Building a Shoffice/Garden Room - value engineering it

Building a Shoffice/Garden Room - value engineering it

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Pheo

Original Poster:

3,324 posts

201 months

Sunday 12th July 2020
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Given COVID, and warnings from my company that we may expect to be WAH well into next year, and having lost my office to the new baby, I’m considering building a combined 6x3.5 office / shed (two separate entrances most likely.

Construction would be timber frame, EDPM roof, windows in standard sizes (hoping to reduce cost)

I’d rather not spend the GDP of a small nation, so I’m trying to work out how to value engineer it, having done a rough calc and ended up with a cost of £2.7k just for wood and a bit of insulation last night (and it was very rough!). I’ve also looked at a garden room SIP kit which is about £3.7k but only would need cladding/roof membrane, a basic base and glazing (and has the speed advantage!)

So looking for advice from those who’ve tackled such a project, about where you can make accommodations to reduce cost. Some thoughts from me:

- Recommendations online seem to say 100mm celotex in the floor. Wow is this stuff expensive even from seconds and co. Could I use 100mm of EPS and suck up the insulation loss (how much difference would it make anyway, our 1930s semi not having any insulation in the timber suspended floor!)

- rockwool the walls instead of celotex again, using 4x2 walls so 100mm rockwool rolls would work

- celotex the ceiling only (probably doing cold roof due to 2.5m permitted development height limit, have seen someone online doing it so they have the celotex hard up against the roof deck, leaving the void under which seems reasonable if unorthadox, given this is a harden room

- foundation plinths from blocks with a light 500-600mm concrete foundation under. Set the spans up to enable 4x3 joists in floor to reduce costs

- buy second hand glazing on eBay. Works well when stick building as I can adjust the frame to accommodate what I can get. Not so good for SIPS

- Clad it in black corrugated steel ( seems lots of modern buildings doing this and if I detail it well it’s relatively good looking), significantly less £sqm than hardieplank, etc. Could also featherboard but would need to fire treat it

- use the structural floor covering as the final floor finish, painted or varnished (can always be changed later)

Any other obvious things I’m missing which could bring the cost down?

Pheo

Original Poster:

3,324 posts

201 months

Monday 13th July 2020
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No takers? Or everyone too busy building their own?

Harry Flashman

19,283 posts

241 months

Monday 13th July 2020
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Watching with interest as I want to do the same, but sadly nothing helpful to contribute!

PhilboSE

4,323 posts

225 months

Monday 13th July 2020
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In terms of value engineering, you seem to have covered most of the bases. To do it the cheapest possible way I guess it's a case of identifying the cheapest possible source for components.

Blank SIPs are often available on ebay cheaply, but you'll have to go to extra effort to work with them. Same goes for solid insulation panels - they can come up on ebay for cheap.

Rather than do a solid slab base, you could just lay 100mm crushed concrete/scalpings as a base and then elevate the room on timber or concrete sleepers.

Otherwise it's just a case of sourcing the cheapest elements e.g. cladding, glazing that will do the job, if you're happy to live with the result.

Personally I wouldn't scrimp on the insulation if you're planning on using it year round as that would be a false economy.

Equus

16,770 posts

100 months

Monday 13th July 2020
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Pheo said:
No takers?
Every building is a compromise.

You have come up with a detailed specification and justification for it that you are clearly happy with (which is fine). I have a feeling that this would be one of those classic threads where you simply argue away anyone who suggests alternatives, because your mind is already set.

The one observation I will offer is that black metal cladding on a thermally lightweight timber frame building is as good a recipe as you'll find for death by heatstroke.

4Q

3,347 posts

143 months

Monday 13th July 2020
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groomi

9,317 posts

242 months

Monday 13th July 2020
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As a reference, I built a 4m x 4m log cabin kit from Dunster House a few years ago for about £5k all in. It's a reasonably high spec as I need to use it all year round and don't want to be replacing it any time soon.

