To tank or not to tank, that is (sort of) the question....

[lowki]

Hi all,

I have the following project lined-up and would greatly appreciate some advise/input on a couple of points.. :

The room in question
We have a porch/utility/outhouse attached to the front of the house (see pic below).. It has a built-up felt flat roof, single-skin +/-95mm thick (half brick?) walls on three sides, a concrete/screed floor, and is not heated by the main house CH system.
The three single-skin walls do have an integral DPC (visible in the first photo, level with the base of the door cill).
There are two doors.. one is the front door (seen in the pic) and the other is inside and leads from the 'outhouse' inside to the dowstairs hallway. The house is an early 70s terrace.



Current make-up inside
The roof is a 'cold roof' so has a plasterboard ceiling with loft-style insulation half-filling the (ventilated) cavity between the rafters/joists beneath the boards.
The walls are currently lined with dot-n-dab fixed plasterboard (probably done in the late 80s-early 90s).
The floor currently has a thin underlay beneath some cheap laminate flooring..

Present issues
The flat roof is old and has leaked in the last couple of months.. (I've made a temp/patch repair in the meantime). The old-style coated chipboard boards are showing signs of failure..
The plasterboard ceiling is failing in places and the boards starting to droop..
The plasterboard dabs have mostly detached from the walls.

My intentions
I intend to have a roofer strip, re-board and re-felt the roof (still a 'cold roof' construction).
I intend to replace the roof insulation with 75mm Celotex and re-line the ceiling.
I intend to re-line the walls and thought best to add some form of insulation whilst the opportunity presents itself as I understand (although unheated) something is better than nothing..
I am not planning to insulate the floor, the laminate flooring will likely be replaced with carpet.
The room will remain unheated and detached from the main house CH.. at best, it will have a portable/panel heater to take the chill-off if/as/when required.

I am yet to finalise which method to use when insulating the walls.. whether to have conventional PIR insulation boards between studwork then p/b over and skim to finish; or, to batten the walls and fix thermal laminate p/b and skim to finish..

My question is..
Having removed some of the existing plasterboard from the walls, I can see the walls have been painted with what looks like a liquid DPM in the past (see pics).

Regarding this coating, should I..
a) try to remove it in order to leave a 'permeable' wall?;
b) leave it as it is?;
c) add something similar (like BlackJack liquid DPM) over the top..?

The application guidance from Celotex for the internal insulation of solid walls seems to suggest that the wall should be permeable and stripped/clear of coatings. I am reading elsewhere that either a liquid DPM or a plastic sheet/tanking should be in-place..?

Is the guidance from Celotex based on the assumption that the solid wall is a double-brick-width thickness and not prone to ingress from the outside..?

Many thanks in advance of any thoughts/input you can offer..
Apologies to ramble on.. I tried to keep it short..!

 

[lowki]

I missed a photo and part of the coating description when adding the pics... Updated/corrected segment as follows :

My question is..
Having removed some of the existing plasterboard from the walls, I can see the walls have been coated with a sand/cement render and painted with what looks like a liquid DPM in the past (see pics)

 

[vinn]

Strip all the walls back to bare brick.
Strip the ceiling back to joists. Drop any roof insulation.
Remove all floor coverings back to concrete.
Then post pics on here.

Attempt to find a membrane (DPM) from below the slab - it will show, if present, at the edges of the floor.
Do you have any indications of damp in the porch walls/floor or the main house area adjacent to the porch?

 

[^woody^]

I suggest you read some of the countless garage conversion threads, as what you are doing is that work content.

 

[lowki]

Hi Vinn,

Thanks for the response..


Ceiling
There's a part of the ceiling I had to cut open when I had the leak.. pics of the rafters/joists as follows.. I wanted to leave the rest of the ceiling/insulation in-place until the roof is sorted. You'll also see an internal downpipe from the flat roof above.. (There was low natural light so the flash made the walls look lighter than they are in some of the pics..)


Walls
I have stripped all the p/b off. The render and 'liquid dpm' coating is applied across all three walls. The coating is about 1-2mm thick.

The walls feel and appear sound/dry. Never any cases of damp in the past.
The wall that abuts the house's outer leaf (of the front cavity wall) has a strip of DPC from top to bottom as seen in these pics :

Never any previous issues with damp in this area of the main house.

/// continued in next post/msg.....

 

[lowki]

Floor
The laminate flooring and poly-foam/styrene underlay went down about 7 years ago.. there was previously a carpet/rug in there (no underlay).
I've lifted parts of the flooring and underlay.
Areas of the floor are damp. There was an old blockboard timber panel. It was sodden/rotten damp and crumbled to bits when trying to lift it. This panel was dry at the time the laminate and underlay went down. Beneath the board panel was a layer of loose sharp sand (about 25mm deep). Beneath the sand, a very old man-hole cover.............

I can't see any evidence of a floor membrane around the perimeter of the room, around where the downpipe enters the floor in the corner, nor around where the manhole cover is.

Additional background
When the houses were built, these 'outhouses' were originally open. I mean open in that they had a roof, but there was no door where the current front door is sited. The original 'front door' would have been where the 'inside door' leading in to the hallway now is.

The original floor level would have been the level the manhole cover is (about 45mm lower than present).
The majority of the outhouses were 'closed-up' in the late seventies and early eighties. (The plasterboard I removed was Knauf wallboard dated 1985). There are one or two houses that didn't have it converted and still have it open.

I have spoken with my neighbour who has lived at his house since the late 70s.. He did some of the work when he converted his... he said that he laid some bricks (loose/dry) on the manhole cover (to leave it 'accessible') and laid a concrete/screed elsewhere to bring the floor level up. He said that a membrane/coating was not laid beneath. His is unheated by the CH, not insulated (just painted brick walls), and has a carpet/fabric underlay floor.. In the 35+ years he's lived there, the manhole cover has never been lifted.

