Building a porch but the walls can only be 18cm thick! HELP!

[andymet87]

Hi all...

I am just i the process of designing (& then building) a small back porch for our house. The problem is that due to existing structures and other constraints, mainly windows, the walls can only be around 18cm thick from the outer face to the finished plaster inside! The porch will house our washing machine and tumble dryer so it needs to be relatively well isulated so it stays above freezing. It will be less than 3m squared.

I was thinking of using the followig wall.

OUTER Surface - - - - - - > INNER Surface

Render (20mm) - Dense Conrete Blocks (100mm) - batton (15mm) - insulated plasterboard (25mm) - Plaster skim (5mm)


I know this is far from ideal however I cant see any other routes and I would rather not have a timber frame structure.... unless anyone thinks that would be cheaper & stronger?

My main concerns are:

a) the strength of the wall being based on a single layer of 100mm blocks

b) how warm the room should stay.... the roof willl let very little heat out... I am planning on using a 60W or 120W tube heater available from toolstation.

any pointers would be much appreciated, Andy

 

[wavetrain]

I would do the following:

OUTER Surface - - - - - - > INNER Surface

Facing bricks (110mm) - vertical studs (40mm) at 400mm centres screwed to wall with standard DPC material inbetween stud and brick - insulated plasterboard (25mm) - Plaster skim (5mm).

You will place 40mm thick insulation inbetween the studs. You will then achieve a wall U value of (I would guess without doing the calcualtion) of around 0.6.

As it is a utilty room, you will not need to heat it if it receives a little heat from the house wall it is attached to, to keep it above freezing

 

[andymet87]

I would do the following:

OUTER Surface - - - - - - > INNER Surface

Facing bricks (110mm) - vertical studs (40mm) at 400mm centres screwed to wall with standard DPC material inbetween stud and brick - insulated plasterboard (25mm) - Plaster skim (5mm).

You will place 40mm thick insulation inbetween the studs. You will then achieve a wall U value of (I would guess without doing the calcualtion) of around 0.6.

many thanks for the reply. I do like the idea of having a brick exterior, I had previously shyed away from them as I cannot get matching bricks for my house. They were taken out of production about 40 years ago.... I do have a few I have reclaimed though from other walls so can try and do something.

If I were to use something like this... http://www.beersltd.co.uk/view_product.php?code=IF000210... between the studs would that avoid the cost of insulated plasterbaord? As in could I just use normal plasterbaord? Every cm counts... its crazy!

thanks

 

[wavetrain]

As you have an issue with buying new bricks:

OUTER Surface - - - - - - > INNER Surface

Render (12.5mm) - Dense Concrete Blocks (100mm) - vertical studs (50mm) at 400mm centres screwed to wall with standard DPC material inbetween stud and brick - Vapour barrier polythene sheet stapled to studwork - plasterboard (12.5mm) - Plaster skim (5mm).

You will place 50mm thick insulation inbetween the studs. You will then achieve a wall U value of (I would guess without doing the calcualtion) of around 0.8.

As it is a utilty room, you will not need to heat it if it receives a little heat from the house wall it is attached to, to keep it above freezing

NOTE I FORGOT YO MENTION THE VAPOUR BARRIER SHEET IN PREVIOUS POSTS, this is a rather important component to this build.

 

[^woody^]

My main concerns are:

a) the strength of the wall being based on a single layer of 100mm blocks

b) how warm the room should stay.... the roof willl let very little heat out... I am planning on using a 60W or 120W tube heater available from toolstation.

There are no problems with strength of a single skin wall

As for warmth, well walls will only keep in heat and will not make the room warmer on their own. Your proposed heaters are not much different in output to an equivalent light bulb, and so this wont be magically keeping the porch warm

Heat will be flying out of any windows or doors in any case

So your main problem is putting enough heat into the room to keep it as warm as you propose, rather than worrying about the walls. Secondly you need to reduce cold surfaces and ventilate properly to prevent condensation and mould issues

I'd use an aerated block rather than a dense concrete, and then line with an insulated plasterboard. This will be the easiest to build and as good as you are going to get

Then reconsider your heating output and type

And ventilate or have a humidistat controlled fan to control humidity

 

[andymet87]

My main concerns are:

a) the strength of the wall being based on a single layer of 100mm blocks

b) how warm the room should stay.... the roof willl let very little heat out... I am planning on using a 60W or 120W tube heater available from toolstation.

There are no problems with strength of a single skin wall

As for warmth, well walls will only keep in heat and will not make the room warmer on their own. Your proposed heaters are not much different in output to an equivalent light bulb, and so this wont be magically keeping the porch warm

Heat will be flying out of any windows or doors in any case

So your main problem is putting enough heat into the room to keep it as warm as you propose, rather than worrying about the walls. Secondly you need to reduce cold surfaces and ventilate properly to prevent condensation and mould issues

I'd use an aerated block rather than a dense concrete, and then line with an insulated plasterboard. This will be the easiest to build and as good as you are going to get

Then reconsider your heating output and type

And ventilate or have a humidistat controlled fan to control humidity

hi, cheers for the reply. the only door will be a new upvc one so shouldn't loose too muchhheat there...
what is the difference between aerated and solid? i was under the impression the aerated blocks aarareararent as strong.

