outside
2. This also means that there is a natutal path for air infiltration/lossses wherever the pipe runs go
3. It is very difficult to seal all air paths around electric cables/sockets.

I am also against any cavity which can exacerabate the risk of deteriation of the existing fabric, which can be very unpredictable in old buildings. If there is no ventilation [which you're obviously trying to avoid] then there is the risk of temperature and humidity levels which can promote rotting of unknown existing timbers.

The latter though is a generic problem with all internal insulation and is why I only use it in some situations.

It still does not sound to me at all like a good idea to have a ventilated cavity behind insulated plasterboard.

You will then effectively be living in tent made of insulated plasterboard and wasting all the insulating and thermal meas features of the walls of the house.

I am not suggesting ventilating the battens cavity . I am suggesting ventilating the cavity between the breeze block and the brick

Just kidding - fair enough Mike . I think there are benefits but I accept you don't

I have to say - another " agree to disagree " point perhaps - that (assuming no insulation in the cavity ) that venting a masonry cavity would help joist ends built into the inner leaf . Damp clammy environment in that cavity - joist ends would be vulnerable

Posted By: sinnerboy
I have to say - another " agree to disagree " point perhaps - that (assuming no insulation in the cavity ) that venting a masonry cavity would help joist ends built into the inner leaf . Damp clammy environment in that cavity - joist ends would be vulnerable

Yes, it would no doubt help, but that ventilation would not get to the airspace in a battened cavity. I wonder what a condensation risk analysis would tell us?

Did you use the Buildesk software Mike ? Vapour barriers are a bit of a "silver bullet" - i.e. include one and we are told condensation risk is eliminated. I am skeptical about the real life situation . And so - to try to achieve the best vapour barrier i can think of - battens , PUR boards over - joints foil taped . Yes the plasterboard fixings do compromise the barrier i accept that . But the butt joint between composite boards - i believe- poses a greater risk of vapour migration .

And Tony - I hate cavity wall construction . ( After 25 years years of dealing with them - as an architectural technician ) I don't believe it ever delivers in practice what we calculate it to . With partial fill boards you can expect to find horizontals joints fouled by mortar droppings, vertical joints not cut true or parallel, boards which lean forward away from the inner leaf . All of which leads to thermal looping and consequent degrading of the thermal performance .