  • Existing concrete slab from old shed.
  • Plastic risers with separate timber bearers, so the whole cabin sit off the slab to prevent rot. There are many different ways of approaching this problem - most of them cheaper than my chosen solution.
  • 40mm pressure treated tongue & groove outer wall, 100mm void filled with Celotex, 30mm tongue & groove inner wall. (I'm now doubting myself and thinking it's 70mm in the walls).
  • 50mm Celotex under 25mm floorboards, then covered with laminate floor tiles.
  • 25mm Celotex on the roof. It's supplied glued to all weather boards. Sit insulation onto the roof planks with the board facing upwards, cover with a plastic membrane and then felt shingle tiles. No voids in this system.
  • Double glazed double doors and single window.
  • Guttering and water butts.
  • DIY electrics.
I've been using it about three and a half years now, all year round. I have a small oil filled radiator for the winter (which I just run on a low temperature set to come on automatically), and a tower fan for the summer.

The only problem I've run into is my layout has the glazed doors and window all on the front. It gets morning sun from about 6am onwards, so if I don't start my day before about 9am it is already baking inside and with all the openings on the front with the sun still beating, it takes a while to move enough air out to cool it down again. In hindsight I'd probably have a side window too so I can move air through it easier.

If you do a lot of conference calls, then another consideration is the roof construction and the possible noise in heavy rain. The noise on mine is noticeable, but not a problem. I wonder if some other constructions might be noisier or quieter - I don't have experience of any others to compare it against though.

Edited by groomi on Monday 13th July 20:43

Carbon Sasquatch

4,582 posts

63 months

Monday 13th July 2020
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Pheo said:
- rockwool the walls instead of celotex again, using 4x2 walls so 100mm rockwool rolls would work
I wouldn't.....

General rule of thumb is that celotex (and similar) are 3x as efficient as rock wool - so you only really use the latter where space isn't an issue, like the average domestic loft. I'd just bite the bullet & celotex the lot.

technodup

7,576 posts

129 months

Monday 13th July 2020
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Pheo said:
- buy second hand glazing on eBay. Works well when stick building as I can adjust the frame to accommodate what I can get. Not so good for SIPS
Ask around local double glazing places for mis-measures. Most will have some lying around and if they're feeling generous you might get one for free. Better than some knackered window someone's ripped out and stuck on Ebay.

Pheo

Original Poster:

3,324 posts

201 months

Monday 13th July 2020
quotequote all
Equus said:
Every building is a compromise.

You have come up with a detailed specification and justification for it that you are clearly happy with (which is fine). I have a feeling that this would be one of those classic threads where you simply argue away anyone who suggests alternatives, because your mind is already set.

The one observation I will offer is that black metal cladding on a thermally lightweight timber frame building is as good a recipe as you'll find for death by heatstroke.
I know better than to argue with you Equus, although I'm pleased you think my spec at least some sense (given I've never specified a timber frame boiling before!).

Hadn't considered the heat problem with the cladding; intuitively you think it would vent given it'd be battened off, but thats just not realistic is it. Might have to have a think about that one then, or find another solution (e.g. fire proofing some wood). Funnily enough when I was in New Zealand a couple of months ago my dad and I remarked about how they loved to paint their wooden frames houses dark colours, we figured perhaps it didn't have the heating effect we thought it would.

RE insulation, point noted from various posters, I wasn't aware rockwool was only 1/3rd the performance, I thought it was better than that tbh, that would only be 30mm, which wouldn't be enough. Does anyone know if EPS is worthwhile considering? 100mm full fill EPS I think would be equivalent to 50mm Celotex but a bit cheaper (although possibly not worth it).

One thing I'm finding hard is to work out the practical difference; I guess adhering to building regs or thereabouts you know you'll end up with a habital quality building, just got a nagging feeling it might be a little OTT and I could trim here and there, its just working out which bits don't matter so much, e.g. is the heat loss through the floor less than the heat loss through the roof, therefore bias towards more insulation there and make compromises in the floor.

Thanks all for the inputs, it helps.

BobSaunders

3,027 posts

154 months

Monday 13th July 2020
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Equus said:
Pheo said:
No takers?
Every building is a compromise.

You have come up with a detailed specification and justification for it that you are clearly happy with (which is fine). I have a feeling that this would be one of those classic threads where you simply argue away anyone who suggests alternatives, because your mind is already set.

The one observation I will offer is that black metal cladding on a thermally lightweight timber frame building is as good a recipe as you'll find for death by heatstroke.
OP on the way to 'work' in his punishment box.