I'm now planning on going with wavetrain and his proposed structure!

i will be creating a vent hole for the tumble dryer exhaust, would that be enough ventilation for the room?

cheers in advance...

 

[wavetrain]

I'm now planning on going with wavetrain and his proposed structure!


cheers in advance...

Not so fast there laddy!

There are even weaknesses with my last proposal

The plasterboard is prone to damage from impacts and these are common in utilty rooms. You could use 12mm or 18mm plywood, but you cannot plaster over that, or can you?

I shy away from airated blocks on a thin wall, since the low weight of the resulting structure makes for a 'lively' building to occupy - you feel the building move when you close doors etc. They don't hold together well with morta going by how easy they can be demolished on TV. Having said that, if you chose a 140 -150mm medium weight airated block (not a turbo or solar as they are difficult to keep the render on) with thin joints and rendered outside and plastered inside, this may be perfectly fine.

An idea that has crossed my mind, and I have not seen this mentioned elslewhere before, would be to cut shallow diagonal grooves in the faces of the airated blocks that will be taking the block laying morta. This would greatly increase the bond strengh of the wall wouldn't it? For your small porch, a trivial task with a small angle grinder.

 

[andymet87]

Not so fast there laddy!

There are even weaknesses with my last proposal

The plasterboard is prone to damage from impacts and these are common in utilty rooms. You could use 12mm or 18mm plywood, but you cannot plaster over that, or can you?

Reading around I think I will stick with the solid block route... it might not be quite as eco efficient but will be much cheaper I think, and stronger...

I am aware that plasterboard can dent, but I think that is a risk I am willing to take. I am hoping the layer of insulation between the timber should provide a solid backing for it. As far as I know you shouldnt plaster onto plywood, it is prone to cracking etc if not done right. I could always tile the bottom metre if I'm worried about that anyway.

Thanks fo the reply ^woody^. I'm now shyig away from the single block and insulated plasterboard route as I like the idea of an internal wooden structure. This will provide more structural strength and provide a good base for the pitched roof.

Just priced it all up, bit of a mission... its come to around £780 all in(Excluding plastering). I think thats about what I was expecting so will be ordering later today once I've established best prices! Probably a combination of Toolstation, Screwfix, and Beers (the local builders yard)!

I had better go and finish the footings...

any other hints/tips/suggestions/pointers please let me know!

Thanks for the help guys

 

[big-all]

i will be creating a vent hole for the tumble dryer exhaust, would that be enough ventilation for the room?

cheers in advance...

no the vents you should use will have fins to self close

keep in mind if you don't heat to a similar level to the adjoining room you will have trouble with condensation unless as said you have good ventilation and you need to keep the door shut unless your passing through it
and calling it a porch will not in my opinion save having to comply with building regs

 

[andymet87]

i will be creating a vent hole for the tumble dryer exhaust, would that be enough ventilation for the room?

cheers in advance...

no the vents you should use will have fins to self close

keep in mind if you don't heat to a similar level to the adjoining room you will have trouble with condensation unless as said you have good ventilation and keep the door shut unless your passing through it

The door to it will be kept shut into the kitchen all the time, unless we are going into/through the room. I was planning on using a tubular heater on a thermostat to keep the room above about 12 degrees. Someone said they werent much good earlier but my dad has one heating a garden building and that keeps it very warm, and that room is much bigger.... We keep the kitchen at around 17 degrees. The kitchen is the other side of a new UPVC back door with argon filled glass, and a 2 skin wall with cavity wall insulation so hopefully it shouldnt cause too many problems.

I will look into the ventilation situation. What is the best way to create adequet ventilation without letting too much heat out? It sounds like your describing a bathrom extractor fan? can you please post a link to the kind of thing your talking about.

Many thanks for the heads up re condensation!



oh and...

...calling it a porch will not in my opinion save having to comply with building regs

...but it is a porch....

Its less than 3m squared, so counts as a porch under building regs. And also easilly comes within permitted development rights as per the 2008 GPDO ammendment!

I was actually worried about whether putting a washing machine in the room would compromise its 'porchness' however a phone call to my local building control department has confirmed that I can build whatever so long as its under the 3m squared as set by the regs and meets the few relevant regs such as windows & doors etc.

 

[andymet87]

is this the kind of fan you were thinking of?

http://www.toolstation.com/shop/Electrical/Fans/100mm+Tile+Fan+Humidistat/d190/sd280/p82261

cheers

 

[big-all]

great you've done your homework

i would suggest a high level and a low level vent away from the heat sources but have them with closures
another choice could be open able windows with restricters or the ability to lock shut but with a few mm gap for ventilation

 

[andymet87]

i would suggest a high level and a low level vent away from the heat sources but have them with closures
another choice could be open able windows with restricters or the ability to lock shut but with a few mm gap for ventilation

cheers for getting back to me.

I will look into natural ventilation, as in unpowered. Would rather that I didnt have to put in a powered fan.