Outbuildings - Damp Proofing & Insulating

[yngndrw]

Hello,

I know the subject has been covered many times but a lot of the time they are specific questions or related to new wooden workshop builds.

I have four buildings to do up: (In order, I'll start with the smallest and work up to the largest and most important)
- An outdoor office building
- A garage (Office / hobby room conversion for my parents to use)
- My workshop
- My house

The construction is slightly different between each of them but they all have a lack of damp proofing in common.

So the first is an outdoor office building - It has solid brick walls (About 240mm thick), appears to have a lime-based mortar with no damp proof course and the concrete slab appears to have a membrane under it as you can see some sheeting at the edges. My plan is to pull all of the plasterboard off the wall, put in a course of Dryrods and paint on some liquid-applied DPM making sure that I go over where the Dryrods are and that I cover a good 150mm of the floor. I was then thinking of sticking 25-50mm of PIR to the wall with adhesive (It's only a small building) and then applying plasterboard with dot and dab. For sockets I'd probably just stick a block of wood in the same thickness as the PIR and foil tape over that, I don't think it has to be perfect for that building but I'd love to have some advice for that. The only complication is that the building will also house a large solar inverter, three phase distribution board and battery which needs some proper support on a fire-resistant wall, so my plan was to use fire-resistant plywood on that wall in place of the plasterboard and to screw through the PIR into the wall with long screws. I'd then use unistrut over the top of that for the heavier items and either fix that into the plywood or use some rigfix bolts into the wall behind depending on how good of a fixing I get for the plywood. Because I plan to paint the liquid DPM onto a section of the floor, I think I should be safe to plaster right down the to floor.

The second task is the garage which is slightly different. This has solid blockwork walls (Single thickness with pillars) with a damp proof course. (At least on three sides, I may need to use Dryrods both horizontally and vertically on one wall which joins onto another building) I don't know if the concrete has membrane under it and I believe it's a cement based mortar. Again my plan was to paint on the liquid-applied DPM and to do very much the same in regards to insulation. Sockets will be surface mount in here so no issues there.

The third task is my workshop and I want to do a slightly better job in here as I'll be air conditioning it to control humidity and I'll also want to fix stuff to the walls. The construction of this is also a solid wall made from single thickness blockwork like the garage with what appears to be a cement based mortar, but sadly this building does not appear to have any damp proof course. Again my plan was to use Dryrods and liquid-applied DPM but this time I need a stronger wall. I was thinking of some thicker PIR (50-100mm) within a studwork frame, tape over the joins and then face with plywood sheets. The roof isn't insulated so that also needs doing, along with ventilation. (I'll add that when the rotten wooden soffits are replaced with UPVC) Sockets will be surface mount in here so no issues there.

Finally there's the house, very much like the first outbuilding in that it's a solid brick wall with lime-based mortar, no damp proof course (Electro-osmosis damp proofing was added about 40 years ago, which I believe was a scam?) and the concrete floor doesn't have any membrane. We want to add underfloor heating and don't have much (any) height available so I think we'll just have to do what we can. Again my plan is generally the same as with the workshop - Dryrods, liquid-applied DPM, studwork with 100mm PIR and then plasterboard. We don't really have much weight to hang from the walls (Kitchen will be moved to an extension) so I'm not even sure if we need the studwork. Also not sure how to do sockets within the house. This is a long way away though, the main thing is to practice and learn the skills on the buildings I don't care as much about, then apply that knowledge to the workshop and house.

Any advice would be greatly appreciated.

 

[HOJ]

I'll jump in as no else has:

they all have a lack of damp proofing in common

Is damp a problem?

You've got a lot going on here, to the extent I would suggest & with respect, you seek advice from more local and direct sources, forums are good, and this is multi skilled one, but in this case you're into an area for consultancy.

 

[yngndrw]

Thank you for your response.

The main reason for tackling the damp proofing is because I want to insulate the buildings. Damp isn't a huge problem right now but that's mostly because they are far from well sealed. There are areas within the main house which are noticeably colder, have some damage from damp and we do have some efflorescence in parts, in the other buildings it's just going to be a future problem as we start to convert them.

We will certainly get proper consultation for the main house, but I was hoping to have a good idea about what was actually required so I don't get scammed. I was hoping that the other projects would be small enough that I could get some hands on experience and save some cash in the process. I think I'm fine with the actual damp proofing for that part - It's just then what to do with the insulation when you start talking about vapour barriers and such. (And to generally get a second set of eyes on my plans to make sure I'm not missing something serious.)

 

[joncooper27]

Did you get this sorted? If so what route did you take?