Pheo

Original Poster:

3,324 posts

201 months

Monday 13th July 2020
quotequote all
BobSaunders said:
OP on the way to 'work' in his punishment box.

rofl

Tin Hat

1,370 posts

208 months

Monday 13th July 2020
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I would avoid EPDM roofs, we stopped installing them about 3 years ago, they are problematic. Kemperol is relatively inexpensive and significantly more reliable.

I have built our room with the deck directly over 100mm celotex, using foil back plasterboard beneath the joists as a token vapour barrier. I personally think that the cold/warm roof dilemma is not the drama that it could be in a garden room environment, as opposed to a living space that may have bathrooms and kitchens where significant moisture is generated.

I would definitely use celotex in the walls.

Cement board can be very effective for cladding, it is highly stable and durable and can often react well to being painted, requiring little long term maintenance.

Buy timber from a reputable timber merchant, if you use general Building goods suppliers, you’ll find 20% is unusable due to twist or warp.

Pheo

Original Poster:

3,324 posts

201 months

Monday 13th July 2020
quotequote all
Tin Hat said:
I would avoid EPDM roofs, we stopped installing them about 3 years ago, they are problematic. Kemperol is relatively inexpensive and significantly more reliable.

I have built our room with the deck directly over 100mm celotex, using foil back plasterboard beneath the joists as a token vapour barrier. I personally think that the cold/warm roof dilemma is not the drama that it could be in a garden room environment, as opposed to a living space that may have bathrooms and kitchens where significant moisture is generated.

I would definitely use celotex in the walls.

Cement board can be very effective for cladding, it is highly stable and durable and can often react well to being painted, requiring little long term maintenance.

Buy timber from a reputable timber merchant, if you use general Building goods suppliers, you’ll find 20% is unusable due to twist or warp.
Are you able to give a bit more detail on the roof makeup you're talking about? Not 100% sure I understand the makeup you're talking about?

Will look into the alternative roof products, and the cement board.

mikeiow

5,287 posts

129 months

Tuesday 14th July 2020
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Tin Hat said:
I would avoid EPDM roofs, we stopped installing them about 3 years ago, they are problematic. Kemperol is relatively inexpensive and significantly more reliable.

(snip)
Just a side question: I’m curious what problems you have had with EPDM....we’ve recently done a 13’ x 5’ storage shed with it, went on a treat, I have high hopes it will outlast me!


trickywoo

11,705 posts

229 months

Tuesday 14th July 2020
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You’ll see no benefit of 100mm celotex in the floor.

25 is more than good enough.

anonymous-user

53 months

Tuesday 14th July 2020
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Is something like this not actually cheaper by the time you have added it all up?

https://www.tuin.co.uk/Helge-with-Shed-Annexe-3.5x...

https://www.tuin.co.uk/Erke-Log-Cabin-5.85x3.3m.ht...

I am looking at similar myself so just wondering which route to go

Pheo

Original Poster:

3,324 posts

201 months

Tuesday 14th July 2020
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anonymous said:
[redacted]
Does not appear to have any insulation whatsoever..: bit of a challenge no? And as it’s solid wood construction I think you’d have a lot of issues boarding it out to add insulation / need to buy the insulation which is then significant additional cost.

anonymous-user

53 months

Tuesday 14th July 2020
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Pheo said:
Does not appear to have any insulation whatsoever..: bit of a challenge no? And as it’s solid wood construction I think you’d have a lot of issues boarding it out to add insulation / need to buy the insulation which is then significant additional cost.
Ahhh ok interesting thanks hadn't considered that

anonymous-user

53 months

Tuesday 14th July 2020
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trickywoo said:
You’ll see no benefit of 100mm celotex in the floor.

25 is more than good enough.
It's true that the first insulation you add will be the most effective, but untrue to say there is no benefit in adding more. There is a reason why building regulations specify U-Values for floors, walls, roofs, glazing etc, it saves money in the long term and, more importantly, increases comfort.

For a summer house it may not be worth it, but for a building you could potentially be spending 40+ hours a week in, at all different times of the year, insulating it properly is a no-brainer. It's much better to spend a few quid extra and do it properly from the start, than have to refit more insulation later or put up with a workspace where you have to wear two pairs of socks to stop them going numb!