New Project - Garden Office - Research Stage, Help Needed!

[Cliffoshay]

Just found out that my wife is expecting, which is wonderful news but will mean that I will be losing my office.

I work from home and not in a position to move so will have to undertake the project of building a garden office. I have done some research of what's available as a kit but due to very specific requirements in terms of size and shape - not to mention cost - I will require something bespoke.

I have an obvious timescale that I need to complete the project in and would very much value any help and suggestions you are willing to give.

As i will be using this space daily, 12 months a year, a priority is to have decent insulation. These are my thoughts so far:
upvc double glazed french doors and windows - 2nd hand
OSB shell on a timber frame
PU foil backed insulation for walls - 50mm?
Timber stud-work for internal walls
plasterboard / dry-line walls
Cedar cladding or similar for visible walls (2 won't be as against boundaries)
Unsure best type of insulation for floor & ceiling
Shingle roof tiles - undecided
Possibly under-floor heating depending on cost.

Does this sound like i'm on the right track?

Many thanks in advance for any pointers you can give me.

 

[noseall]

Don't skimp on the base/foundations.

Try and build the base so that the building's outer facade overhangs the base. I have lost count of the posts on here that are complaints about water ingress at the base of pre-fab garages due to the slab 'toe' protruding allowing rainwater to settle on the slab.

Make sure you have enough insulation and ventilation.

Fit all insulation snug, foam or mastic any gaps and use foil tape on joins as well as foil backed plasterboards.

If you can afford to, then use 100mm foam board (Celotex, Kingspan etc) for the insulation, on all surfaces.

The last thing you want is a sweaty hot-box in the summer and a mouldy unpleasant place to be in the winter.

 

[Cliffoshay]

Don't skimp on the base/foundations.

Try and build the base so that the building's outer facade overhangs the base. I have lost count of the posts on here that are complaints about water ingress at the base of pre-fab garages due to the slab 'toe' protruding allowing rainwater to settle on the slab.

Make sure you have enough insulation and ventilation.

Fit all insulation snug, foam or mastic any gaps and use foil tape on joins as well as foil backed plasterboards.

If you can afford to, then use 100mm foam board (Celotex, Kingspan etc) for the insulation, on all surfaces.

The last thing you want is a sweaty hot-box in the summer and a mouldy unpleasant place to be in the winter.

Thanks, that sounds like great advice and I will fully take that on board and implement into the design... exactly the sort of tips I was hoping for.

I was wondering about what thickness insulation to use. I noticed that some of the 'higher spec' prefab kits you can buy include 45mm or 50mm but I suppose it's a case of the thicker the better?

If I use foil backed insulation boards do I also need separate DPM, foil sheeting and foil backed plasterboard?

Two other things;
-Should I felt the 2 un-clad, unseen sides?
-Can i dispense with 600mm spaced upright stud-work if I use adhesive to fix the insulation boards to the OSB and same to secure the plasterboard to the foam boards?

Thanks again.

 

[handyman77]

Wouldnt bother with the underfloor heating, you might be better getting a small aircon unit so you can use it for heating and cooling. There is some reasonable prices about.

 

[mointainwalker]

I don't know if you are tight for space down to the odd centimetre, but I always favour fibre-wool over insulation board. Use 100 mm roll-insulation ( needs fixing to prevent slump) and it will be 20% more efficient than 50 mm of Celotex and probably 80% cheaper.

The foam-backed pb you mention is a particularly expensive variant. Use the fibre-wool, a plastic-sheet as a vapour-barrier and normal pb.

UFH is simply not sensible in this kind of building. Apart from numerous other reasons, the cost of ( legally ) installing the heating-circuit cables would be rather high

 

[Cliffoshay]

Good to know, thanks. I'm officially abandoning the UFH idea!

The insulation is expensive but I need to get it right due to the usage. I was originally looking at 50mm but seems that may be insufficient. Pricing 100mm is pushing my budget.

Is it viable to use both types by way of compromise? Or is that overkill?

Ventilation wise, I need some pointers on this. There will be trickle vents on both windows and french doors. What else should I consider? Front and back / high or low? Floor / ceiling?

Thanks again.

 

[mointainwalker]

Is it viable to use both types by way of compromise? Or is that overkill

?

And what reason do you have to do that ?

 

[Cliffoshay]

Just concerned that 100mm may not be sufficient if using roll insulation, given the earlier suggestion of using that thickness of foam board.

So, if I use 40 - 50mm foamboard & 100mm fibre-wool, is that reasonable... or stupid!

 

[mointainwalker]

So, if I use 40 - 50mm foamboard & 100mm fibre-wool, is that reasonable... or stupid!

Well the thing is, you don't really seem to know what you want. What is most important - saving space, ease of construction, saving money, achieving best insulation irrespective of cost, achieving a specific U-value ?

To repeat my own opinion as to the question above, if you have 150 mm space, i would use 150 mm of fibre-wool as it is very cheap and 60 % efficient vs PU but only 20% of the price. it will however be more difficult to fix into place.

This may however be over the top especially if you only have a small space - what are the dimensions ?