 

[yngndrw]

The workshop project ended up being pushed to the back of the queue. We decided to go for planning permission for a new kitchen extension as well as change of use on the garage to an office. The planning permission has just been granted so we're currently going through the building control process with our architects which will impact the route taken for the workshop.

While the extension construction is not relevant (Cavity wall with thermalite inside and out, warm roof), the garage also needs to pass building control and these changes very much apply to the workshop. The garage is currently a single-skin blockwork wall with a damp proof course and an uninsulated floor. For building control reasons, we need to pull out the garage floor and put in an insulated floor. (That's an annoying and unexpected change) but they then call for a small cavity with a wooden inner stud-wall so that is probably the route that we'll take for the workshop. They are calling for cavity trays, closers and fire barriers. Here's the current wording for the garage wall construction:

DAMP-PROOF COURSES AND CAVITY TRAYS
Existing block walls to have part of motor grinded out 150mm above external finished ground level, membrane should be folded over and to be IKO 'Hyload' or similar approved PVC type. DPC to overlap with DPM. Minimum 75mm vertical flashings to be provided at all abutments. Cavity Trays with stop ends to be provided above all external openings. Cavity trays should be provided at all interruptions to the cavity, eg window and door openings, air bricks, base of walls and piers, base of wall built off beam or slab etc. Cavity trays to provide an impervious barrier and ensure that water drains outwards, project at least 25mm beyond the outer face of the cavity closure or, where a combined cavity tray and lintel is acceptable, give complete protection to the top of the reveal and vertical dpc where provided, provide drip protection to door and window heads ,have an overall minimum upstand from the inside face of the outer leaf to the outside of the inner leaf of 140mm and be shaped to provide at least a 100mm vertical protection above a point where mortar droppings could collect. Where fairfaced masonry is supported by lintels weep holes should be provided at 450mm (maximum) centres with at least two weep holes per opening, cavity trays or combined lintels should have stop ends. Where full fill insulation is placed in the cavity, a cavity tray should be used above the highest insulation level, unless the insulation is taken to the top of the wall. (Manufacturers' recommendations should be followed.) Cavity trays should be provided at abutments of roofs and cavity walls where the roof is over an enclosed area. Where the roof abuts at an angle with the wall, preformed stepped cavity trays should be provided. For Parapet Walls, Dpcs below the coping should be supported over the cavity to prevent sagging.

EXTERNAL WALLS
To be existing Thermalite Hi Strength 7 block outer leaf 7.3N/m with 40mm Celotex TB4000 with 25mm low emission cavity, on 25 x 47 timber counter battens @ 600 centres finished with 12.5mm Gyproc wall board and plaster skim. All to achieve min 0.30 W/m2K. Actual achieved 0.28W/mK2. All cavities to be maintained and closed with suitable cavity fire barriers at top below wall plate. Cavities around openings to be closed with a suitable thermal cavity closer. Tie new to existing walls with vertical strip ties. All installed to manufacturers recommendations. All to be in accordance with manufacturers recommendations.

 

[Fitzroy]

I’ll get my view out straight away, chemical damp proofing is an industry built on shoddy data.

I lived in a solid wall Victorian house with a slate damp course. We had a patch of damp appear by a door and ‘the industry’ who admittedly I invited in determined I needed to rip off plaster drill holes and inject stuff on every wall at a cost of thousands.

I’m a very evidence based person and extraordinary claims require extraordinary evidence. So I went looking for the evidence for the chemical damp industry and it comes up very short! The two key problems are firstly the instrumentation used to measure damp levels is not designed for the task, and secondly the cause of any damp measurement is not investigated if it is within 50cm of the floor.

At this point I’d opened Pandora’s box, and firmly closed the door on MrDampman. My key takes from my following review of information, studies and practices was the main causes of damp are:
- interstitial condensation due to moist air dropping below its dewpoint, either due to cold surfaces or poor ventilation.
- penetrating damp due to failed rainwater goods, or bounce back from hard surfaces.
- penetrating damp due to elevated external ground levels
- leaks from failed internal water fittings

Rising damp is possible, some will argue it is not, but it is a problem only in a minority of cases.

In most case the fix is solve any leaks, fix rain water goods, lower ground levels and removed plants and structures externally, ensure installed ventilation (air bricks etc) is functional. Then time is what the building needs to dry out, 6-12 months.

Chemical damp proofing and internal tanking will just trap any moisture in the wall, masking the issue and potentially leading to a bigger future issue or building damage.

Sorry essay over but once bitten twice shy.

For your outhouse do you have a damp problem now? You can measure it yourself by drilling a hole and collecting the brick dust, buy a sensitive scale, weigh the dust, dry it in the oven and weigh it again. You can then calculate its moisture content. If you do it needs solving first. If not then you just need to make sure you don’t create one with your modifications.

