Converting oak framed garage into dwelling.

[ElliotP]

Hi All,

 

I've been reading the forum for a few hours, I know this kind of topic has been done to death so I'll just be brief, and include some photos to save reading too much.

 

So in brief, oak framed garage (9m x 6m), roof joists are 125mm, wall studwork is 95mm.  It's a self build, I did plan on PIR between and then 50mm PIR over the top, but as we all know, it's an awful job, especially when coming from underneath.  However, I am keen on 'super insulation' to a passivhaus or near passivehaus standard.  My questions/scenarios are this:

 

1) Extend the studwork out (twin stud), 300mm walls, 400mm ceiling and get cellulose blown in (this is mostly what I see for Passivhaus standards).  Not a fan of this for probable cost, and loss of space with the walls.

 

2) Take the easier(?)/cheaper route of rafterroll between, and then I was thinking perhaps going a bit ott and 100mm PIR over the top of the timber.  Batten screwed through to the joist behind the PIR to create a service void, then plasterboard on top of that. 

 

3) As 2, but maybe put 80mm PIR between the studs, as well as 100mm over the top.

 

In both cases will tape and membrane the hell out of it to make it airtight, and fit a mvhr.  Outside will be OSB, breather membrane, and cladding put back on.  Floor will be 200mm of EPS70 (or is 150 needed?)

 

My main questions are:

 

1) In the roof, do I need to leave a 25mm (or 50mm?) outside air gap between the insulation and the memberane that's under the tiles?  

 

2) What products are best suited for scenario 2?

 

3) Has anyone tracked PIR and other costs of late, has it increased by a lot? And is it worth waiting 6-12 months if I can to see if material costs return to sensible levels?  My last self build was 5 years ago, so I'm a bit out of touch on that side of it.

 

4) Any advice welcomed, perhaps there is a better way.

 

 

 

Thanks,

Elliot

 

 

 

 

 

 

Edited September 20, 2021 by ElliotP

[PeterW]

34 minutes ago, ElliotP said:

My main questions are:

 

1) In the roof, do I need to leave a 25mm (or 50mm?) outside air gap between the insulation and the memberane that's under the tiles?  

Depends on the membrane used. If it is a full breather membrane then none required. If it is not a breather membrane then you need 25mm clear plus vents top and bottom. 
 

36 minutes ago, ElliotP said:

2) Take the easier(?)/cheaper route of rafterroll between, and then I was thinking perhaps going a bit ott and 100mm PIR over the top of the timber.  Batten screwed through to the joist behind the PIR to create a service void, then plasterboard on top of that. 

Fairly standard detail. I would use less PIR and more frametherm or similar. 
 

38 minutes ago, ElliotP said:

However, I am keen on 'super insulation' to a passivhaus or near passivehaus standard.  My questions/scenarios are this:

Air infiltration will be your issue, not insulation. You need a very good airtightness barrier here, and oak frames aren’t good at that as they move. 

[ElliotP]

Hi Peter,

 

Thanks for the reply.  Unfortunately I don't know if it's a full breather membrane or not; the garage was already here when we moved in.  I've just taken a close up photo, this could be a silly question, but it is identifiable?  It looks porous to me.

 

 

 

Also, I just wanted to check I have my 'layers' correct on the wall and ceiling, before I start ordering materials:

 

Wall (from inside out):

 

1) Plasterboard & Skim

2) Service void (25mm timber screwed through to studwork)
3) Airtight membrane (DPM?)
4) 100mm PIR

5) Studwork with Frametherm 32 @ 90mm

6) Vapour Permeable Sheathing Board (Panelvent or something else?)

7) Permeable membrane

8.) Cladding

 

Ceiling (from inside out, no service void needed as no lights in ceiling)

1) Plasterboard & Skim

2) Airtight membrane (DPM)

3) 100mm PIR

4) Joists with with Frametherm 32 @ 90mm (or more if I don't need the air gap)

5) 25mm gap (possibly depending on membrane type)

6) Existing timber and tiles

 

My plan is to airtight the whole interior by having the PIR meet from the ceiling, walls, and upstands from the floor, aluminium taped together.  Then over the top of that, envelope the whole structure in airtight membrane (dpm or similar), from the floor, up the walls onto the ceiling.  Then get the screed laid on top of that.

 

 

Thanks,

 

Elliot

[PeterW]

You would need to find the makers info on it - usually on the outside ..!

 

What is driving you to go Passiv on such a small building …? It doesn’t seem to be an ideal starting point as they are inherently unstable and move when the weather changes with humidity. 

[ElliotP]

Well, with a bit of luck, I managed to peel it back at one of the joins and got a 5 digit number of what I thought was a product code, but it turned out to be a phone number area code.  So a bit of Googling, and i've found a picture which matches the font, colour and phone number:

 

 

It's VapR-free which fortunately looks like a breathable membrane (https://vaprfree.com/vaprfree/)

 

In answer to the Passiv question - a couple of reasons; because I strive efficiency and I'm a bit of a green head, secondly, because eventually I plan to end up building indoor pool in the same style as the garage, and maybe a few years after that renovating the main house - all of which I'd like to achieve the best possible efficiency. 

 

Now this might not be certified Passiv - I don't think I'll achieve that on my own, but I'd like to try to get as close as possible.  Either way, I would probably do the same level of insulation if I wasn't going for it. As you rightly say, it will come down to airtightness as well, so for the sake of a bit of extra time, effort, membrane and tape - why not try I guess.

 

Edit: I should add, I don't think I'll achieve anything near Passiv anyway with the amount of glass going in the front, and the level of insulation, but it can't hurt to get as close as possible.

 

Edited September 20, 2021 by ElliotP

[ElliotP]

I have a quick question, if anyone can answer;

 

Is an external rigid sheathing required? I plan on using Panelvent if so, however i'm wondering if in this case simply a membrane will be enough? The reason I ask is because by adding sheathing, it's pushing the cladding further out.  Currently it sits neatly inside the oak corner posts, but the addition of the Panelvent will push the cladding out and it won't be flush to the edge of the oak.

 

It's not not the end of the world, I can put beading down the edges, but I'm wondering if the sheathing serves a purpose in this case compared to just a membrane?  It'd fit the cladding back over nailing it directly into the studwork through the membrane.

 

Thanks

[Iceverge]

What is your exact wall build up at present on site? Are you planning on removing existing cladding?

 

Passive buildings are a wholistic concept. Super insulating your structure is not a panacea. Airtightness, appropriate glazing and shading are all important. Neglecting anyone and you’ll have an underperforming building. 

[ElliotP]

Hi,

 

Thanks for the reply.  Currently the wall buildup is simply studwork and cladding as per the photo above. Yes, I plan to remove the cladding, and put on breathable membrane (tyvek housewrap probably) and then clad back over. So i'm wondering should i just put the housewrap straight onto the studs and put the cladding back over, or should I/do I need some kind of panel sheathing - a lot of people say osb3, but given the price currently vs panelvent, and the fact panelvent is made for this purpose, I was planning for that - but can i 'get away' with out it?

 

I have to be honest, after doing further research I don't think i'm going to get near PH requirements due to the amount of glass going at the front, and the oak beams thermal bridging. I'm going to PIR over most of the beams internally, but along the front of the building will be only Oak beams and 3g floating glass, which is going to be 9m x 1.8m of glass and oak only.

 

I will still aim for airtightness however, and do the best possible insulation everywhere else, but in this case achieving PH insulation requirements is probably a lost cause. 

 

 

Edited September 23, 2021 by ElliotP