AndyFExtension

Overheating timber frame

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Hi.

I am looking to build a timber frame extension.

 

as a stepping stone to that I built a garden office:

70mm PIR insulation in studs - cedar cladding

100mm PIR warm roof.

 

In the warm weather the internal temperature is +1-2 degrees higher than outside even with a fan on - pretty unbearable today to work in.

 

my concern is that I will be replicating the problem on a south facing extension (assuming 120mm PIR is used).

 

My questions are:

a) what did I do wrong with the garden office

b) what should I have done differently and ensure I reflect in the extension

 

my current thinking on b) is to mix up the insulation types e.g mineral wool and eps/PIR.


many thanks

 

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Some options: More insulation, triple glazing, roof velux to let some heat out? 

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What’s under the office? Is it free standing or on a solid concrete base. Good insulation will keep the heat in and cold out. But without a big thermal mass it will heat up quickly and keep that heat in too.

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Posted (edited)

While PIR performs well with a masonry skin, it's not going be as good in a light weight timber frame structure due to its short decrement delay.

For a light weight structure you need to pick an insulation with a longer decrement delay. Insulating material with low thermal conductivity, high density and high specific heat capacity give a long decrement delay. Cellulose fibre and wood fibre are better matched to a timber frame structure.

 

Edited to ask:
How much glazing does your garden office have? can sunlight come directly in to the room? If it can how are you shading the room?

Solar gain will heat the room up very quickly.

Edited by IanR
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Posted (edited)

You need to look at the volume to solar exposed surface area.

The larger the building, generally, the less exposed area there is, this limits the energy transfer.

Best to just do some calculations and scenarios for different conditions. I think Jeremy Harris's spreadsheet can accept solar inputs.

Edited by SteamyTea

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Thanks for the comments.

 

the office has performed well in the winter. The floor is suspended timber joists with 100mm PIR between.

 

I have a small triple glazed window and I have 2g sliding doors. They are largely facing away from the sun.

 

it is ultimately a small space (3m x 3m) so the P/A is 12/9

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Posted (edited)

Following on from your comments

 

as an alternative to just 120mm in between studs.

 

what do you think to adding the below to improve comfort in summer and maintain a good u value:

 

a - 25mm rock wool (100kg/m3) inboard of pir outside of vcl

 

b) steico universal sarking board either instead of external osb sheathing or in addition to?

 

both of them?

 

I was thinking of adding 100mm EPS as external insulation could this also work in the above build up?

 

many thanks

Edited by AndyFExtension

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On 19/07/2021 at 19:44, AndyFExtension said:

Thanks for the comments.

 

the office has performed well in the winter. The floor is suspended timber joists with 100mm PIR between.

 

I have a small triple glazed window and I have 2g sliding doors. They are largely facing away from the sun.

 

it is ultimately a small space (3m x 3m) so the P/A is 12/9

 

What else is in the office generating heat i.e. computers, displays etc?

 

In this warm weather our passive tf house works well at keeping cool until the sun flips over to the west side where we don't have external shutters on the glazing and by that time the external air is warm so keeping both direct sun and warm air out of the house is a priority.

 

As soon as the sun goes low, air temp drops considerably and the house can be stack ventilated until morning when the cycle starts again.

 

We have blown cellulose so high decrement delay which seems to be working well, external shutters on east windows which is very effective at reducing solar gain. 

 

Need to sort the west aspect where we have large sliders - either a film or external structure to provide more shade. We have some linen drapes inside that are somewhat effective but insufficient.

 

However the home office, in a NE room, is always warmer due to displays, laptops etc chucking out extra heat.

 

 

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7 hours ago, AndyFExtension said:

Following on from your comments

 

as an alternative to just 120mm in between studs.

 

what do you think to adding the below to improve comfort in summer and maintain a good u value:

 

a - 25mm rock wool (100kg/m3) inboard of pir outside of vcl

 

b) steico universal sarking board either instead of external osb sheathing or in addition to?

 

both of them?

 

I was thinking of adding 100mm EPS as external insulation could this also work in the above build up?

 

many thanks

A decent ventilation void on any sun baked elevation will pay dividends. 25mm of rockwool is not worth the effort. EWI would not compensate fully for solar gains, so factor in mitigation against the most prolific problem causer.

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I don't think you have done anything wrong or unexpected.

 

Most of my house at the moment is sitting 3-4 degrees warmer than outside at the moment. The only areas that are not warmer than outside are the garage and a cupboard that have no windows at all. The WC with a small obscured window which faces north is still a couple of degrees warmer than outside.

 

In an insulated space with windows solar gain is likely to make it warmer than outside. I don't think that changing the materials would make much difference to this.

 

You can limit the solar gain via shading, sizing and orientation of windows, but without some kind of active cooling method that is as well as you can probably do.

 

I am finding that the air temperature stays high well into the night at the moment. Opening windows will help during the nigh, but mainly in the say 2-8am period.

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35 minutes ago, AliG said:

Most of my house at the moment is sitting 3-4 degrees warmer than outside at the moment.

That is my experience with my timber frame, 1987 vintage.  I even have that sort of temperature rise in the, in the unheated parts, in winter, usually more.

36 minutes ago, AliG said:

You can limit the solar gain via

The front of my house is SW facing, so the last few days have been tough.

I stuck tin foil on the inside of the bedroom window as a temporary measure.  Works brilliantly and only cost a quid.

39 minutes ago, AliG said:

Opening windows will help during the nigh, but mainly in the say 2-8am period

Last two nights have been killers.  For only the third time in my life I had heat stroke yesterday, got home just wanting to sleep, could I (expletive deleted).

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