Your planned modifications will create a warm side with some moisture due to human respiration, and a cold side behind the insulation. If the warm moist air gets to the cold side damp is likely. Most designs aim to manage this through to elements. Firstly a barrier membrane on the warm side to prevent the moisture getting to the cold side, and secondly ventilation on the cold side so that any moisture can dry out over time. You can easily incorporate a moisture barrier in your design, ventilation is perhaps possible with a 50mm cavity behind the insulation and air bricks to ventilate, but complicated and takes internal space. My workshop has a similar warm/cold damp possibility, and I couldn’t ventilate the cold side, so I did the best job possible on the vapour barrier and reasoned the risk is low as the building is only lightly used for a few hours a day.

Hope some of that is somewhat helpful.

Fitz

 

[yngndrw]

Hello Fitz, thank you for your response - It is very helpful to hear of your experiences and the lessons you've learnt.

You've absolutely right that there are many forms of damp and that there are different approaches to each of them. We seem to have just about all of them around the property.

One thing I've only recently realised is just how far water can track - We had a leak in the roof of the house which tracked across half of the building and appeared on a wall quite some distance away - It must have been tracking a good 5 meters at roof level before it went down the wall.

As mentioned the order in which we are tackling the work has completely changed with the extension, but our general remedial plan before we even start doing any interior work on each building is:
- Make sure the guttering and roofing is sound, including the pointing around the edge of the tiles (Some of our roofs are slate and the pointing around the perimeter is an issue)
- Make sure the walls themselves are in good condition (The house and outbuilding both have really poor pointing, in some places I think water can track straight through)
- Sort out the surrounding ground level, remove close trees and add french drains around the main house.

That will probably do most of the work as you mention, certainly a lot of the damp issues for the main house and the outbuilding are related to ground level and poor wall / roof condition.

You're right I haven't properly tested the damp level and will certainly be using the oven trick so thank you for that, but in some places you can tell from the clusters of woodlice, damp looking plasterboard or peeling plaster / paint.

I'm still unsure about the chemical DPC. I think that a lot of the issues people have are down the the number of different causes of damp, although even issues such as the surrounding ground level should in theory be resolved by a chemical DPC as long as it's not too high. I have seen others mention that they have had excellent results with dryrods though so I don't doubt that they will work if the problem is indeed rising damp. I suppose in our case, it's the question of whether or not we should bother after resolving everything else and if we want to risk doing all of the other work only to have to come back and add a DPC later. I'm leaning towards doing it on the workshop and outbuilding anyway as they are smaller buildings and it's easy to do while they are not full. It's easier to add french drains around the house so we might rely on those instead for the house.

My biggest concern is what then comes after - Not creating a new problem when adding insulation.

The new extension handles that by using a full cavity wall and for the roof we've specced a warm roof even on the pitched section - Bypassing the problem of roof ventilation altogether. For the other roofs, ventilation will need to be added as currently they are quite open but that needs to be retained as insulation is added.

For the various outbuildings, I'm still torn on the walls. For the garage we need to go down the cavity as the building regs call for it to be done properly, so after that work we should have a proper feel for how involved it is to add the required ventilation and cavity trays. I think having the experience from that project will then help with the outbuilding and workshop - But the nagging question for me is whether or not it's necessary to go to that effort for a building with low occupancy, as you've mentioned. If the work is easier than it initially seems then we might as well go down that route so being forced to learn how to do it for the garage isn't a bad thing.

Either way it's not simple - The are many different factors at play so the best we can do is learn what we can with the first projects and then apply that going forward.

It's annoying that there are so many cowboys in the industry - It forces us all to go down the rabbit hole but then again learning new skills is never a bad thing.

 

[MorrisWoodman12]

Your planned modifications will create a warm side with some moisture due to human respiration, and a cold side behind the insulation. If the warm moist air gets to the cold side damp is likely. Most designs aim to manage this through to elements. Firstly a barrier membrane on the warm side to prevent the moisture getting to the cold side, and secondly ventilation on the cold side so that any moisture can dry out over time. You can easily incorporate a moisture barrier in your design, ventilation is perhaps possible with a 50mm cavity behind the insulation and air bricks to ventilate, but complicated and takes internal space. My workshop has a similar warm/cold damp possibility, and I couldn’t ventilate the cold side, so I did the best job possible on the vapour barrier and reasoned the risk is low as the building is only lightly used for a few hours a day.

Great explanation @Fitzroy Think I now understand why it's done the way it is. Wish I'd seen this a few weeks ago.
Martin