My course of action...
What are my options for the floor... Painting a liquid DPM or suchlike over it and up the wall?, or laying a membrane sheeting with laps up the side of the walls..?

 

[lowki]

I suggest you read some of the countless garage conversion threads, as what you are doing is that work content.

Hi Woody,
Thanks for the response.
I've read countless threads re garage conversions on the various forums together with the application guidelines from the likes of kingspan and celotex... a lot of info taken onboard.
I haven't come across any examples with an internal render such as mine.. With what appears to be the liquid dpm on top, how does that affect things when using for example a celotex system where they specify the wall should be permeable..? Aside from me contacting Celotex tech and asking them, any ideas on this..? The majority (if not all) the advice I've so far read on the forums etc suggests to seal the internal side of the wall, not leave it 'clean'.
Thanks

 

[^woody^]

With that solid wall, then it's assumed that some moisture will go across from the outside.

So to prevent this causing a problem on the inside you need a barrier. This can be in one of several forms, and can either go on the internal or external face of the external wall.

Similarly, moisture will be assumed to enter the wall from the internal room - moisture from use of the building. So to deal with this, you either stop it entering in the first place, with a barrier on the room side (aka vapour check or vapour membrane, or you ventilate a cavity within the wall, or you insulate the wall to remove cold spots which could be condensation risk areas. Or a combination of these.

The easiest way forward is to place a big sheet of polythene on the internal face of the wall, then insulation within and across a timber frame with no air gaps, then a vapour membrane, then plasterboard. The details of this you can find in some of the other posts.

But the key thing, is the principle that you stop moisture coming through from the outside, you stop vapour entering from the room side, and you eliminate all voids within the wall to remove the chance of moisture condensing within the wall structure.

The existing render is irrelevant. If the existing waterproof coating is sound, then you could potentially recoat with similar instead of polythene. Either way, pay attention to the detail at floor/wall junction and wall/wall and wall/ceiling junctions.

It's a similar situation for the roof.

A cold unheated room is a condensation risk, and more so with just occasional heating. So ensure to insulate all cold surfaces as much as possible, and ventilate the room.

 

[cjard]

The easiest way forward is to place a big sheet of polythene on the internal face of the wall, then insulation within and across a timber frame with no air gaps, then a vapour membrane,

This is the only part of an otherwise comprehensive and helpful post that I disagree with. You're effectively advocating sandwiching timber between two vapour impermeable membranes. Convention is to have a breathable membrane on the external facing face of a timber structure, and a vapour barrier on the internal. There should also be a ventilated cavity outside the breather membrane. This way any moisture and condensation in the cavity can escape, any moisture in the timber can migrate through the breather membrane and any moisture in the room is prevented from entering the timber wall

 

[^woody^]

The easiest way forward is to place a big sheet of polythene on the internal face of the wall, then insulation within and across a timber frame with no air gaps, then a vapour membrane,

This is the only part of an otherwise comprehensive and helpful post that I disagree with. You're effectively advocating sandwiching timber between two vapour impermeable membranes. Convention is to have a breathable membrane on the external facing face of a timber structure, and a vapour barrier on the internal. There should also be a ventilated cavity outside the breather membrane. This way any moisture and condensation in the cavity can escape, any moisture in the timber can migrate through the breather membrane and any moisture in the room is prevented from entering the timber wall

The convention you describe is for a traditional timber frame. The OP's situation and my description is not. So th is description is the convention for this situation.

The prevention of vapour getting into the structure removes the need to have a membrane to allow it to breathe.

 

[cjard]

If it were possible to prevent vapour entering a structure, breather membranes would have no commercial purpose..

 

[^woody^]

If it were possible to prevent vapour entering a structure, breather membranes would have no commercial purpose..

Like DPM's you mean? Like flat roofing covering? Like the errrr, breather membranes? Yes, complete waste of time.

 

[cjard]

You can't just advise someone to build a timber frame, but skip out small details like providing an escape route for moisture, and pass it off as OK "because it's not constructed like a conventional timber frame because woody said so"

If it was OK to build wooden framed buildings as you propose here, timber frame companies would be all over it. What a simple idea; just put a DPM on both sides of the wall, keep the moisture out and all will be perfect.. Even better, no pesky cavity to provide a home for rodents, a fire spread, inches of lost room space thanks to those nasty planners and their dimension insistence..
It's so simple, I can't believe that no one else has ever tried it. I can't believe that no one builds buildings this way; just think of all that "convention" that flies in the face of your suggestions - it has been established for no good reason..

If this alternative approach to dealing with water vapour were confined to your own head it wouldn't be so bad, but that you're actively promoting it as a sensible way to do things for others to follow is seriously antisocial. One can only hope that the OP takes what you say with the necessary pinch of salt and does some additional reading around the matter.

 

[^woody^]

One can only hope that the OP takes what you say with the necessary pinch of salt and does some additional reading around the matter.

One could also hope that the OP listens to someone who knows what he is talking about, instead of someone who doesn't.

That same someone who does not know the difference between constructing a timber frame (ie a frame made of timber) and timber frame construction ( ie a specific construction method)

As mentioned by the OP in his earlier posts, it's people like you who are actually causing the confusion. If you don't know what you are talking about, then you really should STFU.

 

[cjard]

the difference between constructing a timber frame ... and timber frame construction

Well, at least the longer you talk, the more likely you are to say something that even an inexperienced DIYer will be able to see is just plain nonsensical..

For the benefit of the OP (and the UK construction industry as a whole) please tell us more about this plastic sheet method that allows us to do away with twin skin wall construction methods and also preserve the integrity of the organic inner components