 

[Cliffoshay]

So, if I use 40 - 50mm foamboard & 100mm fibre-wool, is that reasonable... or stupid!

Well the thing is, you don't really seem to know what you want. What is most important - saving space, ease of construction, saving money, achieving best insulation irrespective of cost, achieving a specific U-value ?

To repeat my own opinion as to the question above, if you have 150 mm space, i would use 150 mm of fibre-wool as it is very cheap and 60 % efficient vs PU but only 20% of the price. it will however be more difficult to fix into place.

This may however be over the top especially if you only have a small space - what are the dimensions ?

Fair enough! Don't get me wrong, I appreciate any and all guidance.

The situation is that I am trying to get as much info at this planning stage in order to quantify the materials, cost and design against benefits. I am flexible to some degree but need to weigh up the pro's and con's. I am on a budget and want to make every £ count. I will be annoyed with myself if I skimp and end up regretting not doing it right first time and having to spend more on heating. I hate false economies!

The structure will be built in the corner of my modest back garden. It's a odd angle - about 80 degrees - and dimensions wise, will be between 3m x 3m to 4m x 4.5m.

I'm relatively competent when it comes to DIY and fortunate enough to have lots of practical help from friends and family if needed. But I like to get the theory side 'hammered out' first.

In terms of the insulation, this is a big chunk of my budget which obviously doubles if I opt for 100mm foam board over 50mm. I like the fact that it's foil backed so my previous point was referring to if I can get the best of both worlds in terms of using fibre-wool and PU.

I'm afraid I don't have a frame of reference regarding what u-value to work to.

I hope that helps to clarify things.

 

[mointainwalker]

For whatever reason there doesn't seem to a meeting of minds here, so here's my last attempt to get it across.


1) 1 m2 of 50 mm celotex will cost around GBP 6.50
2) 1 m2 of 100 mm Knauff fibre-wool from B&Q costs GBP 0.35

1) has a U value of 0.5
2) has a U value of 0.4

The fibre wool is 18. 5 times cheaper than the celotex and 1.25 times more efficient in its insulation properties.

Buying the fibre wool is therefore 23 times ( that is 2300 % ) more cost-effective in insulation terms than buying the Celotex in exchange for some extra work fixing it into position. When there is such a huge price difference, I don't understand why you want to mix the two products, especially when you can't give a reason for doing so. You talk of getting the best of both worlds, but what is the big advantage you think you are getting with the Celotex that outweighs the massive cost-penalty, especially when you say pennies are tight ?

 

[Cliffoshay]

Thanks mointainwalker, i did get your point first time round. Here's my thought process.
1) looking at prefab kits available from various companies showed spec including foil backed PU insulation with a thickness of 45mm / 50mm+. So that was my starting point. I like the fact that its foil backed so I wouldn't need to do that separately.
2) The first reply on here advised to use 100mm foam board insulation if I can afford it. That made me think that 50mm may be inadequate but the cost of the extra will seriously dent my budget.
3) Your point on using fibre-wool is well taken but if 50mm pu is inadequate, presumably so would 100mm fibre-wool? The cheaper cost is a big plus though.
4) I'm thinking - but may be wrong - that if I use just fibre-wool, I will need more timber for the uprights. Also, I will not be able to achieve an unbroken 'bank' of insulation on all sides which presumably must impact on the overall insulating properties? I will also have to use a separate barrier to compensate for the lack of foil-backing
5) I can live with 150mm insulation but anymore will look daft for the size of the construction.
6) My suggestion about using both types was centered around the notion of improving on the insulation properties of 50mm with out a big jump in cost

Sorry if I'm not getting my thoughts across very well. I've not tackled anything along these lines before. I'm grateful that you're persevering with me though!

 

[mointainwalker]

but if 50mm pu is inadequate, presumably so would 100mm fibre-wool?

FGS man read the numbers above including the U values and look that up if it is an unfamilar term. That apart how can you say it is inadequate since you have no target and don't know what you want

 

[Cliffoshay]

but if 50mm pu is inadequate, presumably so would 100mm fibre-wool?

FGS man read the numbers above including the U values and look that up if it is an unfamilar term. That apart how can you say it is inadequate since you have no target and don't know what you want

Not my intention to anger you. I'm not sure how else to spell out where I'm coming from.

Regarding not knowing what i want and not having a specific U value to work to... that's why I'm asking for guidance! You seem incapable of understanding my perspective or addressing any of the points I've raised.

 

[mointainwalker]

O.K.

I've said ( not on here but I have ) I want to try to be a better, nicer, more patient person so this seems to be a good opportunity to try out that new resolve !

The problem I have is - bluntly - you don't know a lot but wish to question everything, even if you don't then understand the answer or the ramifications.

Help me to get to an answer for you and answer the following.

1) Where do you live in UK
2) Exposed area or town ?
3) How much heat in kW do you envisage needed to heat shed ?
4) What is your total budget and how much is for insulation ?
5) What percentage of shed will be doors/windows ( big heat-loss items )
6) How many watts of energy will computers/printers/anything in shed put out ?
7) How high will walls be ?
8 ) What is lowest temp you have had during day this winter ?